Pinusponderosa Pinusponderosa, Pinuslabs.bio.unc.edu/Peet/pubs/vegetatio45;3.pdf · 2009-07-07 ·...

Forest vegetat ion o f the Co lorado Front Range

Composition and dynamics*

Robert K. Peet** Department of Botany, University of North Carolina, Chapel Hill NC 27514, USA

Keywords: Colorado, Forests, Front Range, Gradient analysis, Population structure, Rocky Mountains, Succession, Vegetation

Abstract

The forest vegetation of the northern Colorado Front Range was studied using a combination of gradient analysis and classification methods. A graphical model of forest composition based on elevation and topographic-moisture gradients was constructed using 305 0.1 ha samples. To derive the topographic- moisture gradient, stands were stratified into eight 200 m elevation belts, and then ordinated by correspondence analysis using understory (<1 m) data. Each of the resultant gradients was scaled against a standard site moisture scalar derived from incident solar radiation and topographic position. Except for krummholz sites, the vegetation defined gradients fit the moisture scalar closely. Once scaled, these gradients were stacked vertically, sandwich-style, to create the graphical representation shown in Figure 5.

Gradient analysis and ordination (direct and indirect gradient analysis of Whittaker, 1967) are frequently viewed as alternative approaches for analysis of vegetation. With gradient analysis the axes are readily interpretable, but stand placement is often difficult and at times questionable. Ordination defines an optimal arrangement for species and/or stands, but axis interpretation is often impossible. With the present combination of methods, the interpretability of gradient analysis complements the precision of placement obtained with ordination.

Forest vegetation was classified by dividing the gradient model into eight series and 29 types on the basis of similar successional trends in canopy dominants. On dry, low-elevation sites above 1 700 m Pinusponderosa woodlands dominate. With increasing elevation or site moisture, tree density increases and Pinusponderosa, Pseudotsuga forests prevail. At middle elevations on mesic sites forests of mixed composition occur. Pinus

* Nomenclature follows Weber (1972) for most species. In some cases where Weber's narrow generic concept deviates from the main thrust of present-day North American systematic botany, names were changed to conform with Harrington (1954) and Hitchcock & Cronquist (1973). Voucher specimens have been deposited in the herbarium of Rocky Mountain National Park, with a few unusual species being deposited in the herbarium of the University of Colorado, Boulder.

A few species pairs presented consistent problems and their treatment as single species was necessary. Garex rossii and C. brevipes were lumped as Carex rossii. Rosa woodsii and R. acicularis were lumped as Rosa sp. Cirsium scopulorum and C. coloradense were lumped as Cirsium coloradense. Extreme forms of Arnica cordifolia and A. latifolia are easily distin- guishable, but as these species intergrade and hybridize ex-

Vegetatio 45, 3 75 (1981). 0042-3106/81/0451-0003/$14.60. © Dr W. Junk Publishers, The Hague. Printed in The Netherlands.

tensively, they have been lumped as Arnica cordiJolia. The native bluegrass, Poa agassizensis, was lumped with Poa pratensis. Solidago missouriensis includes some S. canadensis. ** Numerous individuals have contributed generously to this project. Among those to whom I am particularly indebted are B. Chabot, R. T. Clausen, C. V. Cogbill, J. Douglas, H. G. Gauch, Jr., D. C. Glenn-Lewin, D. Hamilton, K. H. Hildebrandt, D. Mueller-Dombois, R. L. Peet, D. Stevens, E. L. Stone, J. Vleck, W. A. Weber, T. R. Wentworth, and P. L. Whittaker. 1 especially thank R. H, Whittaker for advice and encouragement. Financial support was provided by grants from the National Science Foundation, the DuPont Foundation, Cornell Uni- versity and the University of North Caroiina Research Council. The cooperation and support of the National Park Service is gratefully acknowledged.

contorta forests dominate at middle elevations over much of the central position of the moisture gradient, though these are primarily post-fire forests. With protection from fire only a small percentage of sites retain dominance by Pinus contorta. Over the lower portion of its range Pinus contorta is succeeded by Pseudotsuga, while at higher elevations Abies lasiocarpa and Picea engelmannii can eventually achieve dominance. At high elevations on all except the driest sites Picea engelmannii and Abies lasiocarpa are exclusive dominants, both after disturbance and in climax forests. Pinusflexilis dominates on the driest high-elevation sites. Above 3 500 m forests are replaced by alpine tundra, often with a transitional krummholz zone.

Structure and post-fire development were examined in the context of the gradient-based classification scheme. Three generalized types of forest development were recognized as reference points in a continuum of developmental patterns var~?ing with both elevation and soil moisture.

On favorable, middle-elevation sites, trees become established rapidly after disturbance. Rapid growth results in severe overcrowding and competitive elimination of reproduction. As a consequence bell-shaped diameter distributions develop. Diversity and productivity appear to drop while biomass remains roughly constant. Following decades or even centuries of stagnation, the forests eventually breakup through mortality of the canopy trees, thereby allowing regeneration to resume. During this period of renewed regeneration, biomass, diversity, and productivity all show dramatic changes in response to the changing population structure (Fig. 9). This type of forest development can be found in forests dominated by Picea engelmannii and Abies lasiocarpa, Pinus contorta, Pseudotsuga menzeisii, Pinus flexilis or Populus tremuloides. On highest elevation forest sites or at middle elevations on the very driest sites reestablishment rates are greatly reduced. These forests dominated by Picea and Abies or Pinusflexilis gradually approach predistur~ance levels of biomass, diversity and productivity, while regeneration remains at a roughly constant level. At lower elevations in the Pinus ponderosa woodlands, regeneration appears episodic, reflecting variation in seed rain and favorable conditions for seedling growth. Here, inter-tree competition is relatively unimportant and diameter distributions show irregular humps resulting from periodic recruitment.

Introduction Background

The Front Range, rising abruptly from the Colorado plains, occupies a central position along the east side of the Rocky Mountain massif. Despite considerable botanical and ecological study, an integrated picture of the composition and dynamics of the forest vegetation of these mountains has not yet emerged.

This monograph represents an attempt to clarify and expand our knowledge of Front Range forests. Specifically, the vegetation of the Colorado Front Range has been studied with regard to the composition of the forest vegetation as related to environ 7 mental gradients. Stand development following disturbance is interpreted relative to the environmental gradients and community types described. Patterns in species diversity and geographical variation in forest composition have been treated in earlier papers (Peet, 1978a, 1978b). Changes in size and age structure of tree populations will be the subject of a future paper.

Location

The Front Range constitutes the major range of the southern Rocky Mountain physiographic province (Fenneman, 193 I; Thornbury, 1965). Over 300 km in length, the Front Range extends from the Arkansas River on the south, north into Wyoming where the Laramie and Medicine Bow Ranges replace it to form the terminus of the province. Bordered on the east by a foothill belt from 5 to 20 km wide, the mountains climb from 1600 m at their base on the plains to nearly 4350 m on the highest peaks. On the side of these mountains is found a broad band of forest vegetation bordered both above (>3 500 m) and below (<I 700 m) by shrub and grassland formations. The present study is an investigation of this belt of forest vegetation as represented in Rocky Mountain National Park and the adjacent foothills.

Rocky Mountain National Park, Colorado, en- compasses an area of over l 060 km 2 between 40 ° 10' and 40032 , north latitude and 105031 ' and 105°41 ' west longitude• In addition, the study area includes the foothills to the east, largely in the Roosevelt National Forest and west of 105 ° 15'west longitude (Fig. 1). The study was confined to the east side of the range as defined by the continental divide south of Fall River Pass, and by the crest of the Mummy Range to the north.

Geology

The study area is underlaid almost entirely by Precambrian granites, gneisses, and schists; the only exception being the easternmost fringe which occurs over the Fontain formation, a Pennsylvanian arkose sandstone and conglomerate (Boos & Boos, 1934; 1957; kovering & Goddard, 1950). The most important rock is Silver Plume granite, a coarse- grained, commonly prophyritic gran-ite which makes up the Long's Peak - St. Vrain batbolith (Peterman et aL, 1967; Boos & Boos, 1934). Second in importance is the Idaho Springs Formation, a set of Preeambrian metamorphic rocks derived from fine-textured sandstones and shales. Moving from west to east, Idaho Springs rocks form a sequence from schist-graphic granite, to biotite-sillimanite schist, to biotite-chlorite schist, to quartz schist. Interspersed are areas of gneiss as well as frequent pegmatite dikes (Fuller, 1924). Also important in the study area is Mt. Olympus granite, a massive, fine to medium grained, even textured granite (Boos & Boos, I934).

Soils

Soils of the Front Range are mostly immature, heterogeneous, slightly acid and coarse-textured, usually very rocky. Steep topography and frequent occurrence of fire with consequent increased erosion have limited the time available for soil maturation on many sites. Most of the more stable soils above 2 300 m occur on till dating only from the last glacial advance, though Richmond (1960) has described localized soil development on isolated tills of greater age. Lithic orthents are found throughout where bedrock approaches the surface, and frequent large rocks and boulders contribute to marked soil heterogeneity as does variable depth to

bedrock. The fine-textured soils which contribute to the formation of mountain grasslands are largely confined to the wider bottomlands.

A detailed soil survey has not been conducted for the study area and an intensive study of the rocky, heterogeneous, immature soils of the region was not undertaken as part of the present study because of time constraints. Instead, a general portrait of soildistribution and composition is presented based on a combination of previously published works (Hanson & Smith, 1928; Johnson & Cline, 1965; Olgeirson, 1974; Retzer, 1974; Smith, 1969) and field reconnaissance (Soil taxonomy follows U.S.D.A., 1975). The landscape is divided into four elevation zones for soil description, according to Johnson & Cline (1965) and Mart (1961),

The lower montane zone (I 800 m - 2 450 m) is dominated by Ustolls and cryoboralfs, As in all the elevation zones, frequent areas of lithic orthents occur where bedrock approaches the surface, and various alluvial soils are found in the bottoms. Ustolls are the most important soils of the zone, being found on all slopes and aspects at low

STUDY AREA

ROCKY MT. NATIONAL PK

NATIONAl,. -- FORES'; LARAMIE

'WYOMING

COLORADO

ROOSEVELT LARIMER \ NATIONAL • FOREST )

WELD

41°I

~u-~ ROOSEVELT / NATIONAL

~S~ FOREST

F~I Boutder I ~i

i ~ INSTAAR

ARAPAHO ) ' NATIONAL GILPIN FO~ST .(

40or,

Fig. 1. Map showing location of study area relative to Rocky Mountain National Park and the State of Colorado USA.

elevations and primarily on south-facing slopes at higher elevations. The dense Pseudotsuga menziesii forests of north-facing slopes are usually encountered on cryoboralfs.

In the upper montane zone (2 450 m - 2 850 m), forest vegetation is more fully developed and cryoboralfs are the most important soil type. At this elevation ustolls are confined to south-facing slopes where open Pinus ponderosa forests of the lower montane extend into the zone. Low grassy meadows are primarily on cryaquolls and various histosols.

Soils of the subalpine zone (2 850 m - 3 500 m) are more homogeneous, as is the vegetation. The widespread Picea engelmannii - Abies lasiocarpa forests occur predominantly on cryorthods, regardless of aspect. Poorly drained sites produce cryaquods. Less dense forest stands of exposed sites are frequently on cryoboralfs. At this elevation boggy soils are extensive, with both woody and sedgy peats common (borohemists and borofi- brists). In the alpine zone (above 3 500 m) cryumbrepts dominate the alpine turf areas, cryaquepts are found in most of the meadows and histosols are important in the boggy areas.

Climate

A relief of 2 600 m coupled with a diverse topography produces complex climatic conditions. While a detailed meteorological study was beyond the scope of the present investigation, the Institute of Arctic and Alpine Research (1NSTAAR), has collected weather data since 1952 at a series of four stations 15 km south of the National Park boundary, located at 2 195, 2 591, 3 048 and 3 750 meters (Marr, 1961, 1967; Marr et al., 1968, 1968; Barry, 1972, 1973). Supplementary data were extracted from the U.S. Weather Bureau records for Estes Park, Aliens Park, and Waterdale. Estes Park is centered on the eastern boundary of the National Park at 2 285 m. Waterdale is located almost due east of Estes Park on the edge of the foothills at 1 603 m. Both stations are approx. 40 km north of the INSTAA R transect. Allens Park (2 591 m) is on the eastern boundary of the National Park, only 20 km north of INSTAAR.

Climatological data are summarized using the Kl imadiagramm of Walter & Lieth (1967). Figure 2

shows 'climate diagrams' for Waterdale and the four INSTAAR stations. The Estes Park and Allens Park data are consistent with the trends in the 1NSTAAR data.

C o m p a r i s o n of the four INSTAAR stations reveals a gradient in mean annual temperature from 8 . 3 ° C a t 2 195into 3.3 ° a t 3 750m, a lapse rate of 7.5 °C per 1 000 m elevation. With increasing elevation the mean daily minimum for January drops f rom 7.8 ° to 16.1 ° and the daily maximum for July drops from 28.3 ° to 19.4 °. The Waterdale station allows extrapolation to the base of the foothills where the mean July daily maximum is 30.7 ° . Barry (1973) examined the mean daily ranges of temperature and found the greatest variation at the lowest elevation site (2 195 m) where the maximum range of 15 °C occurs in July. At 3 750 m no month has a mean daily range in excess of 8 °C. Average frost-free periods are 125, 104, 56, and 47 days for the INSTAAR stations, and are shorter in valleys subject to cold air drainage. At high elevations frost can occur at any time during the year (Griggs, 1956; Ives, 1946).

The climate of the Front Range is strongly continental in character with sudden, extreme changes in weather possible at any time (Marr, 1961; Ives, 1938). Winter weather is the most predictable with cyclonic storms dominating the precipitation patterns, occasionally causing deep snows. With the northward shift of the prevailing westerlies in the summer, cyclonic storms become few in number and erratic in behavior. Summer precipitation is dominated by local, convective valley storms which occur on a regular basis throughout July and early August, diminishing late in the summer as moisture is lost through stream flow,(Ives, 1938). These storms can be very violent producing damaging hail and not infrequent snow above 2 400 m. Lightning generated by such storms is.an important cause of forest fire.

With the exception of the 3 750 m site, all INSTAAR sites show maximum precipitation in May with a secondary increase in July and August reflecting convective summer storms. Average total precipitation rises from 532 mm at 2 195 m to 1 050 mm at 3 750 m. Ives (1942) has postulated that certain favorably located subalpine forests, usually in sheltered cirque-basins, receive daily convective

INSTAAR (AI) 2195 M

h -9.'] i - 35.C

o b d e

Waterdale 1 6 0 3 m 8.8 = 5 9 5 "

( 5 4 - 7 7 ) c " 1 t 37 .8

2 8 . 3

m

-7 .8

-35,9

f 38 .9 g 50.7

INSTAAR (BI) 2 5 8 0 M

34.4 INSTAAR (CI) =,.,050 M , 657

25.0 25.0"

19.4,

- I 0 . 0

-35.6 -I 2.2

-56 .6

19.4

12 2

-16.1

-36.E

Fig. 2. An e levat ional sequence of c l imate d iag rams based on

data col lected at Wate rda le and the Ins t i tu te of Arct ic and

Alp ine Research ( I N S T A A R ) . The d iag rams fol low the fo rma t

p roposed by Wal t e r & Lieth ( [ 967). Abscissa in mon ths s t a r t ing

with January , ord inate wi th one d iv is ion -- 10 ° C or 20 m m

prec ip i ta t ion , a = s ta t ion, b = e leva t ion in m above sea level, c =

yrs of t e m p e r a t u r e and p rec ip i t a t ion records, d - mean a n n u a l

t empera tu re in degrees C, e = mean annua l p rec ip i ta t ion in mm,

f = highest t empera tu re on record, g = mean dai ly m a x i m u m of

the warmes t m o n t h (July) , h = mean dai ly m i n i m u m of the

coldes t m o n t h ( January) , i = lowest t empera tu re on record , j = mean mon th ly prec ip i ta t ion curve, k = mean month ly tempera-

ture curve, I = relat ive humid season (vert ical shading) , m =

relat ive per iod of d rough t (dot ted shading) , n = mon ths with

mean dai ly m i n i m u m below 0 ° C (neut ra l shading), o = months

with abso lu te m i n i m u m below 0 ° C (d iagonal shading) , p =

mean dura t ion of the frost-free per iod in days.

rains of considerably greater magnitude, perhaps 2 500 mm in the course of a summer. Estes Park averages only 405 mm and Aliens Park (2 59t m) 518 mm, both substantially less than would be expected extrapolating from the INSTAAR data. Waterdale, on the edge of the foothills, receives only 395 mm/yr.

Winds can be of major significance for mountain vegetation along the Front Range where 15 to 25 m/s gales occur almost daily in highest elevation forests. Occasional heavy winds in early spring blow down large areas of montane and subalpine forest (major blowdowns within the study area occurred in 1949 and 1973).

History

Quaternary and recent time has been characterized by climatic variability which has played an important role in shaping vegetation. A climatic optimum occurred roughly 7 500 yr ago. This was followed by three cold or neoglaciat periods alternating with warmer periods. During the neoglacial periods small glaciers formed in old cirques, while in the intervening periods the ice completely melted (Richmond, 1972). The most recent neoglacial lasted from roughly 1 500 to 1 900 with the period from 1 600 to 1 650 being the coldest 50 yr period during the last 4 000 yrs (Bray, 1971). As many of the forest trees in the study area became established under these cooler climatic conditions, the torest presently on a site does not necessarily represent the forest which would become established today under otherwise similar conditions.

It is uncertain to what extent Indians frequented the study area before White settlement. Apparently Arapahoes, Utes and Cheyennes used the area for summer hunting grounds but never lived there year round (Mussehnan, 1971). As the area was never heavily populated, the only impact the indigenous people would likely have had on the natural communities would have been through periodic reduction in game or an increase in fire frequency. By the time the region known today as Estes Park (In the southern Rocky Mountains broad valleys and intermountain basins dominated by grassland rather than forest are called parks) was settled in 1860 the Indians had departed, and there was little evidence of their previous occupation.

An important impact of settlement and subse-

quent national park establishment was on game herd size. While elk (Cervus canadensis) were originally abundant, the species was soon extirpated by hunting pressure. In 1912 and 1913 elk were reintroduced into what is now the national park (Packard, 1947), Settlement, however, limited available habitat, and in particular winter range was restricted by cattle grazing. By 1933 the game herds had grown and there was concern for the winter range of deer (Odocoi&us hemionus) as cattle had greatly reduced browse of such preferred genera as Purshia, Prunus and Amelanchier (Thompson, t933). For elk the shortage of winter range encouraged their habit of stripping Poputus tremuloides bark (DeByle, 1979; Packard, I942). While this damages trees considerably and increases the rate of Populus mortality during forest succession, its presettlement importance is unknown.

It is also difficult to determine the extent or impact of early grazing. Cattle were first brought to Estes Park in 1860, and by 1874 there were over I 400 cattle grazing in the basin (Carothers, 195 t). Much of the area outside the national park is grazed today. Consequently, most of the low elevation study sites must have been grazed at one time. Within what is now the National Park, it appears that grazing was limited to peripheral, low-elevation Pinuspona'erosa forests, and the open, grassy parks where hotels maintained horses. With a few exceptions, little evidence remains today.

Fire frequency has been influenced by post- settlement human activities but the extent is unknown. Numerous forest fires burned the slopes near Estes Park in the early post-settlement years of t860.-1910 (see Clements, 1910). Crandall (1897) reported a large increase in fire frequency due to human influence throughout the northern Front Range during the later part of the nineteenth century. Hanna (1934) suggested that part of the fire increase in the Medicine Bow Mountains might have been due to vengeful Indians, but this does not appear to have been the case for the present study area. More recently, fire suppression activities have resulted in a fire frequency far below natural levels.

There has never been extensive logging within Rocky Mountain National Park. A saw mill was operated for a few years in the Mill Creek region to salvage trees killed by the Bear Lake fire of t900. Elsewhere logging appears not to have been important within the park, and it seems safe to assume

that all of the sample sites were on areas where tree population structure was the result of strictly natural processes. Some cutting has taken place in the foothills east of the National Park, but no sites studied contained evidence of cutting within the last twenty years.

Forest disturbance

The coniferous forests of the Rocky Mountains can best be described as disturbance phenomona. Owing to the agencies of fire, wind and insect attack, these forests are periodically destroyed in a patchwork manner, resulting in a mosaic of stands of differing ages and histories. The complex and varied patterns of composition and successional change of these patches constitute an important but frequently neglected aspect of Rocky Mountain forest ecology.

Before European settlement in the middle nineteenth century, fire was by far the most important form of disturbance in the forests of the Rocky Mountains (Biswell, 1973; Houston, 1973; Kesse!l, 1976, 1979; Loope & Gruell, 1973; Weaver, 1974). Working within the present study area in what is today Rocky Mountain National Park, Clements (I910) documented a series of Pinus contorta forests dating from fires which occurred over a period of 200 yr. If all the forests of a region are periodically destroyed by fire, as appears to be the case for much of the Rocky Mountain and western boreal regions of North America, it is inappropriate to ask whether a particular forest has burned; one should ask when it last burned and what the nature of the fire was. In these forests the concept of climax vegetation is of 0nly limited utility. Instead, forest ecology needs to be viewed in the context of stand recovery patterns.

Not all fire-modified forests of the Rocky Moun- tains have alternating episodes of destruction and recovery. Open, grassy forests and woodlands of fire-resistant Pinus ponderosa, and to a lesser extent Pseudotsuga menziesii, burn on a regular basis, but are usually not destroyed. Rather, frequent fires remove accumulated litter, thereby reducing the danger of holocausts which can result where excessive fuel has accumulated (see Dodge, 1972; Lunan & Habeck, t973; Wright, 1974).

Fire is not the only cause of the natural destruction of Rocky Mountain forests; both wind and

insects play important roles. Wind is probably of greatest importance in areas where overmature trees, those most susceptible to breakage, are plentiful. Many small-scale blowdowns occur in the study area each year, and major blowdowns occur at irregular intervals. Unfortunately, little is known of their frequency or significance.

Insect outbreaks can be just as devastating. In the 1940's an epidemic of the Englemann spruce beetle killed all the Picea engelmannii and most of the Abies lasiocarpa over 10 cm dbh (diameter breast height) on the White River Plateau in northwestern Colorado, destroying an estimated I07 m 3 of Picea alone (Alexander, 1974; Miller, 1970). As might be expected, outbreaks are usually associated with mature and overmature stands (Alexander, 1974). Several other insect species can also cause periodic widespread mortality and forest destruction, none of the major tree species being exempt (Amman, 1977; McKnight, 1968; Roe & Amman, 1970).

The pervasive importance of disturbance in Rocky Mountain forests dictates that virtually all ecological questions be viewed in the perspective of stand development. Plant population structure and dynamics can be expected to change with stand age, as can such community and ecosystem properties as diversity, biomass, productivity, and nutrient flow (Peet, 1978a, 1981).

Methods

Sample definition and placement

Forest vegetation was broadly defined as vegetation dominated by arborescent species. Specifically, a st'and had to have either >300 trees and saplings per ha (>2.5 cm dbh) or basal area (Mueller- Dombois & Etlenberg, 1974) of greater than 4 m2/ha. These requirements were designed so as to include both open Pinusponderosa woodlands and subalpine krummholz. Three stands which failed to meet these criteria were included to represent transition from woodland to foothill shrubland.

Sampling was preceded by field reconnaissance and a literature review aimed at determining the major trends in vegetational composition. Eleva- tion, moisture (including topographic position) and successional status were suggested to be most important (Bates, 1924; Daubenmire, 1943; Marr,

10

1961 ; Whittaker, 1956, 1960). Several other factors were seen to be locally significant including soil depth, soil texture, wind exposure and snow related phenomena.

Because it is difficult to obtain a representative sample of vegetation using strictly subjective plot location, random sampling is often suggested for vegetation studies (Gounot, 1969; Smartt & Grainger, 1974). However, random placement fails to include many of the unusual and probably more informative types of vegetation, and is extremely time-consuming (see Moore et al., 1970). Subjective sampling can yield a much broader coverage of vegetational variation in a given amount of time. For the present study a dual approach was used. The majority of the samples were subjectively chosen (259), while a smaller number of samples (46) were randomly selected using a stratified technique similar to that of Seely (1961).

Forest vegetation above 2 400 m (almost all within the National Park) was sampled intensively using 269 0. l ha plots. Potential sampling sites were stratified using seven elevation belts of 150 m width, and six topographic-moisture classes. The topographic-moisture classes ranged from wet, sheltered, bottomland sites to exposed ridge tops with the intermediate classes corresponding roughly to a scale of potential direct-beam solar radiation. At least five plots were placed in each of the resultant 42 categories in such a way as to span the range of successional stages available. For those cells where considerable variation was encountered, additional plots were sampled. Randomly located plots were sampled before the subjective placement phase was completed so as to avoid excessive overlap.

For random sampling the northern half of the study area was divided into six natural drainage basins. One of these, the Black Canyon area, was randomly selected, Aerial photographs were used to delimit homogeneous areas of vegetation. Twenty-three such areas were located within this 20.62 km 2 watershed. These were subsequently divided into 100 X 100 m subunits, and two such subunits were randomly selected in each unit with the center being designated as the sampling point. Of the 46 subunits so selected, all were sampled except two. For these two completely inaccessible sites, random replacements were selected and sampled. When a random sampling point was reached, a random direction was selected. A 50 m

tape was stretched in the selected direction defining the center line ofa 0.1 ha sample plot. Ifa subjective cheek for homogeneity failed, a new direction was selected.

Forest vegetation covers less of the landscape in the foothill region than at higher elevations. In addition, the diversity of successional stages characteristic of high-elevation forests is not encountered. For these reasons, it was considered adequate to employ a low intensity sampling scheme for forests below 2400 m. 36 sample plots were subjectively located in this zone, all to the east of the National Park boundary. Samples were scattered to represent as much variation in forest composition as possible.

All potential sampling locations were inspected for homogeneity; sites with noticeable heterogeneity in either the herbaceous or arborescent vegetation were excluded. Plots were also inspected for continuous trends of variation from one side to the other. Plots with a known history of logging were excluded. All sites appearing currently grazed were rejected as were sites showing residual grazing damage. For lower elevation sites it was not possible to select only ungrazed sites, these being virtually nonexistent. Above 2 400 m all sites with a known history of grazing were excluded.

Sampling procedure

Tenth hectare quadrats were used for vegetation sampling (see Whittaker, 1960). Once the location of a sample plot was determined, a 50 meter tape was placed along the center line and the edges of the plot were located 10 m to either side. Within the resulting 50 X 20 m rectangle all woody stems greater than 10 cm high were counted and recorded by species. Diameters (dbh; 1.37 m above base) were recorded by 2.5 cm (1 inch) classes, with an additional size-class for individuals less than I m high and one for individuals greater than 1 m high but less than 2.5 cm dbh. The actual size of the plot was considered somewhat flexible and in unusual situations was adjusted to compensate for extremely dense or sparse tree populations. Typically adjustments were made if tree (>2.5 cm dbh) density fell below 20 or above 400 per 0.1 ha (24 increases and 56 decreases out of 305 plots sampled). In such cages only the area for stern counts was adjusted and final totals were multiplied by a

correction factor to determine the number of stems expected in 0.1 ha.

The herb stratum (leaf area between the ground surface and 1 m) was sampled using a transect of 25 contiguous 0.5 ?< 2 m subplots running the length of the 50 m center line. Within each subplot the percentage cover of each species was visually estimated to the nearest 1%, or above 20% cover to the nearest 5% (maximum of 100% for one species). As sampling was conducted between July I and August 20 for sites above 2 500 m (well after snow melt), and between June 18 and July 1 for sites between 1 700 and 2 500 m (before the summer drought), phenology should not have contributed significantly to variation in the cover estimates. Data were tabulated as per cent cover and frequency for each species in the plot. All additional herbaceous species occurring within the 0.1 ha quadrat, but not encountered within the subplots were recorded as present.

Basic site data were recorded for a typical point near the center of each plot. Included were location, elevation, slope, aspect and soil conditions (based on a 10 cm soil pit). Four subjective indices were recorded using a scale of one to five: slope position, ranging from valley bottom through concave and convex slopes to ridge or hilltop; exposure, ranging from sheltered draws, through open hillsides to exposed ridgetops; soil drainage from boggy and consistently saturated sites through moist, to dry and excessively drained sites with coarse, sandy soil; and soil rockiness from no surface rocks (rocks >10 cm) to a solid pavement.

The total data set for the vegetation analyses consisted of 305 0.1 ha plots. Included were 545 species of vascular plants, 7 575 0.5 X 2 m herb plots and approximately 42 000 trees (>2.5 cm dbh).

A second data set contained information on tree ages. For 28 stands all trees <2.5 cm diameter were cored to determine age. Parallel adjacent strip plots 10 m wide and of variable length up to 50 m were sampled until at least 100 trees were included. Increment cores were extracted from below l0 cm and were angled so as to intersect the center of the tree either at or immediately above ground level. In the present study these data are used to verify patterns in stand development and age. The details of stand age structure will be the subject of a subsequent paper.

11

Gradient analysis

Approach Vegetation is considered as a continuously vary-

ing, stochastic phenomenon wherein plants respond individualistically to environmental conditions (Gleason, 1926; McIntosh, 1967; Whittaker, 1967). With vegetation varying continuously in many dimensions, it follows, as Webb (1954) has suggested, that 'plant communities should be classified multifactorially rather than hierarchically'. Gra- dient analysis (Whittaker, 1967) provides both an approach for examining patterns of continuous vegetational variation and a means of multi-fac- torial classification.

As visualization of patterns in more than two or three dimensions is at best difficult, gradient re- presentations are usually based on a few 'master' or 'complex' gradients (sensu Whittaker, 1956, 1967). These gradients are composites of covarying environmental factors representing such complexes as soil-nutrients, elevation, continentality, or soil- moisture. Whether studied in the compound form (e.g. Whittaker, 1956, 1960) or synthesized from components (e.g. Loucks, 1962; Walker & Coup- land, 1968), the result is a set of'complex gradients' defining a vegetational response field of small enough dimension that effective visualization and interpretation is possible. The forests of the Front Range were known to reflect primarily three complex factors: moisture-topography, elevation, and disturbance. These factors were selected for initial analysis by gradient analysis methods.

Elevation as an environmental complex gradient has long been understood to be one of the primary determinants of vegetation in most mountainous regions. Within the forests of the Front Range elevation appears equally important. Most workers who have studied the vegetation of these mountains have considered it necessary to classify on the basis of elevation-defined life zones (e.g. Costello, 1954; Daubenmire, 1943; Mart, 1961; Ramaley, 1907, 1908; Rydberg, 1916). For the present study, elevation was quantified using topographic maps and altimeter.

Perhaps the most frequent source of variation identified in vegetation studies is moisture. A moisture gradient is not, however, easily quantified, being the product of numerous environmental factors which vary through the course of the

12

growing season. For the present study no simply measured index was available or even feasible, given the scope of the project. Instead, a combination of ordination and environmental scalars was used to identify a moisture gradient.

The moisture scalar Whittaker (1956, 1960; Whittaker & Niering,

1965, t968) has used a combination of exposure and slope-aspect to provide the basis for a subjective index of 'topographic-moisture'. Gradient analysis studies based on component factors of complex gradients have used similar combinations of site moisture parameters (e.g. Loueks, t962; Minore 1972, Wali & Krajina, 1973; Walker & Coupland, 1968; Waring & Major, 1964; Wikum & Wali, 1974).

As a preliminary means of quantifying the moisture gradient, slope and aspect were recorded at each site, These were used to calculate potential direct-beam solar radiation, a factor closely related to site micro climate and frequently an excellent predictor of vegetation (e.g.I.oucks, 1962; Minore, 1972; Waring & Major, 1964; Wikum & Wali, 1974). Potential solar radiation was determined by interpolation from the tables of Frank & Lee (1966) and relativized to a scale of 0 to 10, Additional aspects of site moisture status were recorded at each site in the form of the previously described subjective scales of site exposure and soil drainage.

A variation on the method of scalars as applied to gradient analysis problems by Loucks (t962) and Walker & Coupland (1968) provided a convenient method for combining the various indicators into a

single, more predictive index. The exposure and moisture scalars were averaged to provide a new scale of 1 to 5 ranging from boggy, saturated bottomlands to dry, exposed ridges. This subjective index was combined with the solar radiation index in a nomogram for determination of the site moisture scalar (Fig. 3). The nomogram was subjectively constructed based on field experience but without reference to vegetation data. For intermediate values of soil moisture and exposure, solar radiation was considered the dominant factor as indicated by the steep slopes of the isopleths. However, for very wet or very dry and exposed sites, solar radiation was less important. Thus, for both high and low soil moisture extremes, the isopleths are of shallow slope. The result is a preliminary indicator of relative site moisture status. It should be emphasized that this indicator is based only on measurements of slope and aspect combined with subjective estimates of site conditions.

Ordination Ordination is the arrangement of stands in a

tow-dimensional, abstract space so as to reveal interrelationships. A useful application is to ordi- nate a set of vegetation samples stratified so that only one major environmental factor influ- ences the variation, tn this way ordination can be used to order stands along a given gradient. This method assures higher compositional continuity than gradients based on arbitrary environmental parameters. In addition, the technique tends to

qn 5

ca

}2

Site Moisture Scalar o

1 O 1 2 3 4 5 6 7 8 9 10

Relative Potential Solar Beam Irradiation

Fig. 3. Nomogram used in construction of the site moisture scalar from incident solar radiation and topographic position. Details are explained in the text.

include unrecognized sources of variation in the resultant gradient so as to yield a complex gradient with increased predictive power. This approach was selected to derive a moisture gradient.

For the present study, correspondence analysis (reciprocal averaging; Hill, 1973, 1974; Gauch et al., 1977) was selected as an ordination technique. Tests by Gauch et al. (1977) suggest that while distortion can present a problem in axis scaling, the method reproduces a primary axis of variation with a very high degree of reliability and is one of the best of the currently available techniques. As data sets with only one major trend of variation (moisture), were desired, the data were stratified into seven subsets with a basal 600 m elevation stratum followed by six 200 m strata.

The ordinations were constructed using understory quadrat data for three reasons. Analysis using trees alone would have been based on only 12 species of which only seven were common, whereas analysis using quadrat data could employ all 545 recorded species. In conifer forests herbaceous species are frequently much more sensitive indicators of site conditions than tree species (Cajander, 1926, 1949; Cormack, 1956; Daubenmire, 1976; Frey, 1978; Minore, 1972; Mueller-Dombois, 1964; Rowe, 1956; Trass & Malmer, 1978; Whittaker, 1962). Coniferous forest tree composition is much more strongly influenced by stand successional history than is the herbaceous stratum. After disturbance, canopy composition and structural recovery can take in excess of 500 yr while the herbaceous vegetation (in terms of species composition) usually recovers shortly after canopy closure (Cajander, 1926, 1949; Frey, 1978; Shimwell, 1971; Trass & Malmer, 1978; Whittaker, 1962).

Various forms of data standardization are possible. Standardization by stands prevents unequal weighting of stands during ordination and to a lesser extent compensates for unequal total cover of different successional stages. Species standardization reduces the influence of the most common species, those likely to be the most widespread and ecologically vague in indicator value. Double standardization includes both adjustments with species standardization occurring first.

Several approaches were initially tested including singly (standwise) and doubly standardized data using both correspondence analysis (CA; which, in addition, includes a different form of

13

double standardization intrinsic to the algorithm) and Bray-Curtis (BC; Bray & Curtis, 1957) ordination. For this test the 2 900 3 100 m elevation stratum was selected, this being intermediate in elevation and composition. Composite endpoints were used for the BC ordination, each being the average of the three dryest or wettest sites. The results were evaluated in terms of Spearman rank correlation with the previously derived site moisture scalar. Correlations were BC single standardization 0.816, BC double standardized 0.829, CA single standardized 0.830, and CA double standardized 0.843. The results were gratifying in that the assumed superiority of double standardization was supported, as was application of CA as an ordination technique. While the BC ordination represented a best effort to define a moisture gradient, the correspondence ordination showed better correlation with the moisture scalar despite the absence of any assumptions about underlying factors.

All seven elevational strata were ordinated by correspondence analysis using cover values doubly standardized. For species present with no cover value, a value of 0.02 (one-half the normal minimum) was assigned. The resulting rank correlations with the moisture scalar are shown in Table 1. Ordination results are summarized in Figure 4.

Gradient scaling CA provides an ordering of stands along a

gradient but does not necessarily provide an appropriate scaling. Distortion remains a major problem. The spacing of stands along the derived axis can be greatly influenced by chance variation in either

Table 1. Spearman rank correlation coefficients between ordination axes and the moisture scalar. Stands were stratified by elevation. Determination of the moisture scalar is explained in the text.

Elevational Correlation Number of Significance St ra tum Coefficient Stands Level

1 700-2 300 m 0.839 26 <0.0001% 2 300-2 500 m 0.900 27 <0.0001% 2 500-2 700 m 0.831 60 <0.0001% 2 700 2 900 m 0.805 57 <0.0001% 2 900 3 100 m 0.815 58 <0.0001% 3 100-3 300 m 0.826 38 <0.0001% 3 300 3 500 m 0.533 35 <0.0001%

t4

stand composition or distribution of stands along the underlying compositional gradient. Conse- quently, a mechanism was needed for scaling the seven derived axes so that their units would be equivalent. The previously derived moisture scalar met this need by serving as an independently derived standard. Each ordination axis was scaled against this standard using curve-fitting techniques with the constraint that the original ordering of the

stands on the axes be preserved. Polynomials of up to second degree along with combinations of loga- rithmic and exponential transformations were evaluated. After rescaling, the ordination position of each site was calculated on a scale adjusted to a range of 0 to 100 for each stratum.

Examination of the resulting moisture axes revealed a few difficulties. Three strata (2 700-2 900, 2 900-3 100, 3 100-3 300 m) had a single bog stand

oo,, '~° ~o~e"" ~o~,,- v~,,e ~°~'- I L.." ~ V ,

oe% °e o,e° '°" oW

u- -~'~° v .-~s~°~° v ..... 133-35oo ! m

.,ooo'o :

~" U~ 1,/ 131-3300 m

I V i f " i f , V I f t,.. / , I f ,, 1.1" 129-3100 m l ..... I "' I I

e~sm o~O 6~O'&s c,OOo

V V " ~ V , 1,7 ~ 2 7 - 2 9 0 0 m } ~," .... V ' i ' - _ t f _ I f I

1 I " / , i f / V V I t , / L / V ...... ~ L.-" I f ~ 2 5 - 2 7 0 0 m

1 ¢~tf O~e , I f ..... ~ V v ~ 1z~.250o m

v " ........ ~ v ' v i f .......... I f ,, 1,7 i V 1 1 7 - 2 0 0 o rn ~fWe, k:~ 4~0 6~ 8 0 Xeric

Fig. 4, Results of the eight final ordinations illustrated using ordination positions of common species. One bog stand had previously been removed from each of the 2 700-2 900, 2 900-3 I00 and the 3 300 3 500 m strata. The 3 300-3 500 m stratum did not include krummhotz stands. These ordinations have not been rescaled against the site-moisture scalar. To see the effect of scaling compare

Table 2.

at the end of the moisture gradient. Bogs are compositionally distinct from the other wet stands, which resulted in a shifting of the remaining stands farther to the xeric side than for equi#alent stands in strata without bog sites. These three sites were temporarily removed from the data matrices and the procedures for ordination and scaling repeated with the gradient starting at five rather than zero, the bogs sites each being given a value of zero. (Gauch et al., 1977 discuss the effects of extreme or 'oddball' stands on CA).

The 1 700 2 300 m stratum proved to be diagonal, incorporating the effects of both moisture and elevation. This stratum was originally of greater elevation width than the others because of the small number of low elevation stands, but the results necessitated division into two narrower strata. The ordination and scaling procedures were repeated using 1 700-2 000 m (n = 12)and 2 000-2 300 m(n = 14) strata. The rank correlations with the moisture scaler were 0.608 and 0.894 resp. The low value for the lowest elevation stratum reflects a failure to locate and include wet and mesie stands below 2 000 m elevation. Virtually all such stands have been destroyed for roads, housing, and agriculture. (Experimentation with correspondence analysis has revealed that the power of the technique to repro- duce an assumed ordering of stands from noisy field data decreases as the length of the gradient is shortened; Gauch et at., 1977).

The 3 300 3 500 m stratum also yielded poor results. This was largely a consequence of the combination of forest and krummholz stands within the same stratum. Krummholz vegetation is influenced by a large number of site variables including moisture, exposure to wind, snow drift and melt, and soil depth (see Smith, 1969; Willard, 1963). Subsequently, the stratum was split into krummholz (n = 10) and forest (n = 25) segments with the extreme mesic and xeric stands included in both. After repeating the ordination and scaling procedure, the forest portion gave a rank correlation of 0.714 with the moisture scaler. Because a distinct krummholz moisture gradient failed to emerge, it was necessary to use weighted-average species moisture values for species from the highest elevation forest zone to calculate stand positions on the krummholz moisture gradient (see Whittaker, 1967). The gradient was then rescaled so that the terminal stands which had been used in both the

15

forest and krummholz sequences had consistent positions.

Classification

Approaches Forest vegetation of the Front Range is a mosaic

of patches of differing age, each developing toward but seldom reaching a steady-state dictated by site conditions. In attempting to classify such vegetation, a dilemma is faced. Should classification be based on the vegetation present, or on the physical aspects of the environment which determine the patterns of vegetational development?

Cajander (1926, 1949; see Frey, 1978) was among the first to advocate using the environmental complex as the basis for classifying vegetation composed of a spatial-temporal mosaic. He also recognized that herbaceous vegetation can be a powerful indicator of site potential.

Daubenmire (1976; Daubenmire & Daubenmire, 1968), building on this Fenno-Scandian approach, has advocated using the 'potential climax vegetation' of a site as the definitive characteristic, an approach now widely applied in western North America (see Daubenmire, 1976; Layser, 1974). As Daubenmire (1968) wrote ' . . . it seems best to take the philosophical viewpoint that the habitat (soil, macroclimate, and topography) is the most durable component of the ecosystem, and that the disturbed vegetation presents varied appearance owing to differences in the degree to which the ecosystem has been thrown out of balance, , .' Conceptually this provides an easily visualized, tractable definition.

Cajander's (1926, 1949; see Frey, 1978)approach was to use understory species as site indicators. While he realized that immediately after disturbance all vegetation appears modified, he found understory species to have excellent indicator value soon after canopy closure.

Structurally, the forests of the Front Range are very similar to those of Fenno-Scandia studied by Cajander, as welt as those studied throughout western North America by adherents to Dauben- mire's methodology. Following these workers a classification based on the environmental situation of the site as interpreted using understory vegetation appears to be the best solution. A gradient analytic representation of forest vegetation derived from understory composition offers an effective

16

basis for such a classification. All that is needed is to dissect the two dimensional representation of forest vegetation into forest types.

Type delimination The two dimensional gradient representation

(Fig. 5) was broken into community-types within which the successional patterns of dominant trees were reasonably homogeneous. For example,

forests with successional trends from Pinusflexitis to Abies lasiocarpa and Picea engelmannii dominance were grouped together. Similarly all Pinus contorta stands were grouped together and this collection was further divided into groups based on whether Abies, Pseudotsuga, or Pinus contorta would follow the initial post-fire Pinus contorta forest. As explained in a subsequent section, sue- cessional trends were inferred by examination of

z o

uJ .J ~J

NORTHERN COLORADO FRONT RANGE VEGETATION of the EAST SLOPE

re~ters

3500

3300

~I00

:~900

2700

~5OO

2300

21OO

1900

1700

Wet

Rovines Shellered Open Slopes Exposed Ridges Slopes NE E SE

Mesic N NW W SW S Xeric

:eel 12,00(

l l,OOC

0,000

9,000

8,000

7,O0O

6,000

MOISTURE GRADIENT

Fig. 5, Communi ty mosaic diagram showing the distribution of communi ty series and types relative to gradients of elevation and topographic-moisture. Communi ty series are distinguished by bold lines with A = Pinus ponderosa woodland series; B = Pinus ponderosa, Pseudotsuga forest series, C = Mesie montane forest series~ D = Pinus eontorta forest series, E --- Pieea, Abies forest series, F = Pinusflexilis forest series, G = Alpine transition series. The dashed line between A-5 and B-4 indicates that these two communi ty

types occupy the same region on the mosaic diagram.

tree diameter distribution, and tree ages determined from increment borings. Groups were examined for homogeneity of the understory data and a few groups were divided. Particularly low homogeneity was encountered at the transition from Pinus ponderosa woodland to Pseudotsuga or Pinus contorta dominated types. This is a consequence of the important role of soil texture on dry sites. Here it was necessary to split one unit into two overlapping groups, separated by edaphic conditions. An additional Populus trernuloides dominated group was defined. The Populus stands, being largely successional, did not fall together on the community mosaic like the other dominance-types; their distribution was more a function of disturbance and edaphic conditions than elevation and moisture. Otherwise all the dominance-types were effectively separated using the two-dimensional gradient representation.

This approach to community classification appears distinctive in the diverse literature of community classification. Pogrebnjak (1930) was one of the first to base a classification on a gradient system. Whittaker (1956, 1960; Whittaker & Nier- ing, 1965) classified vegetation by the dissection of gradient analytic diagrams. The approach employed parallels Aichinger's (1951) recognition of Vegetationsentwicklungstypen in the combined use of dominance-types, successional trends and herbaceous indicator species. However, Aichinger started by recognizing dominance-types, and used herbaceous species only secondarily. The combination of gradient analysis based on composition of the herb-stratum with dominance defined types, as well as the combination of mathematical ordination and direct gradient analysis appears new. The units so defined come close to the habitat-types of Daubenmire which are defined on the basis of potential climax .vegetation, but the gradient approach sets this apart as does inclusion of successional components. For simplicity, the basic units recognized are called 'types', and these are combined into larger units called 'series'.

The present study ted to recognition of 8 series (Fig. 5) named on the basis of physiognomy and dominant tree species. For example, sites initially dominated by Pinus contorta but successional to Pseudotsuga were placed in the Pinus eontorta forest series owing ~o the rarity of old-age stands. Series were also assigned letter codes (A H). Series

17

were in turn divided into community-types on the basis of the successional pattern of the dominants as well as site characteristics. In these cases species names separated by dashes indicate successional trends while commas indicate codominance. 'Mesic Pinus contorta-Abies, Pieea forest' indicates a type initially forested by Pinus contorta after disturbance, but with a climax or steady-state dominated by Abies and Picea, in that order of importance. Mesic distinguishes this from a type with similar canopy characteristics but different understory composition. Types were assigned codes with a letter indicating the series and a number the type within the series.

Community characterization

Data summarization

The continuous, highly stochastic variation of vegetation limits the extent to which community types can be accurately or precisely characterized. A few typical stands cannot represent the range of variation encountered, yet presentation of a sufficiently large number of stands to represent the variation present often obscures underlying patterns. In the present study community floristic composition is summarized in tables designed to present a maximum amount of information While avoiding excessive length. The table format is that of the author, but several aspects are patterned after tables in the works of Curtis (1959) and Dahl (1956). Tables for the 8 recognized series and 29 component types are presented in the Appendix.

Prevalent species are used to characterize community composition (Curtis, 1959). For this pur- pose constancy was first calculated for all species encountered in a type (percentage of stands in a type in which the species is present). Next, the average number of species per stand (d = species density of Curtis) was calculated, fractions being rounded to the next highest integer. Prevalent species were defined as those d species with highest constancy. In case of ties, the order was determined by average cover. A list of prevalent species so defined is considered to characterize the composition of the type. Listed for each prevalent species are the average frequency (in 0.5 X 2.0 m quadrats) for those stands in which the species was present

18

(first column), and the constancy (second column). For each community-type modal species were

determined. These were defined following Curtis (1"959) as species having their highest constancy in the type. Again, ties were decided on the basis of cover values. Modality is indicated for prevalent species by underlining the constancy. Modal, non- prevalent species are listed without frequency values. While available space precluded listing modal species which were never prevalent, these and other supplementary data are summarized in expanded community tables available from the author (see Appendix). Certain limitations must be placed on the interpretation of modality. In cases of small numbers of stands defining a community- type (3-6), modality can be strongly influenced by stochastic variation in sample composition. Also, as this study includes only forest vegetation, modality applies only to forests and not to the region as a whole. For example, in the present study Do'as octopetala is modal in 'Subalpine Pinus flexilis forests' where it has a constancy of only 12.5%, but Willard (1963) recognized an alpine community (Do,asetum octopetalae) where Drvas has a constancy of 100%.

Given stands arranged in types, it is desirable to have an indicator of the relative uniformity of a type, or homotoneity. (Homotoneity refers to between-stand comparison, whereas homogeneity refers to within stand comparison.) An index defined by Curtis (1959) as the sum of the constancy values of the prevalents divided by the sum of the constancy values of all the species (or, average constancy of the prevalents) was calculated for each forest type. This is similar to indices used by Raunkiaer (1934) and Raabe (1952). Itsadvantage is that it is relatively independent of the number of stands in a community type, since species density approaches a constant after the first few stands are sampled (Peet, 1974). Curtis obtained values between 34.5 and 70.3 in his study of Wisconsin vegetation. In the present study using geographi- cally more restricted, less diverse community-types, values ranged from 54.5 to 78.5.

Tree composition is summarized using importance values. These were calculated as the average of relative density (density standardized to total 100) and relative basal area (also standardized to total 100). Where tree density is used, this refers to stems >7.5 cm (3 inches) dbh.

Pinus ponderosa woodlands (A)

The dry foothill vegetation of the Front Range is dominated by open Pinus ponderosa woodland, a formation characterized by scattered trees with less than 50% cover over a graminoid dominated understory. These woodlands are found on the lower slopes over the central portion of the moisture gradient. They grade into cottonwood (Populus sargentii, P. angustifolia) forests in river bottoms, Pseudotsuga forests on steep, north-facing ravine slopes over the central portion of the moisture gradient. They grade into cottonwood (Populus sargentii, P. angustifolia) forests in river bottoms, increasingly confined to the xeric end of the moisture gradient and are eventually replaced by the Pinusponderosa, Pseudotsuga forests (B). Foothill forests dominated by Pinus ponderosa and Pseu- dotsuga were first described by Vestal (1917) working in the vicinity of Boulder. He recognized 18 formations as occurring in the foothills and listed dominant species. Ramaley (1908, Ramaley & Robbins, 1908) also provided species lists for several plant communities.

The lower margin of the foothill woodland grades into grassland, typically dominated by some combination of Agropyron smithii, Andropogon scoparius, Bouteloua curtipendula, B. gracilis, Bromus teetorum and Stipa comata (see Hanson, 1955; Hanson& Dahl, 1957; Ramaley, 1908; Vestal, 1917). On these sites soil texture as determined by parent material (Retzer, 1953) and the erosion deposition cycle appear of central importance in determining community composition. Slope and aspect have only limited influence on the composition of these communities, a result consistent with patterns observed by Hanson & Dahl (1957). As the present study was confined to granitic substrate, parent material induced vegetational variation was largely avoided. However, a shift from grassland to woody vegetation occurred with a change from fine to coarse textured soils. Cercocarpus montanus and Rhus triloba shrubland dominates on rocky sites, and grasses dominate on finer-textured soils.

Robbins & Dodds (1908) and Larsen (1930) have suggested that at low elevations Pinusponderosa is confined to rocky sites; edges of large rocks or coarse-textured soils are necessary for seedling establishment. These observations are consistent

with the behavior of Pinusponderosa in the Front Range. Lower slopes and broad valleys where fine- textured soils accumulate often have few if any trees. These are the so-called 'parks' of the southern Rocky Mountains. A similar situation occurs in Oregon and Washington where Pinus ponderosa woodland and shrub-steppe form an intricate mosaic in ecotonal areas in response to soil texture (Franklin & Dryness, 1973). These examples all conform to a general pattern of semi-arid regions where grasses competitively exclude woody plants on fine-textured soils, whereas the deeper moisture infiltration and the lack of a continuous sod of grasses allows woody plants to succeed on coarse- textured soils (Waiter, 1970; Wells, 1965).

The Front Range woodlands are rich in species averaging 43 per 0.1 ha sample. Of these, grasses are particularly important accounting for 25% of the prevalent species and 42% of the herbaceous cover. Composites are also important accounting for 18% of the prevalent species.

Pinus ponderosa shrubland (A 1) Comprising the transition from woodland to

Cercocarpus shrubland, the Pinusponderosa shrublands are the most xeric of Front Range forest- types. At lower elevations (<1 700 m) the formation dominates a broad range of habitats on rocky slopes. However, at its upper terminus (~2 000 m), the formation is largely confined to rocky ridge tops and south-facing slopes. The community is open in appearance with a matrix of shrubs, predominantly Cercoearpus montanus and Rhus triloba from which emerge 40 to 275 widely spaced trees per hectare. Pinus ponderosa dominates the tree stratum with an average importance value (I.V.,.average of relative density and relative basal area) of 95. Juniperus scopulorum is regularly present but rarely attains a diameter greater than 12 cm dbh. Occasionally individuals of Pseudotsuga are encountered. Basal area ranges up. to 12 mZ/ha depending on site and disturbance history.

Despite xeric conditions, understory cover averages 64% with grasses providing almost half. Suc- culents reach their greatest importance in the Front Range forests in this community-type, as do suf- frutescents and annuals. Richness is high with an average of 45 species occurring per plot. Cover is largely dominated by Bromus tectorum, Rhus triloba and Cercocarpus montanus. On the more

19

mesic sites Purshia tridentata and Agropyron albicans also contribute substantially.

This community-type appears to represent a northern extension and attenuation of theQuercus gambelii chaparral found at the base of the foothills from Denver south (Peet, 1978b). Many of the species are shared including those with the greatest cover values. This attenuation continues into Wyoming where, in the forests of the Wind River and Teton ranges, both the shrub formation and the Pinus ponderosa woodlands are missing, these being replaced by Artemisia tridentata steppe grad- ing directly into Pinus contorta forest (Reed, 1952; Reed, 1969). In the Big Horn Mountains to the east is found a low shrub formation dominated by Cercocarpus led(folius and confined almost exclusively to calcareous substrate (Despain, 1973).

Mesie foothill woodland (A2) Broadly occupying the center of the moisture

gradient between 1 900 and 2 200 m, the mesic foothill woodlands are structurally open with between 250 and 500 trees per ha. Pinus ponderosa and Pseudotsuga are the major tree species with average importance values of 75 and 22 resp. Again, Juniperus scopulorum is commonly present but rarely of significant size or importance. Total basal area varies between 10 and 20 m2/ha.

Understory cover is low compared with the shrubland type, averaging only 28% with grasses contributing a third of the total. An average stand includes 45 species making this one of the richer community-types of the Front Range. Understory dominance is patchy, varying between the shrubs Purshia tridentata and Ribes eereum and the graminoids Leueopoa kingii, Carex rossii and Muhlenbergia montana.

Xeric foothill woodland (A3) Between 2 200 and 2 350 m the dry end of the

moisture gradient is occupied by xeric foothill woodland. This community, like Pinus ponderosa shrubland (A1), has a sparse canopy, tree density ranging between 75 and 150 per hectare. Basal area ranges from 6 to 12 m2/ha. Pinusponderosa with an average IV of 92 is the only important tree species, though Juniperus scopulorum attains its highest importance in this type (averaging 8 but occasionally reaching 25 on very rocky sites). These stands are the closest approximation in the study

20

area to the Pinyon Juniper (Pinus edulis, Juni- perus monosperma, J. scopulorum) woodlands commonly found south of Denver along the base of the Front Range.

Due to xeric conditions and shallow soil with frequent exposure of underlying rock, understory cover values are low, averaging only 34%. In xeric foothill woodland, like xeric montane woodland (A4), the dominant species is the grass Muhlen- bergia montana which here a~erages 10% cover and ~ occurs with a constancy of 100%. Though grasses dominate the understory (40% of total cover), small shrubs are again important with Ribes cereum and Rubus deliciosus both having 100% constancy. Cercocarpus montanus occurs occasionally, sug 7 gesting affinities with the lower-elevation shrublands. Despite the xeric nature of the community- type, the large areas of protruding rock contain frequent seeps and moist cracks. Mesic herbs such as Parietaria pensylvanica and Mimulus glabratus occupy these microhabitats.

Mesic montane woodland (A4) Mesic montane woodlands suggest a higher ele-

vation variation of mesic foothill woodland (A2), usually on fine-textured soils. Tree density varies considerably with between 60 and 500 trees per hectare depending on disturbance history and soil texture. Basal area usually varies between 10 and 20 m2/ha. Pinus ponderosa is dominant with an average IV of 62, though Pseudotsuga is also important averaging 30.

Understory cover values are moderate, averaging 38% with grasses comprising a third of.this. Like mesic foothill woodlands (A2), this type is rich in species averaging 45 per 0. ! ha. Purshia tridentata and Ribes cereum are the dominants among an average of six shrub species per stand. The graminoids Muhlenbergia montana, Carex rossii and Leueopoa kingii also contribute greatly to the total cover.

Xeric montane woodland (A5) Xeric montane woodland, and xeric Pinus pon-

derosa forest (B4) occupy the same position on the community mosaic diagram (Fig. 5), the xeric end of the moisture gradient between 2 450 and 2-850 m. The two types are ecologically differentiated primarily by soil texture. Rocky sites with coarse- textured soils are covered by forest while fine-

textured soils support woodland. This is consistent with observations on the influence of soil texture on competition between grasses and woody species. Nearly half of the total woodland understory cover of 46% is provided by grasses.

Xeric montane woodlands are dominated by Pinus ponderosa which has an average IV of 83, and Pseudotsuga is frequently present. Total basal area ranges between 5 and 25 m-~/ha, depending again on disturbance history and site conditions. Tree density is low in this woodland type ranging from 40 to 160 trees per hectare, thus allowing a highly developed herbaceous stratum. However, due to dominance by a few grass species and in particular Muhlenbergia montana, the diversity is the lowest found in the woodland types. Again Purshia and Ribes cereum dominate the shrub stratum.

Pinus ponderosa, Pseudotsuga forests (B)

A band of vegetation stretching from mesic valleys and north-facing slopes at 1 700-2 200 m across the mosaic diagram (Fig. 5) to xeric south- facing slopes and ridge tops at 2 400 2 800 m, the Pinus ponderosa, Pseudotsuga forest series represents the transition from foothill woodland to dense, high-elevation forest. Being on the border of the woodland formation, these forests were originally subject to a high natural fire frequency. Reflecting the varied disturbance history of the landscape, the present vegetation is composed of a mixture of old groves of large trees with dense sapling populations, younger stands with dense, even-aged populations of small trees representing post-fire recovery, and stands of mixed age (c.f. Cooper, 1960). Recently fire suppression activities have resulted in unnaturally high sapling densities, as well as increased fuel loads. Sapling densities have resulted in a decreasing reproductive success of the shade intolerant Pinus ponderosa relative to Pseudotsuga, while high fuel loads have lead to an increasing probability of devastating forest fires (see Lunan & Habeck, 1973; Weaver, 1974; West, 1969). Marr (1961) suggests an alternative explana- tion for the changing forest structure based on heavy grazing and consequent reduced competition from herbaceous species. While perhaps of local importance, such overgrazing appears relatively restricted in the present study area.

In terms of understory cover, shrubs are very important in this community series. On the average the understory contains over six shrub species per 0.1 ha with Juniperus communis and Ribes cereum having the highest constancies. Among the prevalents, Physocarpus monogynus, Juniperus communis. Arctostaphylos uva-ursi and Jamesia americana have the highest cover values. Graminoids, especially Leucopoa kingii and Carex rossii, also have high cover, but are much less important than in the woodland types.

Foothill ravine forest (B1) Foothill ravine forests are found in cool, moist

draws and on sheltered north-facing foothill slopes. This forest-type provides the lower elevation limit of many montane and subalpine species. A combination of cold air drainage, low incident solar radiation and consequently low potential evapo- transpiration produces localized conditions favorable to species of higher-elevation forests.

Pseudotsuga dominates the tree stratum with an average IV of 65 compared with 32 for Pinus ponderosa, the only other important tree species. Examination of average relative seedling, sapling, and tree density suggests the more shade tolerant Pseudotsuga to be increasing at the expense of Pinus ponderosa. The implication is that post- settlement suppression has prevented fire from keeping these types open, allowing abnormally high seedling establishment and changing forest dominance.

The dense, coniferous canopy and thick litter layer contribute to the lowest species diversity among the foothill types. The average stand has an understory cover of slightly under 30% with approximately 28 species represented. The majority of the cover in the understory stratum (62%) is provided by shrub species. Physoearpus monogynus is the most characteristic shrub species with the highest cover and a constancy of 100%. Juni- perus communis, Jamesia americana and Areto- staphylos uva-ursi also contribute substantially to shrub cover.

Foothill Pseudotsuga, Pinus ponderosa forest (B2) The central portion of the moisture gradient

between Pinusponderosa woodland (A) and Pinus contorta forest (D) is occupied by foothill Pseu-

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dotsuga, Pinus ponderosa forest. This community- type shares many features with the foothill ravine forest (B1). Pseudotsuga and Pinus ponderosa share dominance in both, and in both frequent fire played an important role before settlement. Juni- perus scopulorum is a frequent associate and Pinus flexilis and Pinus contorta occur occasionally on the higher-elevation sites. Basal area is variable ranging from 8 to 40 m2/ha depending on disturbance history.

Shrubs and graminoids are the principal understory growth forms with 40% and 22% of the total cover resp. Shrub cover is dominated by Physo- carpus monogynus, Juniperus communis and Arctostaphylos uva-ursi. Understory cover for this type averages only 19%, reflecting the influence of canopy closure on the competitive ability of tree seedlings, shrubs and herbs. Among the herbaceous species Leucopoa kingii has the highest cover. Despite low cover values, species richness averages 33/0.1 ha with 24 herb species and 7 shrubs.

Xeric Pinus ponderosa forest (B3) Xeric Pinus ponderosa forest occupies the same

portion of the community-mosaic as xeric montane woodland (A5), that area between 2 350 and 2 750 m at the xeric extreme of the moisture gradient. The primary difference between these types is edaphic, the forest community being characteristic of rockier substrates.

Pinus ponderosa is the dominant tree species with an average IV of 77. Pseudotsuga is commonly present but with an average IV of only 18. Seedling success differs due to the greater shade tolerance of Pseudotsuga which has a relative density of 33% compared to 40% for Pinus ponderosa.

The combination of xeric conditions, rocky soil, and typically heavy shade from the dense canopy results in the lowest average understory cover of any type delimited during the study, 9%. Of this tot~tl, 53% is contributed by graminoids, and 15% by shrubs. The average site has 28 species despite the low cover values. Understory dominants in terms of cover are the graminoids Leucopoa kingii and Carex rossii. The shrub Physocarpus monogynus is also important. The high importance of Purshia tridentata in the shrub stratum underlines the affinities with the xeric montane woodland (A5) type.

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Xeric Pseudotsuga forest (B4) On xeric sites between 2 700 and 2 950 m, a

narrow zone of xeric Pseudotsuga forest (B4) separates Pinus ponderosa and Pinus flexilis dominated communities. As expected, these two species preempt a portion of the dominance in the peripheral regions of the type. Like the community-types immediately below it, xeric Pseudotsuga forests are strongly influenced by edaphic conditions with dense stands of usually even-aged Pseudotsuga O n the rocky sites, and open stands on the sites with finer-textured soils.

Because the tree component consists of two edaphic phases, is hard to characterize. In both phases the dominant tree is Pseudotsuga (average IV of 74). Pinusflexil'is and Pinusponderosa which occasionally share dominance have average IV's of 7 and 14. On open sites basal area ranges between 12 and 20 m2/ha and tree density ranges between 200 and 450/ha. In contrast, on the more densely forested, rocky sites, basal area ranges between 25 and 55 m2/ha and tree density between 1 000 and 2 000 stems/ha.

Thirty-two species occur in the understory of a typical stand including 24 herbs and 7 shrubs. Altogether, understory cover averages 19%. Shrubs make up one-third of the cover with Juniperus comrnunis, Artemisia tridentata and Physocarpus monogynus comprising the largest portion. While Artemisia tridentata has a constancy of only 33%, it dominates those stands in which it occurs. Among the herbaceous species, Muhtenbergia montana usually dominates the ground cover.

Mesic montane forests (C)

Mesic montane forests make up a heterogeneous group of stands characteristic of moist, relatively low elevation sites. At their upper limit they are dominated by Picea engehnannii and Abies tasio- carpa, usually mixed with Pseudotsuga. On more xeric sites Pinus contorta dominates, frequently being succeeded by either Abies or Pseudotsuga. The lower elevational limits are defined by foothill ravine forest (B 1) and foothill cottonwood forest.

The central positio n of this group at the intersec- tion of several forest series dominated by different tree species contributes to its heterogeneous composition. Dominance is determined not primarily by disturbance as is the case for most forest-types in

the study area, but by subtleties of environmental variation including drainage, soil aeration, exposure and cold air drainage. These forests are the most diverse of the Front Range in woody species, the average stand containing 5 tree and 8 shrub species in addition to 28 herb species.

Due to the pervasive influence of economic development at low elevations, foothill cottonwood forests are little known botanically, though they probably belong in this series. A combination of road building, grazing, and agriculture has greatly reduced the number of such forests available for study. Casual observation indicates dominance of Populus sargentii below 1 950 m and Poputus angustifolia at higher elevations, both being associated with various Salix species. This appears consistent with the brief reports of Marr (1961), Vestal (1917) and Young (1907). Additional observations were not made for lack of undisturbed sites within the study area. At elevations above 2 200 m and extending to about 2 800 m, floristically similar though nonforested vegetation occurs on very wet, usually saturated soils. Combinations of Alnus tenuifolia, Betula occidentalis and various Satix species mark the occurrence of these latter communities which provide the hydric limit of the mixed wet forest (C1) (Young, 1907).

Mixed wet forest (C1) Occurring at the wet end of the forest moisture

gradient between the foothill cottonwood forests and the start of the high-elevation Picea, Abies forests, the mixed wet forests (CI) are composed of a variety of tree species combined in heterogeneous assemblages. Two topographic variants can be distinguished, characteristic of cool, sheltered ravine bottoms, and warm, moist floodplains.

The ravine forest variant is at least partially dominated by Pseudotsuga with scattered Populus tremuloides. In wet pockets Betula occidentatis, Populus angustijolia, Alnus tenuifolia and Pieea pungens are also important. On the dryer, rocky microsites Pinus ponderosa and Juniperus scopulorum dominate. Basal area ranges from 20 to 30 m2/ha and tree density is typically between 300 and 500 stems per hectare. The floodplain variant is dominated by a combination of species including Populus angust(folia and Picea pungens with the former achieving highest dominance on warmer sites. Both species can attain substantial size, and

basal area occasionally reaches as high as 80 m2/ha. Diversity, understory cover, and tree reproduction are all reduced in such high basal area stands.

The average of 60 species per 0.1 ha including 8 tree species is the highest encountered in the study area (see Peet, 1978a). However, the other two types in the mesic montane series have higher shrub richness. Whittaker (1965, 1972; Whittaker & Niering, 1975) has suggested the Santa Catalina Mountains of Arizona to have among the most diverse communities in North America, yet the richness values he reported are lower than those found in this community-type.

With its high species diversity, the mixed wet forest type is hard to characterize. Grasses contribute 30% of the cover with three species primarily responsible: Calamagrostis canadensis. Phleum pratense and Poa pratensis. Other important species include Equisetum arvense, Rosa spec., Thermopsis divaricarpa, Geranium richardsonii, Arnica cordifolia and Rudbeckia laciniata.

This community-type, in addition to its high diversity, has the highest affinity with the flora of eastern North America of all the community-types studied. The marly shared species may represent remnants of a more mesic, transcontinental flora, or may simply be the result of migration along the prairie border. The mesic Populus tremuloides forests described by Severson & Thilenius (1976) from the Black Hills of South Dakota are intermediate between this type and the true eastern deciduous forests.

Montane ravine forest (C2) On cool ravine slopes and sheltered well-drained

bottomlands between 2 300 and 2 550 m, montane ravine forest is dominant. This, like the mixed wet forest, is a heterogeneous forest type, the composition of which is greatly influenced by subtleties of site quality and history, On exposed sites Populus tremuloides is the first species to invade after disturbance. The eventual dominants can be Picea engehnannii, P. pungens, Pseudotsuga, or Pinus ponderosa depending on moisture and temperature conditions. Rocky sites usually have Pinus ponderosa or Juniperus scopulorum present. On cool, moist sites Picea pungens and Pseudotsuga dominate with Betula occidentalis and AhTus tenuifolia occurring in wet pockets. With increasing elevation Picea engelmannii and Abies lasiocarpa increase in

23

dominance and Pinus contorta assumes an early successional role.

In addition to being second only to the wet montane forest in species richness, this community-type has the highest average number of shrub species (9.5). Only Rosa has a constancy of 100%, but several shrubs contribute substantially to total cover including Physocarpus monogynus, Jamesia americana, and Juniperus communis. Ribes cereum and Symphoricarpus oreophilus are also frequent. When present, the grasses Calamagrostis canadensis and Poa pratensis are among the leading dominants in understory cover.

Mixed rnesic forest (C3) Mixed mesic forest occurs at the junction of

several community-types on the mosaic diagram. After a disturbance such as fire, simultaneous establishment of Populus tremuloides, Abies lasiocarpa, Pinus contorta, Picea engelmannii, Pseu- dotsuga menziesii, and Alnus tenuifolia can occur, thus leading to high tree diversity. This wealth of tree species precludes description of any simple pattern of forest development. The steady-state forest can be composed of Pseudotsuga or Picea, Abies or a mixture depending on the site. Pseu- dotsuga can act as both a pioneer and climax species.

Total understory cover is 50% but 30% of the total represents coniferous regeneration and another 30% Ericaceous shrubs, mostly Vacciniurn myrtillus, Shrub diversity is high with an average of 8 species per 0.1 ha. Linnaea borealis is important as are Rosa and Juniperus communis. Of the herbs, only Arnica cordifolia and Haplopappus parryi have a constancy of 100%, though Pyrola secunda and Goodyera oblongifolia can be considered characteristic.

Pinus contorta forests (D)

Pinus contorta forests are the most central and perhaps the most widely distributed forests in the northern Front Range. Occurring between 2 400 and 3 200 m and intermediate in moisture requirements, they occupy a central position on the community mosaic (Fig. 5). Clements (1910) located, aged and characterized a series of burns dating from 1707. Moir (1969, 1972; Moir & Francis, 1972) conducted a series of short studies on

24

the Pinus contorta forests near Boulder, south of the study area. Brief descriptions of some of these forests have been published by Marr (1961). More complete descriptions of stands in the Medicine Bow Mountains to the north and the Frasier Experimental Forest southwest of the study area have been provided by Romme (1977), Whipple

(1973, 1975) and Wirsing (1973). Pinus contorta is a successional species par

excellence. On favorable sites the species seeds in quickly after fire forming dense, even-aged stands. On some sites Populus tremuloides shares dominance as a successional species. While repeated burning can increase Pinus contorta dominance, Pinus contorta forests usually revert to forests of other, more shade tolerant species when given protection from fire. At~lower elevations succession favors Pseudotsuga. while at higher elevations Abies and to a lesser extent Picea replace Pinus contorta. In the central portion of their elevational range, the Pinus contorta forests form a narrow band of self-maintaining forests.

Successional variation in Pinus contorta forests makes description of composition difficult. For the tree stratum, age sequences must be examined. Although the herbaceous vegetation is more constant through time than is the tree component, cover and frequency values vary tremendously and must be interpreted with care.

The dark, dry understory conditions of successional stands (between 30 and 200 years old) result in the lowest average number of species per 0.1 ha and the lowest average understory cover of any series. In overall appearance, the understory is nearly devoid of herbaceous growth in most successional stands, and is relatively barren in steady- state stands as well. Juniperus communis, Vacci- nium myrtillus, Abies, and Arctostaphylos uva-ursi have the highest understory cover values and are the only woody species with constancies over 50%. No herbaceous species has an average cover of over 1% and Carex rossii alone has a constancy over 5O%.

Pinus contorta forest (DI) Unlike forest-types on environmentally more

extreme sites, Pinus contorta forests are almost impossible to describe in a generalized way, the successional context being critical. Virtually all aspects of stand structure and composition change

through the course of succession. Initial, post-fire forests usually consist of a mixture of Pinus contorta and Populus tremuloides. In a typical stand, Pinus quickly overtops the Populus to become the exclusive dominant. Steady-state composition is more variable for while Pinus contorta dominates the typical site, Picea, Pinus flexilis and Pseu- dotsuga can share dominance.

An average of 19 species is found in the understory of a Pinus contorta forest. Juniperus communis, Arctostaphylos ura-ursi and Carex rossii are the only species with over 70% constancy. Only Juniperus and Arctostaphylos have average cover values over 2%.

Mesic and xeric Pinus contorta - Pseudotsuga forests (D2, D3)

The lower elevation portion of the Pinus contorta forest series is potentially dominated by steady-state Pseudotsuga, though a high natural fire frequency causes such stands to be exceedingly rare. Post-fire successional stands are dominated by Pinus contorta with varying amounts of Populus tremuloides and Pseudotsuga, depending on seed supply, weather, and site conditions. The overstory structure and dynamics of these two types are similar but the understory species are more respon- sive to varying moisture conditions making it desirable to recognize two community-types.

An average of 24 species is found in the mesic group including 14 herbs and 7 shrubs. Understory cover averages 17%. Shrubs are an important component of this type contributing over half of the understory cover. Both Juniperus communis and Jamesia americana have high constancy and high cover values. Physocarpus monog)'nus occurs in somewhat over half the stands and has high cover when present. In contrast, herbs are heterogeneous. Only Carex rossii has a n average cover greater than 0.2%, and only two other species have a constancy above 50, Potentilla fissa and Penstemon virens.

Twenty-seven species occur in an average stand of the xeric Pinus contorta Pseudotsuga forest: 3 trees, 20 herbs and 4 shrubs. In contrast to the mesic type, only 14% of the total understory cover is contributed by erect shrubs, but an additional 23% of the total is in the form of prostrate shrubs, mostly Arctostaphylos. Total understory cover is 21%. Juniperus communis has the second highest cover and is present in most stands.

Mesic and xeric Pinus contorta - Abies, Picea forests (D4, D5)

These community-types form the upper portion of the Pinus contorta series. Abies lasioearpa and to a lesser extent Picea engelmannii replace Pinus contorta during succession. Again, tree composition and dynamics are similar in these types but herbaceous vegetation varies sufficiently with moisture status to warrant recognition of separate types.

On mesic sites high cover values of Vaeeinium myrtillus and V. scoparium along with Abies contribute to an average understory cover of 42%, substantially higher than the 17% average of the Pinus eontorta forest type immediately below on the community-mosaic. Havas ( 1971) has suggested the distribution of the green-stemmed, deciduous Vaccinium species to be limited by winter snow accumulation and persistence; snow being necessary to prevent desiccation.

Species richness is relatively low withan average of 25 species per plot including 17 herbs and 5 shrubs. In the shrub group Vaccinium myrtilIus has an average cover of 8. 1%, 9 times that of the next highest species, Rosa. The only other shrub species with high constancy is Juniperus communis.

The most distinctive aspect of the xeric Pinus eontorta Abies. Picea forest is its low diversity. The average stand has only 13.5 species including 4 shrubs and 7 herbs; the lowest average species richness encountered. Understory cover remains much higher than in the Pinus contorta forest, but is composed almost exclusively of Abies seedlings and Vaeeinium myrtillus.

In addition to Vaccinium myrtillus, Vaccinium scoparium occasionally shares understory dominance at high elevations, though it has a constancy of only 35. No herbaceous species contributes significantly to cover, the maximum value being 0.35%. The four most frequently encountered herb species are Carex rossii, Epilobiurn angustifolium, Pyrola secunda and Arnica cordifolia.

Picea, Abies forests (E)

Dominated by Picea engelmannii and Abies lasiocarpa, Picea, Abies forests occupy much of what has traditionally been called the subalpine zone. This series forms the climax forest vegetation above 3 100 m on all but the most xeric sites, and in the cool, sheltered valleys down to 2 500 m. Be-

25

cause of the high natural fire frequency, stands between 2 900 and 3 100 m on open slopes have been placed in the Pinus contorta series despite the dominance of Abies and Picea in old-age, steady- state stands. Similarly, stands with Pinus flexilis successional to Picea engelrnannii and Abies lasiocarpa have been placed in the Pinusflexilis series.

The earliest work on the Picea, Abies forests was that of Young (1907) who delimited Pinusflexilis, Pinus eontorta, Pseudotsuga-Picea, and Picea- Abies formations and a Populus tremuloides so- ciety. More detailed is the work of Amundsen (1967) on the subalpine forests of Wild Basin in the southern portion of the study area. Amundsen recognized four forest types within Wild Basin: Pinus contorta successional to Pseudotsuga, Picea- Abies climax, Pinus flexilis-Pseudotsuga successional to Picea-Abies, and Pinus contorta. Marr (196 l) found a similar series of forest types during his studies at INSTAAR. He recognized climax regions of Picea-Abies and Pinus JTexilis with successional stands of Pinus contorta and Populus tremuloides. In the more sheltered valleys he reported thickets of Salix and Betula glandulosa. Oosting & Reed (1952) examined the Picea-Abies forests of the Medicine Bow Mountains, studying eight stands in detail. They found these forests to be floristically simple with no significant phytosociological differences with changing site, exposure or altitude. Subsequently, Romme (1977), Whipple (1973, 1975) and Wirsing (1973) have reexamined Medicine Bow Picea, Abies forests and have reported considerable variation corresponding to site conditions.

Picea, Abies forests are remarkably homogeneous along the length of the Rocky Mountains. Comparisons with stands described from New Mexico (Dye & Moir, 1977; Peet, 1978b), central Colorado (Langenheim, 1962; Whipple, 1975; Whitfield, 1933), Wyoming (Despain, 1973; Hoff- man & Alexander, 1976; Oosting & Reed, 1952; Whipple, 1975), Montana (Pfister et al., 1977), Idaho (Daubenmire & Daubenmire, 1968) and Alberta (Horton, 1959; Moss, 1955) suggest a uniform structure and a relatively low rate of species change or turnover with latitude.

Montane Picea, Abies forest (El) Forests on frequently saturated soils in cool,

sheltered locations between 2 500 and 2 900 m

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belong to the montane Picea, Abies forest type. This type differs sharply from the other Picea, Abies forests in herb and shrub composition, and differs from the lower elevation mixed wet forest in that the trees are exclusively Picea engehnannii and A bies lasiocarpa.

Excepting early successional stands, basal area ranges between 35 and 55 m2/ha and tree density between 800 and 1 100 stems/ha. Picea engelmannii is dominant with an average IV of 68 compared to 21 for Abies lasiocarpa. Abies appears to favor the cool conditions of high-elevation sites and is less successful at low elevations than Picea. Comparison of relative seedling, sapling and tree density of the two species suggests the composition to be relatively stable with a ratio of roughly 2 to 1. In terms of basal area, Picea dominates 4 to 1.

Cover in the understory averages 56% and species richness 34/0.1 ha. The shrub laYer with such prevalent species as Lonicera involucrata, Ribes lacustre and Sambucus racemosa readily distinguishes this type from the other high elevation forest-types. Over six shrub species occur in the average stand.

No less distinctive are the herbaceous species, Galium triflorum, Pyrola secunda, Carex disperma and Streptopus amplexifofius all have high constancies. Particularly important in terms of cover are Carex disperma, Streptopus, Equisetum arvense, and when present, Calamagrostis canadensis. Prevalents with high indicator value include Moneses uniflora, Cinna lat~otia and Gymnocar- pium dryopteris.

Pieea, Abies bog forest (E2) Bog forests occasionally develop on flat, poorly

drained sites above 2 800 m. These sites are characterized by saturated soils with only limited surface flow of water, and by a thick accumulation of organic material. Boggy sites above 3 300 m usually are not forested. Bog forests are most often fo.und behind cirque lakes or in areas of extensive beaver (Castor) activity. Because of the rugged topography of the Front Range, boggy areas are uncommon, but "the distinctive composition of the vegetation necessitates their recognition as a community-type. Of the 71 species encountered on the three sites studied, 40 were modal. Cover averaged 93% with half this attributable to graminoids (including Carices). Mosses also covered much of the ground surface.

The bogs appear as a mosaic of forest and sedge meadow, the forest phase being more frequent on slightly raised portions and Carex aquatilis dominating the swales. The relative importance of the two phases of the mosaic as well as tree density and basal area vary with site conditions. Abies has an average density roughly twice that of Picea, but basal area is nearly equal for the two species because of the greater size of the Picea.

The understory vegetation is distinctive. Of the shrubs, Vaccinium myrtillus has the highest cover value, while Gaultheria humifusa and Kalmia pofi- foliaare characteristic, occurringahnost exclusively in this type. Carex aquatilis is the most important herbaceous species with its average cover of 30%, over four times the cover of the next highest species, Caltha teptosepala. The graminoids Eleocharis pauciftora, Calarnagrostis canadensis and Agrostis variabilis also have high cover. Twenty-nine herb species occur in the average stand,

Affinities of the Picea, Abies bog forest with the arctic flora (as measured by percent of species in common) is the highest of any Front Range type. In comparison with the lower arctic affinities of alpine and timberline vegetation, this suggests the endemic character of the alpine vegetation of the southern Rocky Mountains and the cosmopolitan character of bog vegetation.

Wet Picea, Abies forest (E3) These forests between 2 900 m and timberline

with periodically saturated soils are found on mineral substrate rather than the organic deposits of the bog forests. Because of the large size of the dominant trees together with the well-developed herbaceous stratum, this type is among the most impressive of the Front Range forests. Moist conditions result in a particularly long fire-cycle and the sheltered locations in which this forest-type usually occur serve to reduce the incidence of extensive blowdowns. Consequently, many forests of this type appear to be approaching steady-state conditions.

The only tree species are the two dominants, and both were present in all stands sampled. The normal pattern is for old-growth stands to have many small Abies and a few large Picea. Steady- state basal area ranges between 50 and 70 m2/ha, though in some old, even-aged stands basal area in excess of 85 mZ/ha can be found.

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Despite high basal area, the canopy is usually sufficiently open for both abundant tree reproduction and vigorous herbaceous growth. The average understory cover is 62% with 35 species including 30 herbs. The understory is patchy with soil moisture, substrate and light intensity strongly influencing composition. Senecio triangularis and Mer- tensia eiliata from dense patches in very wet but sunny spots, especially near running water. Similar locations with more slowly moving water and an accumulation of sediment and organic material frequently support Calamagrostis canadensis and Carex aquatifis. In equally wet, but shaded locations Saxifraga odontoloma, Trollius taxa and several small Epilobiums are important. Dry hum- mocks with accumulated coniferous litter usually support Erigeron peregrinus, Polemonium delica- turn and Arnica eordifofia. On shaded, moist patches of raw humus, Adoxa moschatettina, Ra- nunculus eschscholtzii and Osmorhiza depauperata are found. Shrubs are largely confined to/the dryer, shadier portions of the understory, especially the common Vaccinium myrtillus and V. seopariurn. Ribes montiginum has the highest constancy and cover values of the shrub species and is somewhat less specific in its choice of intra-community habitat.

Mesic Picea, Abies forest (E4) Cool, sheltered, well-drained sites above 2 700 m

support mesic Picea, Abies forest. Though relatively restricted in distribution at its lower limit, the type occupies an increasingly broad portion of the moisture gradient with increasing elevation. Be- tween 3 100 and 3 300 m this forest and the xeric Picea, Abies forest are the two most widespread forest types. The canopy is dominated exclusively by Picea engelmannii and Abies lasiocarpa with density and basal area varying dramatically with successional state. Picea dominates initially after most fires but Abies tends to dominate in older stands and blowdowns.

Mesic Picea, Abies forests, like Pinus contorta forests, are near the center of the community- mosaic and have early successional stages with poor understory development. The type averages 21 species per 0.1 ha including 15 herb species and 4 shrubs. These values can be misinterpreted, however, as species richness varies dramatically across the successional sequence. The central portion of

the sequence has an average of only l0 species per 0. I ha with frequently only 3 herbaceous species, Pyrola seeunda and Pyrola virens being the only herbs regularly encountered. Vaccinium myrtillus is the only major shrub species, and it provides the majority of the understory cover. Mosses and the lichen Pehigera apthosa are also important in these forests.

Considering the type as a Whole, over 80% of the understory cover of 51% is from woody species, one half being Vaecinium myrtillus and V. scoparium. Vaccinium myrtillus is over twice as abundant as V. seoparium; the reverse of the situation in the xeric and subalpine Picea, Abies forests. Among the herbaceous species, Arnica cordifolia, Pyrola secunda and Epilobium angustifolium are the only species with high constancies. Where moist pockets occur. Osmorhiza depauperata, Mertensia ciliata, Luzula parvijlora and Moneses uniflora are found. At high elevations, or on dry sites where the canopy is more open, Hieracium gracite and Pedicutaris racemosa can be important.

Xeric Picea, Abies forest (E5) Open slopes above 3 100 m typically are dom-

inated by xeric Picea, Abies forest. Superficially, this type resembles mesic Picea, Abies forest, both in the codominance of Picea and Abies, and the solid understory of Vaccinium. The Vaccinium, however, is primarily V. scoparium rather than the V, myrtillus of mesic sites. Also, trees are smaller on the xeric type than on the mesic and wet types. Very few Picea were recorded over 50 cm dbh with the largest Abies being 43 cm. In contrast, Abies occasionally reaches 75 cm on the wet and mesic sites with Picea commonly surpassing 75 cm and occasionally reaching l l0 cm dbh. Steady-state basal area is only 30 to 40 m2/ha on the xeric type (occasionally reaching 55 in even-aged stands) versus 40 to 45 m2/ha on mesic and 50 to 70 on wet sites (reaching 85 in even-aged stands). An occasional individual of Pinusflexilis or Pinus contorta can be found on lower, peripheral sites.

Understory cover averages 51% but in excess of 90% of this is provided by woody species. An average of three shrub species occurs per 0.1 ha, but virtually all of the understory cover is concentrated in the two Vaccinium species. Sixteen herb species occur in the average stand, but the composition is not distinctive and the total cover is suppressed by

28

competition from the more abundant shrub group. Only Carex rossii has an average cover over 1%,

Subalpine Picea, Abies forest (E6) These forests form the transition from mesic

Picea, Abies forest to tundra or mesic krummholz, depending on exposure. Except for an increase in diversity caused by an infusion of alpine species, loss of Vaccinium myrtittus from the understory, or a more open canopy, the forest generally resembles mesic Picea, Abies forest. Except for a rare individual of Pinusflexilis, only Picea and Abies occur in the canopy.

The community appears as a fuzzy mosaic with open areas resembling alpine meadow and tundra mixed with denser forest areas dominated by mats of tree regeneration resulting from layering of Abies. Vaceinium is the most important genus for understory cover, being almost exclusively V. scoparium. Ribes montigenum has the second highest average cover. Among the herbs Polemonium delicatum, Penstemon whippleanus, Carex rossii, Poa nervosa and Potentilla diversifo#a are most common. In the open phase Artemisia arctica, Androsace septentrionalis, Sedum lanceolatum, Hieracium gracile, Potentitla diversifolia, Festuca brachyphylla and Carex foenea dominate. The understory averages 34 species/0. I ha.

Pinus flexilis forests (F)

Pinus flexilis dominates forest communities on exposed, xeric sites above 2 800 m. These sites typically have shallow, coarse-textured soils, often with large areas of protruding bedrock, and are usually located on south-facing slopes, ridges or sites otherwise exposed to desiccating winds. At high elevations (>3 100 m) Pinusflexilis can be either successional to Picea, Abies forest or se l f maintaining, depending on severity of site conditions. Pinus flexilis, however, is not restricted to these high elevation sites. Occasional individuals can be found on xeric, rocky sites anywhere between 2 100 and 3 500 m.

The role of Pinus flexilis as a high elevation climax species on xeric sites is somewhat atypical for Rocky Mountain forests. From James Peak south along the Front Range a different 5-needle pine ( Pinus subgenus haploxyton), Pinus aristata, is usually dominant on xeric, high-elevation sites,

with Pinus flexilis either sharing dominance or restricted to lower elevations (Peet, 1978b). North of the study area Pinusj7exilis is usually found as a low elevation species. On the east slope of the Medicine Bow Mountains Pinus flexilis shares dominance with Juniperus scoputorum, Pinus ponderosa and Pseudotsuga at the transition from grassland to woodland. Pinus flexitis does not commonly dominate above the Pinus contorta zone in the Medicine Bows. Despain (1973) and H offman & Alexander (1976) report Pinusflexilis to share dominance at lower timberline with Juniperus scopulorum in the Big Horn Mountains of Wyoming, while still another 5-needle pine dominates the xeric, high-elevation sites, Pinus albicaulis. In the Wind River Range of Wyoming, Pinus flexilis occasionally forms stands on low rocky ridges well below the normal lower tree line, while at higher elevations it shares dominance on the xeric sites with Pinus albicaulis (Reed, 1969, 1976). Further north Pinus albicautis completely replaces P. flexitis as the xeric, high-elevation dominant. Weaver & Dale (1974) report Pinus albicaulis stands in Montana with herbaceous composition, ecological position and tree population structure all similar to the Pinus flexitis - Picea, Abies forest of the Front Range. It appears that for the southern and central Rocky Mountains resp., Pinus aristata and Pinus albicaulis are the dominant xeric, high-elevation pines, and only in their absence in the northern Front Range is Pinus flexitis comPetitively released to play this ecological role.

PinusJlexilis forests have poorly developed understory vegetation. An average of only 24 species occurs per stand. Understory cover averages 28% with 71% of this attributable to woody species. Few species have high constancy.in these stands. Picea engehnannii seedlings are usually present as is Juniperus communis. The herbaceous species with highest cover are Saxifraga bronchialis, Calama- grostis purpuraseens, Selaginella densa, Arenaria fendleri and Carex rossii.

Montane Pinus flexitis forest (F1) Montane Pinus fIexilis forest comprises the

lower-elevation half of the potentially steady-state Pinus flexilis forests. This is distinguished from the subalpine Pinus flexilis forest largely by the absence of alpine species in the understory. While

other tree species occasionally share dominance, PinusJlexilis is the most important species with an average IV of 75%. On more mesic sites dominance is occasionally shared with Pinus contorta, though on the driest sites only Pinusflexilis occurs. At its lower elevational limit the type grades into Pseu- dotsuga dominated forest.

Large numbers of Populus trernuloides sprouts are present on some sites. On these usually successional sites, Populus rarely exceeds 3 cm dbh but can attain densities of over 5 000 stems/ha. In addition, seedlings of Picea, Abies, and Pseudotsu- ga all occur frequently.

Montane Pinus flexilis forest characteristically occurs on the rockiest, most exposed sites. Roughly one third of the ground surface is covered by rock (>10 cm diameter). Where soil has developed its texture is coarse with large quantities of sand and broken rock present. Because of the rocky substrate, understory cover is relatively low, only 23%. Three-fourths of the un,derstory cover is made up of woody species, and tree species alone constitute 40%. Juniperus communis, Arctostaphylos uva- ursi and Jamesia americana are the most important shrub species. A typical stand contains 23 species including 14 herbs. Among the herbs, the highest cover values are attained by Saxifraga bronchialis and Calamagrostis purpurascens.

Pinus flexilis - Picea, Abiesforest (F2) Pinus flexilis - Picea, Abies forest occurs from

3 150 to 3 450 m elevation between potentially steady-state Pinusflexilis and the xeric Picea, Abies forest (E5). Here Pinusflexilis acts as a post-fire successional species with Picea and Abies dominating those rare stands where fire has been absent for a sufficient time for steady-state forest to develop.

Understory cover is higher than in the other Pinus flexi6s types, owing to the more mesic conditions and the absence of extensive rock out- croppings. Average understory cover is 37% with one third from trees species and one third shrubs. The average stand contains 26 species including 2! herbs. Among the shrub species Juniperus communis and Vaccinium myrtillus have the highest cover and constancy values. Herbaceous species with high cover values include Arenaria fendleri, Carex foenea, Saxifraga bronchialis, Carex rossii and Trifolium dasyphyllum.

29

Subalpine Pinus flexilis forest (F3) As Pinusflexilis does not usually assume stunted,

krummholz forms, the subalpine Pinus flexilis forest represents the upper limit for forest growth on the most xeric and exposed sites. On somewhat more mesic sites the forest grades into xeric krummholz (G2). The subalpine Pinusflexilis forest is not greatly different from the montane type, the most obvious characteristic feature being the abun- dance of alpine species. As with the montane Pinus flexilis forest, this community type occurs on very rocky sites with shallow, coarse-textured soils, usually on ridgetops exposed to strong winds and vulnerable to severe erosion.

Pinusflexilis is the dominant canopy species of this type having an average IV of 61, and on the more extreme sites it is the exclusive dominant. On less extreme sites succession proceeds toward shared dominance with Picea and Abies. Pinus contorta occasionally occurs on warm, southwest- facing slopes in the lower-elevation portion of the type. Basal area appears to reach a steady-state maximum of 30 to 35 m2/ha, somewhat lower than the 35 40 m2/ha steady-state value of the montane Pinus flexilis forest.

Understory cover averages 22% with 42% of this being tree species and 30% shrubs. While the average stand has 3 shrub species, Juniperus eommunis is the only one with high constancy. Vacci- nium myrtillus and Arctostaphylos uva-ursi occasionally have high cover values. Among the herbaceous species, Calamagrostispurpurascens has the highest average cover of the prevalents.

Forest - alpine transition (G)

While most of the forests of the Front Range can be interpreted in terms of three or at most four complex gradients (moisture, elevation, disturbance-history, and substrate), timberline communities are more complex. Among other factors, wind exposure and patterns, snow drift and melt, and temperature must be considered (Willard, 1963; Smith, 1969; Bollinger, 1973; Marr, 1977). It was this complexity which lead to failure of ordination analysis to reveal a simple timberline moisture gradient. Moreover, none of the above mentioned factors can be effectively evaluated and quantified during a single site visit.

The 0.1 ha sampling units used in the study were

30

designed for forest vegetation and were too large to include only homogeneous vegetation when applied to timberline communities. Islands, or clusters of A bies or Picea, form where one tree initially becomes established and reproduces by layering. Alternatively, trees can be stunted as krummholz, forming ribbons or patches where subtleties of wind and exposure dictate (see Billings, I969; Marr, 1977). 1 nterdigitating with these various formations is alpine tundra vegetation. All of these variations could fall inside a single 0..1 ha unit despite the desirability of dividing them into distinct community-types. No pretense is made of trying to interprete the complex patterns. Rather, the results are presented as a preliminary interpretation to facilitate comparison with forest and alpine vegetation types. Two groups are recognized on the basis of position on a complex gradient incorporating moisture, soil texture and exposure. In addition subalpine Picea, Abies forest and the subalpine Pinusflexilis forest must be considered as ecotonal to alpine vegetation. The present series is distinguished from these latter types on the basis of frequent wind-trained or krummholz treeforms, and the interdigitation of forest and tundra.

Alpine vegetation, while not specifically treated in the present monograph, has received considerable previous attention. Both Kiener (1967) and Willard (1963, 1979) have published major mono- graphs on the alpine of Rocky Mountain National Park, and Kom~irkov~t (1979; Kom~irkowi & Webber, 1978) has examined in detail the alpine of Niwot Ridge, an area 15 km south of the study area.

Mesic krummholz (GI) Mesic krummholz takes many forms with the

physiognomy determined by site conditions. At one extreme all trees are less than 2 m high (usually less than 1 m) and are strongly shaped by wind-caused ice abrasion. The canopy has a smooth, apparently ice-sheared surface, with a varying but continuous cover of Abies, Picea and Salix. This vegetation often takes the form of ribbons or clumps o1~ vegetation which have formed behind small rocks or depressions where seedlings, sheltered from the wind, can become established. On sheltered slopes are forests composed of groups of large trees bordered by small Abies produced through layering. While the initial tree of these clumps is frequently Picea, Abies usually becomes established

in the shelter of the lower limbs and a dense mat develops.

Understory cover is high averaging 75%. Only under a dense tree crown does the cover appear less than continuous. Cover by tree species is high in this lower stratum providing over half of the total, though much of this results from layering or low krummholz individuals. Of the shrubs, Salix brachycarpa and S. planifolia frequently dominate edges of krummholz patches. In contrast, Vacci- nium scopulorum and Ribes montigenum are associated with the understory of tree patches. Of those herbaceous species occurring with high cover, Po- lemonium delicatum, Penstemon whippleanus and Epilobium angustifolium are typically forest species, Artemisia arctica, Arenaria obtusiloba, Bis- torta bistortoides, Potentilla diversifolia and Trifo- #um dasyphyllum are more characteristic of tundra communities and occur predominantly between krummholz patches. Sedum lanceolatum, Selagi- nella densa, Antennaria rosea and Trisetum spicatum can be considered typical of both phases. A typical stand contains 38 species including 3 shrub and 33 herb species.

Xeric krummholz (G2) Xeric krummholz is the shallow soil, well-

drained, high-exposure counterpart of the mesic krummholz (GI). It too is characterized by an interdigitation of wind-trained tree forms and alpine species. However, the alpine element is not the mesic forb group found in mesic krummholz, but is a mixture of scattered individuals characteristic of alpine fell-fields or, on areas with soil accumulation, dry Kobresia myosuroides, Carex rupestris turf. Cushion forms such as Silene acaulis, Arenaria obtusiloba and Trifolium dasyphyllum are common.

Large arborescent individuals of Pieea and Abies are mostly absent from this type. Shrub forms of Abies are typically more important than Picea, or Salix brachycarpa which often defines the edges of krummholz mats. The diversity of the microclimatic situations, from dry and exposed rock, to Kobresia sod, to coniferous understory, allows a rich assem- blage of plants to grow in this type, the average stand containing 40 species. Graminoids are an important component of the herbaceous cover in xeric krummholz and include Kobresia myosuroides, Poa lettermannii, and Poa alpina. Where

substrate permits, Kobresia myosuroides and Carex rupestris form solid turf, together with Carex albonigra and C.foenea. Dominant forbs in the turf areas include Trifolium dasyphyllum, Arenaria obtusiloba, Arternisia borealis, Geum rossii, and Hymenoxis acaulis.

Populus tremuloides forests (14)

Populus tremuloides stands occur isolated in dry, high-elevation grasslands, as successional stands on both wet and dry sites, and as climax vegetation in certain poorly drained areas. This complex pattern is complicated further by the apparent ability of Pinus contorta to grow on many of the same sites and out compete Populus. Typically, Populus- dominated sites have better soil conditions than Pinus contorta sites (Hoff, 1957; Morgan, 1969; Reed, 1971). When they do occur on xeric sites they are usually on deep and relatively fertile soils, situations such as talus slopes, moraines and allu- vium. On shallow soils with underlying bedrock, Pinus contorta quickly attains dominance and Populus is usually present only as an early successional species rarely exceeding 3 cm dbh. At middle elevations on moist sites (often mixed mesic forest) Populus is frequently successional to coniferous species. On these sites Populus attains its greatest size locally (>50 cm dbh) but is succeeded by Picea and Abies. On the edges of meadows and other wet sites with saturated soils and well developed herbaceous vegetation, Populus appears to maintain dominance rather than succeeding to conifer forest. Root sprouting allows continued establishment and spread in dense herbaceous undergrowth, a mechanism not available to the local coniferous species. Similar stable Populus stands have been reported by Marr (1961) and others (Reed, 1971; Wirsing, 1973).

While none of the stands included in the Populus group show strong successional trends toward coniferous dominance, all appear relatively young and slow conifer invasion remains likely. A few individuals of Picea, Abies, Pinus contorta and Pinusflexilis were found in many of the stands. The relative dominance of Populus varies from site to site in response to edaphic factors, and competition from the rich herbaceous layer is also important.

A few small Populus stems are present in most forest-types and after fire these root sprout to

31

produce many small trees (see Patten & Avant, 1970; Steneker, 1974). Frequent and repeated burning can severely reduce the local conifer populations, while the root-sprouting Populus increases in dominance. Dominance after fire can also depend on seed supply. While there is some question as to the ability of Populus to regenerate from seed in this region, it seems likely that if the time of year and weather were favorable, many Populus seeds could blow into a stand and become established. Pinus seeds are heavier and are not dispersed as readily. Thus, on large burns lack of seed source could restrict initial Pinus establishment.

The enigmatic gradient pattern of Populus tremuloides can be clarified by viewing its ecological response in geographic perspective (Peet, 1978b). South of the range of Pinus contorta in the Rocky Mountains, Populus trernuloides is the dominant post-fire, successional species for almost all middle- elevation forests. With the northward increase of Pinus contorta, the central portion of the range of environmental conditions potentially available to Populus tremuloides is preempted. In the northern Front Range Pinus contorta is widespread and Populus tremuloides appears restricted to peripheral habitats through competitive exclusion. This restriction of Populus to varied but relatively extreme sites explains the failure of investigators working in only one locality to resolve the competitive relationships of these two species.

The confinement of Populus tremuloides to peripheral sites results in a heterogeneous assem- blage of stands. However, these stands do have the unifying characteristic of rich herbaceous vegetation in relation to adjacent coniferous forests. This in turn makes Populus stands even more distinctive and precludes effective inclusion in environmentally similar coniferous types. However, on those sites where a clear successional pattern had developed, the stand was placed in the type containing the presumed steady-state. Only where Populus was the dominant and no clear successional pattern was yet apparent was the stand placed in the undif- ferentiated Populus group. In effect, the stands grouped as Populus tremuloides forests constitute a heterogeneous group of misfits. These stands occur between 2 650 and 3 150 m elevation and cover a broad range of moisture conditions. On the community mosaic they overlap most of the Pinus contorta series and limited portions of adjacent series.

32

Average understory cover in Populus tremuloides forests is 57% with leafy herbs and graminoids dominant. This is indicative of the lush understory of the Populus forests. An average of 34 species occurs per stand. Shrubs are well represented with an average of six species per stand. Dominant among these in terms of cover are Arctostaphylos uva-ursi and Juniperus communis, though Rosa leads in frequency and constancy. The numerous Populusroot suckers also contribute a large portion of the understory cover. N.o herbaceous species could be considered characteristic, a result which reflects the diversity of environmental conditions represented in the group. Highest cover is attained by Thermopsis divaricarpa, Arnica cordifolia, and Haplopappus paro,i.

Gradient patterns

F o r e s t z o n e s

Rocky Mountain vegetation has traditionally been described in terms of elevational belts or life zones. Ramaley (1907, 1908) and Rydberg (1916) were among the first to propose such classifications. The various early schemes have been re- viewed and revised by Daubenmire (1938, 1943) who suggested use of six vegetation-based zones: alpine, spruce-fir (Picea, Abies), Douglas fir (Pseu- dotsuga), Ponderosa pine (Pinus ponderosa), juniper-pinyon (Juniperus, Pinus edulis) and oak- mountain mahagony (Quercus gambelii, Cercocar- pus montanus). Similar zones have been described specifically for Colorado by Costello (1954), though he combined the Pinusponderosa and Pseudotsuga zones. Moir (1969) proposed addition of a Pinus contorta zone, arguing that this species forms a climax forest type between the Pseudotsuga and Picea-Abies forests.

Despite numerous attempts to refine life-zone classifications, no concensus exists or is likely to exist (Peet, 1978b). As with any form of vegetation" classification, no particular set of zones can be considered to be a priori correct. Shortcomings will always be evident as the complex vegetation of a mountainous region can never be accurately represented by a simple set of classes.

Examination of the gradient model presented in Figure 5 shows the traditional elevation zones to be

related to both moisture and elevation gradients. Grasslands and shrublands reach highest elevations on the driest sites. Bordering these open communities, the Pinus ponderosa zone extends from sheltered slopes at 1 700 m to dry ridges and south- slopes at 2 700 m. The traditional Pseudotsuga zone forms a diagonal from ravine forests at 1 700 m to xeric ridges at 2 800 m. The Picea, Abies zone includes a pronounced extension down to 2 500 m in the cool ravines but is rarely found below 2 900 m on open slopes. Between the Pseudotsuga and Picea, Abies zones is a heterogeneous diagonal group of vegetation types including the mesic montane, Pinus contorta, and Pinus flexilis series.

Species successional positions vary with site but can be clarified if viewed in relation to elevation and moisture gradients. Pinus contorta dominates the central portion of the community mosaic and is successional to Pseudotsuga at low elevations, to Abies and Picea at high elevations, and is stable on certain intermediate sites. The successional position of Pseudotsuga is equally complex. At low elevations it is a climax species, replacing Pinus ponderosa. At higher elevations Pseudotsuga is a climax species replacing Pinus contorta. On xeric sites the species often occurs alone. In some mesic montane forests Pseudotsuga is a post-fire, pioneer species giving way to climax stands of Picea and Abies. However, on other mesic sites, Pseudotsuga is both a pioneer and a climax species. Similarly, Abies plays a variable successional role. Abies is often the climax species of stands initially colonized by Pinus contorta or Pieea engelmannii after fire. However, under certain conditions of seed rain, Abies can be the first species to dominate a site after fire, and is with time partially replaced by Picea. Species successional roles, like patterns in diversity, need to be studied in a multidimensional perspective.

Understorv vegetation

The small number of tree species occurring in Front Range forests allowed communities to be readily characterized using dominant trees. Like trees, herb and shrub species vary continuously and independently in importance with respect to the elevation and moisture gradients, but their much greater number (>530) precludes a simple, gradient-based mosaic representation such as was used

for tree composi t ion. As an a l te rnat ive , represen-

tative species can be examined individual ly and

c o m m o n patterns can be sought. Table 2 illustrates

for 26 selected species var ia t ion in the weighted

average posit ion on the final ordinat ion-based

moisture gradient across a series of 7 elevation belts

(the 1 700-2 000 m elevation belt was not included

because of the small n u m b e r of samples).

The species listed in Table 2 have been divided

into two groups. The first group is composed of

species which show a s t rong temperature-correla ted

dis tr ibut ion. Specifically, with increasing elevation

the species are found farther toward the xeric end of

the topographic-mois ture gradient. For example,

Jamesia americana and Physoearpus monogynus are both largely confined to cool, nor th-facing

ravine slopes at 2 000 m, but at 3 000 m are limited

to exposed south-facing slopes at the xeric end of

the moisture gradient. As the xeric sites are largely

33

defined on the basis of having high levels of incident

solar radiat ion, they are also warm sites. In contrast

ravine slopes at the moist end of the gradient are

cool, being sheltered from direct sunlight and often

being subject to cold air drainage. Thus, the

decrease in tempera ture resulting from increased

elevation is compensated for by a shift in posi t ion

along the topographic-mois ture gradient.

The second group contains species which are

largely constant in their moisture gradient posit ions

with changing elevation. Apparent ly these species

respond much more strongly to moisture related

aspects of the env i ronmen t than to temperature.

Species like Calamagrostis canadensis, Carex aquatilis and Equisetum arvense are virtually restricted

to saturated soils and appearent ly cannot tolerate

even short periods of drought . A number of other

species are limited to xeric sites, probably in part

because of a requi rement for the high light levels

Table 2. Positions of representative species on ordination axes after scaling.

Elevation Belt 2 000- 2 300 2 500- 2 700- 2 900 3 100 3 300 2300m 2500m 2700m 2900m 3 100m 3300m 3500m

Temperature specific species Achillea lanulosa 1.83 24.98 49.60 66.64 65.64 62.34 68.39 Antennaria rosea 18.50 40.60 64.59 81.53 74.82 63.23 67.92 Arctostaphylos uva-ursi 21.48 54.26 63.46 81.55 82.29 98.55 Arnica cordifolia 4.39 12.79 15.19 47.21 34.77 29.79 49.00 Fraseria speciosa 15.69 55.55 52.37 85.81 85.95 84.99 - Geranium fremontii 47.46 63.48 80.50 95.68 93.17 - - Jamesia americana 18.71 31.36 32.99 82.05 77.95 100.00 - Physocarpus monogynus 19.08 27.69 39.47 82.99 96.51 - - Pyrola secunda - 12.21 35.61 46.76 61.13 61.61 Ribes cereum 50.12 65.18 74.37 98.42 75.74 Rosa sp. 2.58 24.26 22.09 58.68 60.98 89.30 - Vaccinium myrtillus - 15.60 44.51 48.32 67.35 64.77

Moisture specific species Artemisia frigida 72.98 99.00 96.32 98.39 - Calamagrostis canadensis 0.04 3.50 4.99 18.23 15.13 3.30 4.54 Calamagrostis purpurascens - 90.96 84.36 88.84 80.06 Caltha leptosepala - - 4.44 5.32 4.54 Carex aquatilis 4.75 5.00 2.69 11.50 Cercocarpus montanus 100.00 100.00 Equisetum arvense 1.55 4.20 0.26 8.90 5.63 - Erigeron perigrinus 20.75 23.85 39.05 24.39 14.06 25.72 Galium triflorum 0.04 3.61 0.44 16.29 5.63 - Leucopoa kingii 54.04 84.78 72.59 91.46 66.67 - Mertensia ciliata 0.97 20.75 0.00 30.22 17.71 8.65 28.46 Muhlenbergia montana 89.67 79.33 100.00 99.37 100.00 - Sedum lanceolata 43.97 77.75 88.80 87.88 78.85 77.73 71.31 Selaginella densa - 78.07 59.87 89.65 84.52 76.82 67.80

34

found in these areas with very open tree canopies. lncluded in this group are species like Selaginella densa, Muhlenbergia montana and Artemisia frigida which are found over a broad range of elevations, but only on dry, exposed sites.

An alternative approach to the study of understory vegetation is to recognize groups of similar, potentially competing species, or guilds (sensu Root, 1967; Schimper, 1903). Growth forms provide a useful basis for such a classification. Fol- lowing this approach the distributions of the major

species in two of the most important groups, the shrub and graminoid (Poaceae, Cvperaceae) guilds, are examined.

Several characteristic groups of shrub species are evident in Rocky Mountain forests, the groups reflecting similar responses to the macrogradients of moisture and elevation. The distributions of the most important members of these groups are illustrated in Figure 6.

The major shrubs of dry, low-elevation woodlands are Purshia tridentata on the sunny micro-

Melers

3500

J o/ix planifoha V myrtillis

I Gaulthena hurmlus V scoparium

Vocc/nturn scoparium i Salix brachycaroa Ribes mont iqenum~

3300

3100

Z 2900 W O <

2700 Z 0

2500 w

2300

2100

1900

1700

" I Vaccmium rnyrtil lus

] ~ scoporium I I

I I I !

I I I I

I /

/Lonicera I e involucrata [

Ribes 1 / lacustre

lRJbes I ,~o~.~e" / Z Vnccinium m ~ / rnontigen urn scopor~ urn / ]-L . . . . . . . i " . oio,om on,0.;o,o . . . . , ,

I I soaper,urn L Vacciniurn V. myrtillus~ ~ l ~ r~ ~....

myrtillus ] ~ V. scoparmm ~ " ~ ~ ~Tumperus

RJbes lacustre mlum myrtillus ~Tamesio 3-uniperus Arctostophylos

1

n /d'umperus communis "~ ~ ~" Arctostaphylos ~

ium myrhlhs / ~ ..~""~ Rosa sp. / ~" ~ i 3'~4 /

, ~ / 1 1 / Arctostophylo ~ ~ /.Tuniperus J / . ,

HJgmperu s j communis / cornmunis~lY F • Linnaea, Rosa • Juniperos • J'amesia • Physocarpus 1 8 ~ I

I_ ~ °~a°~° - I I ,~

/ I I I I /

Purshia tridentata Ribes cereum /

/

I Phy-socarous rnonogynuss.~ ~ ~ ~ ~ - - Ribes cereum ! I Ribes cereum

I Arct°s t °phyt°s~ r / Cercocarpus

/ I

/ P u r s ~ i a I I Physocarpus tridentota ~ f i J-uniperus communis Ribes cereum i_ I O'amesia americana / Arctostaphylos / Ribes cereum j f

J / / Rhus trilobo

/ / /

/ /

Wet Ravines Open Slopes Exposed Ridges Xeric

TOPOGRAPHIC-MOISTURE GRADIENT

Fig. 6. Community mosaic diagram showing dominant shrub species relative to gradients of elevation and topographic-moisture. The community series and types indicated by solid and dashed lines respectively are identified in Fig. 5 and its legend.

sites with fine-textured soils and Ribes cereum of the cooler, rockier sites. The most xeric sites at the lower woodland border are often dominated by two larger shrubs, Cercocarpus montanus and Rhus trilobata. In ravines and on cool, north slopes where true forest replaces woodland, a well-developed shrub group contains Acer glabrum, Jamesia americana, Mahonia repens, Physocarpus monogynus, Ribes inerme, Rosa, Shepherdia canadensis and Symphoricarpus oreophilus. At higher elevations, near 2 500 m, the ravine guild overlaps with a cool, moist forest guild containing Lonicera in-

35

volucrata, Ribes lacustre, Sambucus racemosa and Linnaea borealis to produce the greatest shrub diversity in the region, an average of 11 species per 0.1 ha. How these species are differentiated with respect to microhabitat factors is not yet clear, but certainly all are broadly overlapping in their distributions.

The central portion of the mosaic diagram dominated by Pseudotsuga and Pinus contorta forest is low in shrub diversity. Here the only species consistently important are Juniperus communis, Arctostaphylos uva-ursi and Salix scouleriana.

Meters

5 5 0 0

Ca~bx aquat~lis

Calamagrostis ~ 3 0 0 - Iconadens~s

I

5100

t - z 2900 LI.J

n," £.9

2700 Z 0

w 2500

ILl

2500 t-,ph t

2100

1900

• I E eochoms I

I pouciflora I I I I I

I / i i

I t I / r... . . . . -]

~r O$1 Is eoSl s

1 Wet

1700

"~ldP". ~c . ~°~ /" I~ ~. IC. rossii, 9o ° .,.,,.I ~b . rossft ~. . . . . . . ~ -i I \

/

/

/ Carex ross l i

/

! /

/ /

I

, I I

,

~o I . . . / I ~o I / - -

\" ~" t f 0 O0

o, ,..;<5,(o>,:o . /

Rovines Open Slopes Exposed Ridges Xeri

TOPOGRAPHIC-MOISTURE GRADIENT

Fig, 7. Community mosaic diagram showing dominant graminoid species (Poaceae, Cyperaceae) relative to gradients of elevation and topographic-moisture. The community series and types indicated by solid and dashed lines resp. are identified in Fig. 5 and its legend.

36

With increasing elevation Abies and Picea replace Pseudotsuga as potential climax dominants, and there is a parallel shift in the shrub synusia toward overwhelming dominance by Vaccinium. Vacci- nium myrtillus essentially carpets the understory of all the lower elevation Picea, Abies forests, but with increasing elevation or dryness it is replaced by V. scoparium. At the transition from forest to alpine tundra and meadow, shrub diversity again increases, probably in response to higher light levels. At this transition the edges of krummholz mats are often dominated by Salix brachycarpa and Ribes montigenum, with Vaccinium remaining important throughout.

Graminoids present different patterns than those shown by shrubs (Fig. 7). The central portion of the mosaic diagram is again characterized by low diversity with few if any species other than the ubiquitous Carex rossii being found in the dense shade of the Pinus contorta or Picea and Abies forests. However, on the periphery of the diagram, on moist sites or where the canopy is open, numerous species are found. Primarily grasses of the plains and low foothills dominate on the low elevation, exposed slopes of the Cercocarpus shrublands. These include Stipa comata, Agropyron albicans, Andropogon seoparius and on more disturbed sites the smaller Bromus tectorum and Bouteloua hirsuta. In the woodlands of the Pinus ponderosa Pseudotsuga zone, Muhlenbergia montana is the dominant species of open, dry sites, and Leucopoa kingii is equally important on somewhat higher and cooler woodland sites.

Middle elevation wet sites are often dominated by Calamagrostis canadensis and the weedy ad- ventives, Poa pratensis and Phleum pratense. At higher elevations (>2 700 m) Calamagrostis continues to dominate the understory on saturated soils but with codominance by Carex aquatilis. Carex disperma also codominates on wet sites between 2 700 and 2 900 m, and Eleocharis parvi- flora above 2 900 m in the bog forests.

The transition to alpine is rich in graminoids. Carexfoenea is almost as abundant as Carex rossii in the wooded areas while Calamagrostispurpuras- tens and Trisetum spieatum are the most important species of the dryer sites. With continued increase in elevation, a sod of alpine species can be found interdigitating with forest. Here dominant species include Kobresia myosuroides, Carex ru-

pestris, Carex albonigra, Poa nervosa and many other species.

Analysis of vegetation in terms of component guilds such as graminoids and shrubs appears to have much promise for improving our understand- ing of community structure. Future studies might profitably focus on the interactions and differentia- tion of similar species occurring over a portion of the mosaic diagram such as the rich shrub communities of the mesic montane forests, or the graminoids of the alpine transition.

Species richness

In Figure 8 species richness, defined as the number of species per 0.1 ha sample, is plotted simultaneously against the elevation and topographic-moisture gradients. Appearing as a bowl- shaped surface, species richness is lowest in the central portion of the figure corresponding to areas dominated primarily by dense, even-aged forests. In contrast to the predictions of Terborgh (1973), the most centrally located (and probably the most widespread) forest types appear the least rich in species. Highest richness is found in the relatively restricted mesic montane forests where moisture is abundant and temperatures are moderate. In addition, moderately high richness can be found across

5 5 0 0 m

l-- z LI2

QZ (.9

Z O ~ 2 6 0 0 r n :> L.d ._1 UJ

1 7 0 0 m

................... "..Alpine ( ~ T r e n s i t i o i ' i ~ ;....Alpine • '.,+%+..+.. "' +.

Picea, Abies Forests ..Forests

o . . . . . . .

................ Forests )...~ .... / ........ /

."7 / p( rosa+ ,.." p ....... ~ / / P s e u d o t s u g ' a Woodlands ........... . ! I / F o r e s t s ,.'" / ....... ,~ ." 40 ...... I / / . . " / / ~ 0 ..... Shrub onds ............... ~ ~rasslan " ~ ' - ' ~ .' ~ u - "" h~ acl Wel Xeric

TOPOGRAPHIC- MOISTURE GRADIENT

Fig. 8. Species richness (number of species per O. 1 ha) plotted against gradients of elevation and site moisture•

the entire moisture gradient at both the high and low elevation transitions from forest to grass or shrub dominated communities.

Various authors have reported richness to be maximal at low elevations (e.g. Reed, 1969; Whit- taker, 1956, 1960; Yoda, 1967) and at middle elevations (e.g. Daubenmire & Daubenmire, 1968; Whittaker & Niering, 1965, 1975). Richness has been reported to peak at the mesic end of a moisture gradient (e.g. Daubenmire & Daubenmire, 1968; Glenn-Lewin, 1975) as well as in the central portion ofa mesic to xeric moisture gradient (e.g. Auclair & Goff, 1971; Whittaker, 1956; Whittaker & Niering, 1965). All these patterns, and several others, can be found in Front Range forests. Here the effects of elevation and moisture interact in such a way that is is essential to consider responses to both factor- complexes simultaneously when examining species richness (Poet, 1978a). Species richness patterns are further complicated by the interaction of succession with the environmental gradients as will be dis- cussed in the following section.

Community dynamics

Structural analysis

Harper (1967, 1977) considered plasticity of plant response to be one of the major factors delaying development of a theory of plant population dynamics, largely because plasticity limits use of age-based theory. He concluded size rather than age to be the most important aspect of plant population structure. Rabotnov (1969; see Harper & White, 1974) and his co-workers have used an approach based on life-stages as an alternative to age in studies of plant population structure. To apply this approach to trees, life-stages need only be assigned on the basis of uniform diameter-classes (e.g. Hartshorn, 1975). Because size relates to canopy position and survival probability more strongly than does age, examination of diameter classes in preference to age-classes appears to be a productive approach for examining forest dynamics.

A stable or steady-state forest should be characterized by balanced birth and death rates. Such forests which typically have many small trees and a few large ones are referred to as having reversed

37

J-shaped diameter distributions (Meyer, 1952; Leak, 1964, 1965). If the probability of an arbitrary individual dying is constant for all size-classes, and the birth rate is constant, the diameter distribution can be described by a negative exponential distribution which has this reversed-J shape. Leak (1965, 1969) considered this distribution typical of the deciduous forests he studied in New England. Similarly, if the probability of dying decreases at a constant rate relative to size, the resultant distribution is a power function which is a more concave reversed-J. Hett (1971; also Hett & Loucks, 1971) suggested that for Acer saccharum (and probably most other forest tree species) early mortality is high because of low light levels and intense understory competition. She further suggested that mortality decreases with increasing age until senescence is reached. Goff & West (1975) concurred, adding that if a senescence phase is included, sigmoidal diameter distribution curves should be expected.

Regardless of which generating model is accepted for a given forest, or whether the model is applied to size or age-classes, the result is typically a reversed-Jdiameter distribution indicating the presence of many more small trees than large.

Diameter distributions of successional stands are more variable in form. When disturbance removes a major portion of the original tree population, a large number of young trees can usually invade. With growth, these young trees soon preempt critical resources and inhibit additional regeneration. Skewed bell-shaped diameter distributions appear most characteristic of such stands (see Bailey & Dell, 1973; Bliss & Reinker, 1964; Day, 1972; Ilvessalo, 1937; Nelson, 1964), the heights and breadths of the curves being influenced by initial density and synchrony of seedlings establishment. The essential pattern is that the more favorable the site and the more intense the competition, the greater the initial preemption of resources. This leads to suppression of the smaller trees and elimination of regeneration until such time as mortality of established trees reduces competitive pressures. Based on these considerations, a stand with a reversed-J diameter distribution for each of its species can be considered to be near the steady- state condition. Deviation can be interpreted as evidence of reduced reproductive success and prior disturbance (e.g., Jackson & Faller, t973; Johnson & Bell, 1975; Schmelz & Lindsey, 1965).

38

The grad ien t -based class i f icat ion was used to

provide sets of s tands of var iable age but with

relat ively cons tant site condi t ions and develop-

menta l po ten t ia l within which progress ive changes

in tree popu la t i on s t ructure could be examined .

Bell-shaped curves were considered indicat ive of

even-aged s tands with the b read th of the bell a

consequence of the length of the es tab l i shment

per iod, the init ial densi ty and site quali ty. Negative

exponent ia l d i s t r ibu t ions (reversed-J) were consid-

ered indicat ive of susta ined rep lacement and thus of

a p p r o a c h to s teady-s ta te condi t ions . All s tands

within a c o m m u n i t y - t y p e were examined together and a r ranged in what appea red to be plausible successional sequences. Because var ia t ion in site

condi t ion , seed rain, and d is turbance his tory can all

influence the rate of . success ion , a r r angemen t of

s tands based on d i ame te r d is t r ibut ions should serve

to make s tands more readi ly c o m p a r a b l e than

a r rangement on the basis of s tand age alone. Orde r ing forest s tands grown under s imilar site

condi t ions into sequences of increasing age can

provide a v a l u a b l e means of examin ing forest

deve lopment . However , even with the large da ta

base employed , an e lement of subject ivi ty is un-

avo idab le in the cons t ruc t ion of such develop- menta l sequences. The highly s tochast ic na ture of

tree es tab l i shment and growth as well as var ia t ion

in seed rain and c l imate lead to cons iderable

in ters tand var ia t ion , thus obscur ing under lying

pat terns . W i t h o u t a large number of s tands for

compar i son , any conclus ions reached using this

me thod would be suspect . Given the l imi ta t ions of the method , the pa t te rns repor ted must be con-

s idered somewha t qual i ta t ive; only average condi -

t ions and suggest ion of the range of possible var ia t ion should be inferred.

Patterns o f forest deve lopment

Pinus contorta forests

Typica l d i ame te r d i s t r ibu t ions for an age se-

quence of s tands of the Pinus contorta forest type

are shown in Table 3. The deve lopmenta l pa t te rn

suggested by these s tands conforms to tha t normal ly repor ted for even-aged conifer forests. A bell-

shaped d i ame te r d i s t r ibu t ion appears ear ly in s tand

d e v e l o p m e n t and, given absence of fur ther forest

pe r tu rba t ion , remains for the lifetime of the init ial cohor t , usual ly 250-300 yr. As the s tand develops,

bo th mean tree d i ame te r and the var iance in tree d iamete r increase.

The be l l - shaped curves and increasing mean tree

size are in large par t a direct result of r eproduc t ive

fai lure. Low compet i t ive pressures immedia t e ly

after d i s tu rbance a l low a high rate of tree seedling

es tabl ishment . Because of the result ing high densi ty of seedlings, c a n o p y c losure is fo l lowed by a per iod

of intense intraspecif ic compe t i t ion with low aver-

age growth rates and high morta l i ty . Vir tual ly no

new seedlings become es tabl ished after init ial

c anopy closure. The length of t ime until c anopy

Table 3. Diameter distributions for an age sequence of five Pinus contorta forest stands.

Species (Inches)* S 0 1 2 3 4 5 6 7 8 9 l0 11 12 13 14 15 16 17 18 (Centimeters) 5 10 15 20 26 31 36 41 46

Stand 183: Stage 1,42 years old Pinus contorta 14 25 55 59 1

Stand 116: Stage 2, 73 years old Pinus contorta 16 8 20 24 46 46 32 10 18 12 2

Stand 282: Stage 3, ~150 years old Pinus contorta 3 0 0 3 5 9 1l 12 13 17 15 10

Stand 108: Stage 4, --225 years old Pinus contorta 47 2 3 l 6 7 3 6 5 13 13 9

Stand304: Stage 5, --3 l0 years old Pinus contorta 83 64 18 23 14 6 3 3 4 5 8 6

4 3 0 1

6 7 1 1 0

4 1 1

* 'S" refers to seedlings defined as stems >10 cm but <1 m tall. '0' refers to stems >1 m tall but <2.5 cm dbh. Diameters were initially recorded by inch size classes to facilitate comparison with existing North American data. Centimeter equivalents are shown below to aid in comparisons. A few additional species occurred in these stands but were of only minor importance. Stem counts in this and subsequent diameter distribution tables refer to stems per 0.1 ha plot unless otherwise specified.

closure and the associated reproductive failure varies. Under extremely favorable conditions 10 yr is adequate, while in some cases 50 or more yr may be required. Regardless of the time period, all stands with this developmental pattern are referred to initially as 'even-aged'. In stand 183 shown in Table 3, none of the individuals >10 cm tall were less than 24 yr old while the oldest individual was only 42. The smaller individuals, although competitively suppressed, were close to the same age as the canopy dominants. Even-age stand structure cannot persist indefinitely. Eventually a sufficient number of trees will have died so that openings which appear in the canopy cannot be filled by suppressed individuals from the understory. When competitive pressures have declined sufficiently, regeneration will resume. Stand 108 in Table 3 shows the start of this process with an increase in seedling density. Stand 304 represents a transition phase with relict members of the original population persisting during the establishment of a new but less even-aged cohort of future canopy dominants.

Each of the 23 stands of Pinus contorta forest sampled during the study was classified as belonging to one of five developmental classes. Class 1 consists of stands with a unimodal diameter distribution and mean diameter between 2.5 and 7.5 cm. Class 2 contains similar stands with a mean diameter between 7.5 cm and 15 cm. Class 3 is composed of unimodal stands with mean diameters between 15 and 22.5 cm. Class 4 contains over-

39

mature, even-aged stands, including both those stands with a bimodal distribution suggesting renewed regeneration and those with unimodat distribution with mean diameters greater than 22.5 cm. Class 5 contains stands approaching an all-size diameter distribution. The average characteristics of the stands belonging to each of these classes are summarized in 'Fable 4.

Across the developmental sequence suggested by Table 4, basal area increases steadily to a site specific maximum (--32 ma/ha), and then remains relatively constant for roughly 150 yr. This latter period is one of stand stagnation with little net production. Diameter increase is often less than 2 cm in 100 yr for all except the dominant canopy trees. While basal area remains reasonably constant, large, compensatory changes occur in tree density and size. Average diameter increases from 11.3 to 22.25 cm while the density of Pinus contorta

(>7.5 cm) drops from 1 578 to 315 per ha. Despite a high variance in initial stockage, most

stands converge toward a site specific basal area maximum within 100 yrs. Density, however, is much more variable as is suggested by the standard deviations in Table 4. Weather patterns, proximity of seed sources, time of disturbance, intensity of fire and numerous other factors can strongly influence initial stocking levels. Because Pinus contorta is tolerant of extreme crowding and capable of pro- longed suppression (Alexander, 1974; Clements, 1910; Horton, 1956; Mason, 1915), effects of initial density variation can persist for long periods, often

Table 4, Developmental stages of Pinus comorta forest.

Number of Typical Average Seedlings~ Saplings2 Trees~ Basal Area Diversity4 Stage Samples Ages DBH (cm) (n/ha) (n/ha) (n/ha) (m2/ha) Herbaceous Total

t 2 20---70 3.99 250 710 60 2.35 16,0 24,0 (198) (608) (71) (1.2) (4,2) (4.2)

2 5 70-125 11.30 74 406 1578 31.8 II,8 17,4 (70) (295) (1095) (11.6) (3.8) (4.7)

3 7 125-175 17.93 111 50 816 30.5 7,7 14,6 (168) (63) (276) (2,6) (4.3) (5,8)

4 6 175-250 22.25 150 58 315 29.1 10.5 18.5 (225) (83) (113) (7.3) (3.7) (4.7)

5 3 250-350 8.43 747 310 506 17.5 20,0 27.0 (669) (87) (35) (3.1) (6.6) (8.0)

Standard deviations shown in parentheses ~ Seedlings are stems > 10 cm tall and <2.5 cm DBH, 2 Saplings are stems >2.5 em DBH and <7.5 cm DBIt, 3 Trees are stems >7,5 cm DBH, a Species per 0. l ha.

40

until death of the original generation of canopy trees. Nonetheless, there is a pattern of continually decreasing density due to natural thinning.

Because of the combined effects of canopy tree mortality and reproductive failure, the opening of the canopy is inevitable with this usually taking place when the stand is between 250 and 325 yr old. Old trees are frequently infected with heart rot and become increasingly susceptible to wind breakage. Mistletoe and insect infestation (Amman, 1977) are also common in older trees. These factors combined with the greater exposure of the remaining trees to storm damage can concentrate canopy breakup in a relatively brief interval, often less than 30 yr. In such cases an abrupt drop in basal area occurs. Similar dramatic transitions have been reported for the conifer forests of Fenno-Scandia (ilvessalo, 1937; Sir6n, 1955), and Alberta (Ploch- mann, 1956).

The structure of a forest following death of the first generation of dominants will be strongly influenced by the synchrony of mortality in the previous stand. If mortality is rapid, due perhaps to a wind storm or a severe insect outbreak, the new generation will tend to be even-aged and structurally similar to the previous forest. If, however, mortality is gradual, a mixed-age stand will develop which can be expected to more closely approximate a steady-state, all-sized structure. A continuing sequence of oscillations in basal area and cover could result (Plochmann, 1956) or, more likely, natural damping could lead to a steady-state structure soon after the demise of the first generation trees. Because Pinus contorta seedlings and saplings have the capability of growing to maturity only in open forests, steady-state forests can be expected to be

more open and to have lower biomass and canopy cover than the even-aged Pinus contorta forests which predominate in the study area. The basal area of 17.5 m2/ha shown for near steady-state (Class 5) stands in Table 4 is probably abnormally low, owing to the small number of new generation trees which had reached maturity in the stands studied. More likely, steady-state basal area is around 24 m2/ha. The actual value is a question of only academic interest, however, as the natural fire frequency is sufficiently high that second and third generation forests are rarely if ever encountered.

Just as the high density of trees near the middle of the age sequence precludes seedling growth, so too is the remainder of the understory stratum suppressed. Cover of herbaceous vegetation (Table 4) drops from 25% in the early stages of establishment to under 1%. Similarly, the number of species in the sequence (Table 4) drops from an early high of 24 to a low near 14, only to increase to 27 again during the breakup and subsequent recovery phases of stand development. Although the results shown in Table 4 suggest this variation in diversity to be attributable almost exclusively to changes in the success of herbaceous species, it is not clear why the competitive impact should be of less importance for woody species, unless they simply have greater longevity.

Structural development in other forest types of the Pinus contorta series appears not greatly unlike that in the Pinus contorta type. In Pinus contorta-

Pseudotsuga forest initial composition can be dominated by either Pinus contorta or Pseudotsuga

depending on seed supply and site conditions. A frequently encountered pattern is for both Pinus and Pseudotsuga to become established early, but

Table 5. Typical diameter distributions for Pinus contorta - Pseudotsuga forest.

Species (Inches) S 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

(Centimeters) 5 10 15 20 26 31 36 41 46 51

Stand 53 Pinus contorta 0 0 1 8 23 24 15 20 8 0 1 Pseudotsuga 2 10 25 14 18 8 8 0 0 l 0 0 0 0 1

Stand 261 Pinus contorta 0 0 0 1 4 9 11 14 8 8 5 4 2 Pseud0tsuga l 8 15 17 18 17 7 7 4 2 3 1 1

Stand 247 Pinus contorta 0 0 0 1 2 1 1 1 0 0 2 2 1 2 5 Pseudotsuga 15 7 5 1 3 2 1 I 0 1 0 0 1 1 2

3 l 0 1 0 0 1 I 0 0 1

for Pinus to overtop the Pseudotsuga, leaving it suppressed in the understory. In addition, Pseu-

dotsuga establishment probably continues somewhat longer than does that of Pinus due to greater shade tolerance. This pattern is illustrated by stand 53 (Table 5) which has bell-shaped diameter distributions for both Pinus contorta and Pseudotsuga.

The plants of the two species have roughly the same mean age but have mean diameters of 11.2 and 5.8 cm respectively.

Mean diameter increases with time, but the size distribution retains its belt-shaped form and Pseudotsuga remains confined to the understory during most of the stand development period (e.g., stand 261). Pinus contorta typically fails to repro- duce on these sites, thus allowing the longer-lived Pseudotsuga to increase in dominance whenever openings appear in the canopy. Stand 247 is an example where many of the canopy Pinus and Pseudotsuga have died and in which Pseudotsuga is becoming increasingly dominant.

Initial post-fire forest development in Pinus

contorta - Abies, Picea forest is again similar to that encountered in the Pinus contorta forest type. Table 6 summarizes a developmental sequence based on 24 stands classified using the same 5 stages employed for the Pinus contorta forest. As in the Pinus eontorta forest, basal area increases steadily to a site specific max imum and then stagnates. In the latter stages of stand development basal area again drops, though not as dramatically as in the

41

Pinus contorta stands. Basal area is typically higher than in the Pinus contorta forest, with the site specific max imum close to 36 m2/ha compared to 32, and a steady-state value near 32 m2/ha compared to 24 in the Pinus contorta forest.

The primary difference between Pinus contorta

forest and Pinus contorla - Abies, Picea forest is that a successional shift in species composit ion occurs in the latter type. Pinus contorta dominates initially though other species are often present. With growth of the Pinus, most of the other species are crowded out, though some Abies and Picea will remain suppressed in the understory. With increasing stand age Abies and Pieea become established in the understory and become increasingly important as is indicated by the shift in importance values in Table 6.

The diameter sequences shown in Table 7 illustrate the later stages of stand development. Stands 29,239, and 71 can be viewed as a developmental sequence on xeric sites. After canopy break-up, which occurs around 300 yrs, Abies assumes dominance. Note the lack of advanced Abies regeneration in stand 29 where the onset of canopy break-up is suggested by the large number of standing dead Pinus contorta. Here Abies is present only in the smallest size class. Because stand 239 represents a xeric extreme of the type, it has less advanced regeneration than most stands in the type. Stand 107 (Table 7) is typical of more mesic conditions with considerable advanced regeneration. In such

Table 6. Developmental stages of Xeric Pinus contorta-Abies, Picea forest.

Relative Importance ~ Number of Typical Basal Area

Stage Samples Ages (m•/ha) Pinus contorta Abies lasiocarpa Picea engelmannii

Diversity z

Herbaceous Total

l 3 20-70 [ 1.8 72.1 4.0 20.9 (11.4) (47.9) (6.6) (36.2)

2 3 70-125 34,9 97.5 0.1 0.2 (9.1) (3.6) (9.2) (0.2)

3 7 125-175 35.9 55.3 30.0 8.6 (13.2) (32.7) (31.0) (7.0)

4 7 t75-250 38.6 42.5 19.5 20.9 (7.2) (30.8) (18.3) (11.6)

5 4 250-350 32.3 13.3 32.1 53.9 (1.3) (12.8) (9.9) (7.9)

21.7 30.7 (1.5) (4.2) 6.0 12.7

(4.6) (6.4) 4.0 10.1

(3.6) (5.5) 5.1 11.0

(2.9) (4.2) 5.2 9.7

(3.5) (4.0)

Standard deviations shown in parentheses. Relative importance is the average of the relative density (>7.5 cm dbh) and the

~' Species per 0.1 ha. relative basal area.

42

Table 7. Typical diameter distributions for Pinus contorta - Abies, Picea forests.

Species (Inches) S 0 ! 2 3 4 5 6 7 8 9 10 !I 12 13 14 15 16 17 18 (Centimeters) 5 I0 15 20 26 31 36 41 46

Stand 29 Pinus contorta 3 0 0 6 2 7 6 8 6 8 7 2 0 1 Pinus contorta (standing dead) 2 4 4 1 9 1l 5 6 3 Pieea engelmannii 4 0 0 0 t 1 0 0 1 1 I 0 0 1 Abies lasiocarpa 64 2 4 0 0 0 0 2 0 0 0

Stand 239 Pinus contorta 3 0 0 1 0 0 0 2 0 0 2 6 2 I 4 4 3 Abies lasiocarpa 14 7 9 3 5 1 6 2 0 1 1 0 0 1 0 1 Picea engelmannii 5 0 2 5 0 1 2 2 l I 0 1 0 !

Stand 7l Abieslasiocarpa 75 40 29 17 13 4 I1 0 3 4 I 2 2 1 l t 0 Picea engetmannii 19 5 4 4 8 0 5 0 3 4 2 I 1 I 0 1 0 Pinus contorta 11 3 3 1 3 4 1 1 0 0 1 1 0 t 0 0 0

Stand 107 Pinus contorta 0 0 3 0 4 0 5 5 5 12 9 11 8 8 4 l 1 Abies lasiocarpa 215 68 25 12 8 7 1 3 2 Picea engelmannii 53 9 1 0 0 0 0 t 0 0 1

I 1 0 0 1

mesic stands Abies and to a lesser extent Picea become established early and remain suppressed in the understory. It is c o m m o n to find individuals well over 100 yr old and only I m tall. Large numbers of seedlings and small saplings are present

by the time canopy break-up starts with the consequence that very little Pinus contorta regeneration occurs and the drop in basal area and cover is not as dramat ic as that described for the other

Pinus contorta types. Stand 239 (Table 7) represents an older phase o f

the forest type represented by stand 29, but with larger and more abundan t regeneration. The final stand in this sequence, number 107, represents near steady-state condit ions with all three species having roughly negative exponential diameter distribu-

tions. Pinus contorta frequently must compete with

Populus tremuloides for dominance after fire. While the precise nature of the ecological relation- ship between these species remains uncertain, stand structural compar isons reveal strong similarities. All of the Populus diameter distributions examined showed evidence o f the stands being even-aged, none appear ing structural ly stable. In most cases the size distribution was bell-shaped with the mode less than 18 cm dbh, but with a large number of seedling class individuals produced f rom root sprouts~ In stands over 30 yrs of age where Pinus

contorta and Populus tremutoides were co-dominants, Populus was consistently overtopped and showed evidence of substantial recent mortality.

Species diversity is initially high, a round 31/0.1

ha, in the Pinus contorta - Abies, Picea forest, but drops off rapidly with canopy closure to a round 10/0.1 ha (Table 6), a pat tern similar to that of the Pinus eontorta forest. This forest differs f rom Pinus contorta forest, however, in that little if any recovery can be seen in the latter stages of s tand development. A combina t ion of a well-developed groundcover of Vaccinium myrtillus and dense reproduct ion of Abies and Picea provides a highly competi t ive envi ronment even after death of the original canopy trees. Again, the change in diversity appears to reflect only a change in the herbaceous component .

Picea, Abies forests Located above the Pinus contorta forests on the

elevation gradient and occurr ing on all but the driest sites, Picea, Abies forests show considerable var ia t ion in structure despite the presence of only two major tree species. Tables 8 and 9 illustrate

s o m e of this structural richness in the form of representative diameter distributions.

~,s a means of examining stand development, all samples f rom wet, mesic, and xeric Picea, Abies forests were classified as belonging to one of four

Table 8. Typical diameter distributions for Picea, Abies forests.

Species (lnches) S 0 1 2 3 4 5 6 7 8 9 l0 11 12 13 14 15 16 17 18 19 20 2t 22 23 24 25 Larger (Centimeters) 5 10 I5 20 26 31 36 41 46 51 56 61 Larger

43

Stand 27, Wet montane forest Piceaengelmannii ? 14 5 8 7 6 1 5 1 3 0 4 2 2 1 I 1 l 1 3 0 1 0 0 0 0 l l Abieslasiocarpa '~ 0 3 5 5 5 t 2 3 2 2 2 5 2 0 2 1 Populus t remuloides ? 11 0 1 0 0 0 0 0 1 2 t 0 2 4 1 2 0 0 3

Stand 300, Picea, Abies bog forest Abieslasiocarpa 76 46 41 26 10 3 2 1 I 3 2 3 0 1 1 2 0 0 1 Piceaengelmannii 57 23 17 5 7 1 1 2 0 l 1 3 1 4 1 1

Stand 235, Wet Picea, Abies forest Piceaengelmannii 12 1 0 0 3 4 2 3 4 3 2 6 4 7 10 7 5 5 6 3 3 2 1 2 I 1 Abieslasiocarpa 61 12 9 8 3 5 2 2 2 1 1 2 1 1 1

Stand 226, Wet Picea, Abies forest Piceaengelmannii 32 20 16 6 2 3 0 I 1 1 0 1 2 0 0 1 3 2 2 3 0 3 4 0 1 0 t Abieslasiocarpa 127 55 28 16 21 9 7 7 5 4 1 0 1 1 ] 3 1 1 0 1 0 0 1

Stand 198, Xeric Picem Abies forest Piceaengelmannii 17 2 8 6 l 2 1 4 4 3 1 4 4 6 0 l 3 2 2 0 I 0 1 Abieslasiocarpa 42 53 35 30 19 11 9 6 3 2 0 2 1 0 1 0 I 0 l

Stand 305, Xeric Picea, Abies forest Piceaengelmannii 99 11 16 10 1 4 5 2 6 0 2 4 0 1 Abieslasiocarpa 368 I8 9 7 3 0 0 0 0 2 0 1

developmental stages. Stage one includes all stands with bell-shaped diameter distributions (indicative of even-age structure) with average diameters of Picea less than or equal to 12.5 cm. Stage two contains similar stands with average diameter of Picea between 12.5 and 25 cm. Stage three includes the remaining even-aged stands, those with mean diameters >25 cm. The final stage is composed of all stands close to an all-size diameter distribu-

tion, though occasionally a few large, relictual individuals are included, Average characteristics of stands in these four stages are summarized in Table 10, and representative diameter distributions for Picea, Abies forests are shown in Table 8 and 9. Summary tables were not constructed for the other forest types in the Picea, Abies series because of the smaller numbers of available samples.

Picea, Abies forests located near the wet end of

Table 9. Typical diameter distributions for Mesic Picea, Abies forests,

Species (Inches) S 0 1 2 3 4 5 6 7 8 9 l0 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 (Centimeters) 5 10 !5 20 26 31 36 41 46 51 54 61

Stand 199: Stage 1, 73 years old Piceaengelmannii 174 88 82 38 40 22 24 8 2 2 Abies lasiocarpa 176 30 26 12 4 6 4 6

Stand I85: Stage 2, --200 years old Pieeaengelmannii 48 28 24 16 4 9 7 12 10 8 I0 7 2" 2 3 0 0 0 0 0 1 Abieslasiocarpa 119 34 31 18 11 6 12 9 2 3 5 0 1 1

Stand t89: Stage 3, ~350 years old Piceaengehnannii 42 5 4 3 3 7 9 4 1 5 3 5 4 5 4 5 2 2 2 4 I 2 1 0 0 2 Abieslasiocarpa 131 29 17 5 5 4 5 l 4 2 1 5 1 2 t t 0 2

Stand t 38: Stage 4, >500 years old Piceaengelmannii 49 28 12 tl 6 2 1 1 0 l 0 0 0 0 2 2 3 0 1 0 1 0 0 0 2 0 1 Abieslasiocarpa 236196 39 24 11 7 1 2 2 4 2 2 0 1 1 0 2 2 2 0 0 0 1 0 0 1

Stand 202: Blowdown, 24 years old (.05 ha) Piceaengelmannil 17 2 6 5 4 2 1 0 0 0 I Abieslasiocarpa 29 77 56 63 I2 9 5 2 2 0 1

44

the moisture gradient (bog forest and wet montane forest) are largely free from the otherwise all pervasive influence of fire. Wind, however, plays an important role here as a disturbance factor. For bog forests in particular, an open canopy and shallow rooting can lead to a high incidence of wind throw. The largest tree found in the bog forests had a diameter of only 44 cm, while in the more sheltered but closely related wet Picea, Abies forest, individuals with a diameter near 100 cm dbh were not uncommon. In the bog and montane Picea, Abies forests studied, diameter distributions were mostly near the steady-state, reversed-J shape, though often with irregularities suggestive of prior canopy disruption. In both stands 27 and 300 shown in Table 8, Picea appears to be retaining constant dominance relative to Abies.

Wet Picea, Abies forest is generally more productive than bog forest and often exhibits evi-

dence of either past or present even-aged structure. Stand 235 (Table 8) is a successional stand with apparent residual effects of disturbance, both in an extremely high basal area (84 m2/ha) and a somewhat bell-shaped diameter distribution of Picea. Such a stand can be expected to eventually undergo a dramatic decline in basal area and cover similar to that which characterizes Pinus contorta forests during death of the canopy dominants. Stand 226 represents a still older stage where only a small number of the original trees persist and where basal area has dropped to only 57.5 m2/ha. With approach to near steady-state conditions, average basal area drops still further to around 50 m2/ha (Table I0).

In mesic Picea, Abies forests the developmental pattern after fire is not unlike that in Pinus contorta forests. The first stage in forest development is establishment of Picea and Abies seedlings. Es-

Table 10. Picea, Abies forest developmental sequences.

Number of Basal Area Diversity 2 Stage Samples (m2/ha) Rel IV Picea ~ Rel IV Abies Herbaceous Total

Wet Picea, Abies forest l 2 9.8 27.6 72.4

(1.1) (29.7) (29.7) 2 2 73.9 66.9 33.1

(14.5) (27.6) (27.6) 3 5 51.9 65.8 34.2

(12.0) (10.7) (10.7) 4 5 49.7 59.5 40.5

(18.0) (8.1) (8.1) Mesic Picea, Abies forest 1 3 12.2 37.0 67.3

(1.0) (37.1) 38.3) 2 3 41.2 76.5 22.5

(7.9) (14.0) (13.8) 3 4 53.8 63.1 34.0

(15.5) (14.1) (12.8) 4 7 40.5 56.6 40.8

(8.2) (17.2) (17.5) Xeric Picea, Abies forest 1 2 12.2 60.4 39.

(7.2) (28.4) (28.4 2 6 29.7 52.1 36.9

(3.7) (17.8) (18.8 3 6 45.3 59.2 39.7

(10.6) (10.5) (13.0 4 3 37.5 56.1 32.5

(4.3) (21.4) (11.2

42.5 49.0 (13.4) (15.6) 20.5 24.5 (4.9) (4.9) 30.0 34.2 (8.9) (9.9) 28.4 34.8 (3.6) (3.8)

16.7 23.7 (3.5) (5.1) 5.0 10,3

(3.5) (5.8) 11.0 15.7 (5.5) (6.2) 20.7 28.7 (9.7) (12.2)

29,0 35.0 (1.4) (4.2) 17.0 22.3 (4.6) (5.2) 14.0 8.7 (5.2) (5.0) 10.7 15.7 (4.5) (3.0)

Standard deviations shown in parentheses t Relative 1V or importance value is the average of relative density of stems >7.5 cm dbh, and relative basal area, 2 Species per 0.1 ha.

tablishment rates are usually lower than in the Pinus contorta forests with the result that diameter distributions are broader and not infrequently negatively skewed (see Table 9, stands 199, 185). Here even-age typically means a 20 to 70 year period of establishment, a situation similar to that reported by Franklin & Hemstrom (1981) for Pseu- dotsuga in Oregon. Low elevation stands, however, were observed to have diameter distributions virtually identical to those found in the Pinus contorta - Abies, Picea forest suggesting rapid establishment rates under favorable environmental conditions.

Establishment rates also vary with seed supply. Day (1963) suggested that bursts of Picea and Abies regeneration frequently correspond to favorable years for seed production. If a site is adjacent to unburned Picea, Abies forest, the establishment rates are likely to be high (Horton, 1956, 1959). For example, in the present study a gradient was observed in Picea and Abies density on a 75-year- old burn with stands adjacent to relic old-growth having a high tree density and stands only 200 m away remaining very open.

During the second stage in stand development rapidly growing Picea dominate, having overtopped and competitively suppressed many of the contemporaneously established Abies. Stand 185 (Table 9) illustrates a bell-shaped diameter distribution typical of this stage and usually best developed between 200 and 250 yrs after fire. Cover and basal area are high but Abies has remained suppressed in the understory. With increasing age, basal area slowly increases toward a site-specific maximum near 55 m2/ha while tree density decreases. After around 350 yr, owing to continued mortality, the canopy starts to thin and regeneration resumes, thus marking the start of the third stage. With thinning and eventual loss of the original dominant individuals, both Abies and Picea approach all-size distributions. This pattern is consistent with the age structure reported by Whipple & Dix (1979) for Picea, Abies forests of the west slope.

In the absence of further disturbance, the greater reproductive success of Abies in the shaded understory favors its steady increase in dominance. The fourth stage, a second generation Abies forest, occurs only after in excess of 500 yrs have passed. Stand 138 (Table 9) is such a stand where Abies has greatly increased at the expense of Pieea. Basal areas of the two species are roughly equal in this

45

stand with Abies far ahead in tree density. It is reasonable to expect this trend to continue with the virtual elimination of Picea after perhaps three generations. Of course, the natural fire-cycle would normally prevent such a process from reaching conclusion. Even in the unlikely event that a small piece of forest managed to remain unburned for a millennium, adjacent forests would surely burn providing the necessary continued seed source for Picea to maintain at least a small role in the structure of the unburned forest.

Large blowdowns provide another, not uncommon, form of disturbance in Picea, Abies forests. Blowd owns give the large populations of suppressed A bies seedlings and saplings opportunity for release with the result that trees of Abies soon outnumber Picea (6 to 1 after 24 yrs of recovery for plot 202, Table 9). However, given additional time the tess numerous but faster growing Picea can be expected to overtop some of these Abies with the net result being a more equitable division of importance.

On xeric sites Picea and Abies share dominance with an occasional Pinus contorta or P. flexilis being present in peripheral stands. Here again successional patterns are variable. On the mesic edge of the series, stand development is similar to that described for mesie Pieea, Abies forest. At low elevations, pronounced bell-shaped diameter distributions often occur while at middle to high elevations and on the driest sites, such bell-shaped distributions are absent. Specifically, on environmentally extreme, high-elevation sites establishment rates are typically very low and few individuals are established in any one yr. Chronic reproductive failure has been reported previously for similar sites (Bollinger, 1973; Fonda & Bliss, 1969; Habeck & Mutch, 1973; Noble & Alexander, 1977; Stahelin, 1943).

Stand 305 (Table 8) depicts a forest known to have burned in excess of 85 yrs before sampling. Here density of Picea and Abies seedlings is low, the canopy is open; basal area is only 7 m2/ha. Yet, this stand has had a higher recovery rate than other portions of the same burn where basal area remains below 1 m2/ha after 85 yr. Of particular interest on these slow establishment sites is the formation of a reversed-J diameter distribution early in stand development with the competitive inhibition of regeneration typical of even-aged stands rarely occurring. In such stands basal area

46

and density only slowly increase with no evidence of the overshoot observed with even-aged development. Stand 198 (Table 8) illustrates an older stand on a slow establishment site with Picea still dominant. Given sufficient protection from fire it is likely that Abies will steadily increase in dominance, just as on the more mesic sites.

The effects of fire at or near timberline can be dramatic and long-lasting owing largely to the low establishment rates of tree seedlings. Several old burns in excess of 100 yr in' age were observed during the study on which very few trees had become reestablished. Bollinger (1973) studied establishment at the alpine-forest ecotone farther south along the Front Range and concluded that competition from herbaceous species had effectively halted tree establishment on many old burns. Fonda & Bliss (1969) reported a similar phenomenon in the Olympic Mountains of Washington. It appears likely that timberline in the Front Range is often the result of a dynamic equilibrium between fire and climate, rather than being a purely climatic phenomenon. Fire periodically removes the high elevation forest vegetation which returns, but only very slowly. Because of the high natural frequency of fire, it is probable that timberline rarely reaches its true climatic limit.

Following the initial stage of stand development, species richness drops dramatically in all three of the community types summarized in Table 10. This is consistent with the changes observed in Pinus contorta forests both in absolute drop, and in being attributable almost entirely to changes in the number of herbaceous species. The degree of recovery

with approach to steady-state conditions is less consistent. On the dryest types including xeric Picea, Abies forest and xeric Pinus contorta - Abies, Picea forest diversity appears to exhibit little recovery with approach to steady-state, while more mesic types including wet Picea, Abies forest, mesic Picea, A bies forest, and Pinus contorta forest show marked increases.

Pinus flexilis forests Early successional, post-fire stands of Pinus

flexilis - Picea, A bies forest are typically even-aged, and composed of either pure Pinus flexilis or a combination of Pinus and Picea. With increasing age Pinus assumes dominance, with Picea when present remaining suppressed in the lower portion of the canopy. With maturation and death of the initial Pinusflexilis population some 200 to 300 yr later, Picea is released to assume dominance. During this period regeneration of both Picea and Abies resumes. On the dryest sites few Picea and Abies are established before canopy breakup. In either case Picea and Abies eventually dominate with Pinus flexilis being retained mostly in small patches on particularly rocky substrate.

Diameter distributions for three developmental stages of Pinus flexilis forest are illustrated by stands 228,229, and 210 (Table 11). In these stands sufficient Pinusflexilis became established early in stand development to significantly inhibit subsequent seedling establishment, thus inducing typical bell-shaped frequency distributions. As in the other forest types described, the bell-shaped diameter distributions become damped with increasing age,

Table 11. Diameter distributions for an age sequence of Pinusflexilis Pieea, Abies forest stands.

Species (Inches) S 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Large1 (Centimeters) 5 I0 15 20 26 31 36 41 46 LargeJ

Stand 228 Pinus flexilis 12 21 26 23 13 3 3 2 1 Picea engelmannii 13 10 12 3 1 Abies lasiocarpa 2 4 1 1

Stand 229 Pinus flexilis 2 2 9 25 45 64 45 24 10 8 5 2 Picea engelmannii 8 I 2 3 2 3 1 1 0 1 1

Stand 210 Pinus flexilis 3 0 0 0 0 0 0 0 1 4 5 3 Abies lasiocarpa 49 47 20 19 5 12 6 4 1 4 1 2 Picea engelmannii 31 6 4 8 5 1 3 1 3 1 0 3

6 4 2 1 1 2

2 2 2 2 2 1

47

Table 12. Typical diameter distributions for Montane Pinusflexilis forest.

Species (Inches) S 0 l 2 3 4 5 6 7 8 9 10 11 I2 13 I4 15 16 17 18 19 20 (Centimeters) 5 10 15 20 26 31 36 41 46 51

Stand 118 Pinus flexilis 12 4 5 5 21 14 7 6 2 6 4 0 2 1

Stand 113 Pinus flexilis 30 8 6 12 1 3 1 t I 0 0 0 1 2

Stand 112 Pinusflexilis 24 I1 11 9 7 6 7 5 10 2 0 4 1 I 1

Stand 122 Pinus flexilis 8 15 3 9 6 6 6 9 5 7 4 4 1 2 2

Stand 97 Pinusflexilis 17 5 13 6 t l 7 8 8 9 7 9 2 3 8 1 Picea engelmannii 6 7 7 5 5 3 3 4 5 5 2 2 0 0 0 Abies lasiocarpa 18 8 9 I 3 3 0 1 2 0 1 1

!

3 0 1

l 0 0 0 t

eventually disappearing due to ingrowth and canopy tree mortality. The oldest of the three stands in Table 9 (210; N240 yr) has no Pinus regeneration, and reversed-J distributions are present for Picea and Abies. Basal area in such forests appears to increase steadily toward approx. 45 m2/ha (though stands of this type were recorded with over 60 m2/ha). Other stand data suggest a subsequent drop to a steady-state level of perhaps 35 to 40 m 2 with death of the initial cohort of Pinusflexilis,

Structural dynamics of the montane Pinusflexi- lis forest vary with substrate. On relatively deep-soil sites patterns similar to those of the Pinus contorta forest can be observed. Very few seedlings occur in stand 118 (Table 12) which has a regular, bell- shaped diameter distribution. On xeric, shallow- soil sites the pattern is different; post-fire regeneration is gradual and similar to that of the environmentally extreme xeric Picea, Abies forests.

An age sequence of four Pinusflexilis stahds is shown in Table 12. Stand 113 is an early (--70 yr), post-fire forest containing a few relic trees. Relic individuals are not uncommon on these sites, largely because the sparce vegetation on rocky soils does not carry ground fire particuNrly well. With continued stand development (e.g., stand 112) basal area and mean diameter increase as does density of mature trees. The diameter sequence of stand 112, has a bulge reflecting the nearly synchronous initial establishment and subsequent overcrowding. These trends are continued in stand 122, and culminate with stand 97 where the diameter sequence is approaching the standard reverse-J shape. Here the

number of seedlings per hectare has increased, in part due to the presence of Picea and Abies in the understory. There is little evidence that Picea or Abies will replace Pinus Jlexilis, but they may eventually share dominance on more mesic sites. On the more xeric sites most Picea and Abies stems die before reaching 12 cm dbh. In the final stage basal area increases to what appears to be a steady- state level between 35 and 40 m2/ha.

Unlike Pinus contorta forests, Pinus ftexitis forests rarely show an extreme overshoot in basal area or cover. Rather, initial establishment is slow due to severe environmental conditions, thus reducing the likelihood that a large initial cohort will severely reduce seedling and sapling growth. Seed- lings become established slowly, reaching peak density only after the canopy has started to close. Thereafter basal area continues to increase with a noticeable but minor decrease in seedling density resulting. With maturation and natural thinning of canopy trees, Pinus flexilis regeneration attains steady-state levels.

Pinus ponderosa woodlands The types of size structure encountered in foothill

woodland communities are markedly different from those of higher elevation forests. First, negative exponential or reverse-Jdiameter distributions typical of supposed steady-state populations are rarely encountered. Second, the bell-shaped distribution type, usually resulting from an early burst of regeneration following disturbance with subsequent competitive suppression of regeneration, is also

48

Table 13. Typical diameter distributions for Pinus ponderosa woodlands

Species (Inches) S 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Larger (Centimeters) 5 10 15 20 26 31 36 41 46 51 56 61 66 Larger

Stand155, Pinusponderosashrubland(0.2ha) Pinusponderosa 0 0 1 l 0 3 l 3 2 3 0 l 0 0 0 l 0 l 0 0 l

Standl05, Mesicfoothillwoodland(0.1ha) Pinusponderosa 2 2 2 6 4 7 3 1 5 0 0 0 2 3 1 1 0 1 0 0 1 0 1 Pseudotsuga 1 2 2 0 1 2 2 0 1 0 0 0 0 0 0 1 0 0 1

Stand177, Xeric~othillwoodland(0.2ha) Pinusponderosa 0 0 0. 2 0 2 3 0 2 1 1 0 0 0 1 1 3 2 0 1 0 0 0

Stand269, Xericmontanewoodland(0.2ha) Pinusponderosa 8 3 2 1 1 2 1 1 1 2 1 1 1 1 2 2 1 2 1 0 1 0 1

Standl71,Xeric~othillwoodland(0.2ha) Pinusponderosa 7 2 2 6 2 0 3 0 1 1 0 2 1 1 0 0 0 l 1 1 1 1 1

Stand3, Mesicmontanewoodland(0.1ha) Pinusponderosa ? 0 0 0 0 1 2 1 0 0 0 1 2 0 1 3 0 2 l Pseudotsuga ? 5 1 3 9 2 3 6 6 4 0 0 2

1 0 0 0 0

2 1 2

uncommon. Instead, patterns largely restricted to woodland vegetation are dominant . Often a mixture of tree sizes is encountered, but with several bulges in the diameter distribution, suggesting irregular episodes of successful regeneration (Table

13, stands 155,105, 177). On some sites uniform size distributions are encountered such as illustrated by stand 269 (Table 13). Stand 171 represents a more typical intermediate case.

Examinat ion of age structure for two representative woodland stands confirmed the correspondence between periodic establishment and groups of similar diameter. The only major result not evident f rom the diameter distributions was that establishment periods were much shorter than indicated by the broad humps in the diameter distribution. Such humps can, in fact, consist of several bursts of regeneration. For this reason only stands with long periods between establishment episodes (>50 yr) show a clear pulsed diameter structure. Stands with frequent establishment episodes usually exhibit more even diameter distributions (e.g., stand 271).

Pulsed regeneration appears most characteristic of the least favorable of the low-elevation sites, the Pinusponderosa shrublands and the mesic foothill woodlands which together form the transition to grassland. On these chronically drought-stressed sites with limited potential for tree growth and establishment, it is likely that only in an unusual year will both weather and seed product ion be appropr ia te for abundan t tree regeneration. In

Arizona similar episodic establishment has been shown to correlate with years climatically favorable for both establishment and seed product ion (Cooper , 1960; Schubert , 1974). Hoffman &

Alexander (1976) suggested episodic establishment for two widespread, low elevation Pinusponderosa woodland types of the Bighorn Mountains of northern Wyoming.

Disturbance factors provide an alternative expla- nation for periodic establishment. Woodland communities with open, grassy understories doubtless burned regularly before settlement. Many trees still show multiple firescars as evidence of repeated burning. The irregular frequency of these fires un- doubtedly influenced seedling establishment. Dau- benmire (1943) reported distinct age groups of saplings corresponding to series of consecutive sum- mers without fire during which seedlings had time to grow into fire resistant saplings. However, obser-

vations of both unburned and recently burned stands in the Front Range revealed no bursts of regeneration on either type. In addition, there was no evidence of a change in regeneration dating f rom the drastic increase and subsequent decrease in fire frequency which occurred in the early part of the present century. Such release has been documented in neighboring regions (e.g., Cooper, 1960, 1961; Marr, 1961; Weaver, 1959) and can be found in the higher elevation Pinus ponderosa forests of the study area. While fire may increase the interval between regenerat ion events, it can not be considered the sole factor responsible for episodic

49

Table 14. Typical diameter distributions for Pinus ponderosa, Pseudotsuga forests.

Species (lnches) S 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 I6 17 18 19 20 2I 22 (Centimeters) 5 10 15 20 26 31 36 41 46 5 t 56

Stand 139, Foothill ravine forest Pseudotsuga 14 5 3 4 2 1 0 1 1 4 5 3 2 2 1 0 2 Pinus ponderosa I 1 5 4 1 5 1 1 1 1 3 2 0 ! 0 0 0

Stand 161, Foothill ravine forest Pseudotsuga 14 12 26 30 29 33 22 23 6 4 0 2 Pinus ponderosa 8 I5 12 l0 3 4 4 7 2 2 3 5 3

Stand 223, Foothill Pseudotsuga, Pinus ponderosa forest Pseudotsuga 11 6 5 6 4 4 3 7 5 1 2 0 0 1 0 2 0 Pinus ponderosa 1 0 0 0 1 0 1 2 2 1 0 ! 1 ! 2

Stand 48, Foothill Pseudotsuga, Pinus ponderosa forest Pseudotsuga 15 t 3 3 3 3 4 2 9 12 16 15 13 9 4 3 2 Pinus ponderosa 0 0 0 0 0 1 0 0 6 2 2 2 4 3 1

l 0 1 1 0 0 l

regeneration in Front Range woodland communities.

On some sites all tree size-classes have roughly equivalent representation (Table 13, stand 269). This pattern is difficult to explain based on short- term observations, but most likely very old, stable stands are involved, with little mortality between achievement of sapling size (>5 cm diameter) and attainment of a diameter in excess of 50 cm. A small but consistent decrease in radial growth with diameter could reconcile the inevitable death of some intermediate aged individuals with the retention of the flat diameter distribution. Alternatively, a series of frequent and regularly spaced regeneration pulses could produce these distributions. It is not uncommon for competition from grasses to limit Pinus ponderosa regeneration to a limited number of micro sites found next to rocks and in other gaps in the grass sod. If availability of micro sites rather than competition or climate were the critical factor limiting Pinus establishment, recruitment rates could be expected 'to be closely coupled to mortality, thus generating the observed regular or flat diameter distribution.

The mesic montane woodland is the only community-type in the Pinus ponderosa woodland series in which the impact of inter-tree competition is evident. Here bell-shaped diameter distributions resulting from competitive inhibition of regeneration can sometimes be seen. For example, stand 3 (Table 13) has a Pinus ponderosa, Pseudotsuga canopy with a dense cohort of smaller Pseudotsuga which probably became established after cessation

of periodic ground fires. In aU of the other woodland community-types the overriding factor con- trolling size structure appears to be establishment as influenced by site and climatic variables.

Pinus ponderosa, Pseudotsugafi)rests Forests of the Pinus ponderosa, Pseudotsuga

series are transitional between Pinus ponderosa woodlands and Pinus contorta forests. Not surpris- ingly, characteristics of both groups can be seen in their development.

Two edaphic phases of most Pinus ponderosa, Pseudotsuga forests can be recognized. Fine-textured soils support relatively open forests, similar to the lower elevation woodlands. Here the well- established sod of grasses effectively precludes most tree establishment. In contrast, rocky, coarse-textured soils generally support dense standsr usually with bell-shaped diameter distributions indicative of prior disturbance.

Pinus ponderosa and Pseudotsuga share dominance on open-phase sites, tree densities ranging between 150 and 600 per ha. Diameter distributions are not unlike those of lower elevation woodland types with bulges resulting from episodes of high establishment, though diameters appear more evenly distributed (e.g., Table 14; stands I39, 223). Competition-induced natural thinning is relatively unimportant on these sites and mortality is low except during the establishment period.

The dense-phase sites usually support even-aged stands of Pseudotsuga and Pinus ponderosa with densities ranging up to 2 000 trees/ha in the

50

younger, 50-100 yr old stands. Pseudotsuga is usually dominant in the foothill ravine forests but on drier sites the two species often codominate. Both the bell-shaped diameter distributions (e.g., Table 14; stands 48, 161) and the frequent occurrence of charcoal are suggestive of a post-fire origin for most of these stands. Not infrequently stands will contain a few large Pinus ponderosa or Pseu- dotsuga as relics from a previous stand.

None of the dense-phase stands encountered were old enough to support the eventual steady- state structure. Natural thinning had not reduced tree densities to the point where regeneration was possible. While the eventual composition of the steady-state is uncertain, the dominance of the smaller size-classes by Pseudotsuga suggests it to be increasing in importance. Given sufficient time without fire, these stands will most likely develop into more open, all-aged forests of reduced basal area and with minor persistence of Pinus ponderosa. If the present fire suppression activities continue on lower elevation sites, such steady-state forests should become increasingly prevalent.

Mesic montane forests Of all Front Range forest types, conditions

appear most favorable for tree growth in the mesic montane forests with the result that after disturbance even-aged stands develop rapidly. As in the Pinus contorta forests, an overshoot in basal area and cover occurs with a subsequent decline due to inhibition of tree regeneration. The major distin- guishing characteristic of these forests is the high diversity of tree species. After fire, when competitive pressures are low, several tree species usually become established. Abies lasiocarpa, Pi- nus contorta, Picea engelmannii, Picea pungens, Pinus ponderosa, Populus tremuloides, Pseudo- tsuga and Populus angustifolia can all be important in first generation forests. This pattern of high initial diversity is strikingly similar to that observed for post-fire forests on favorable sites in the Lake States by Goffand colleagues (Goff& Zedler, 1968; Auclair & Goff, 1971). The eventual steady-state forest is usually dominated by some combination of Abies lasiocarpa, Pseudotsuga and to a lesser extent Picea engelmannii and Picea pungens.

Variation in forest development

Forest structural development and the dynamics

of tree populations are not uniform throughout the study area, or even within a species or community type. Rather, they vary in response to environment and chance historical events. To understand these forests it is necessary to understand not only general patterns of development but also the variation in the patterns.

Although dominated almost exclusively by two species, the Picea engelmannii, Abies lasiocarpa forests exhibit considerable structural variation relative to both site factors and successional development. Failure to recognize this variability has led to considerable confusion in the literature. In particular, the compositional and structural stability of the Picea, Abies forests has been variously interpreted. Working in the Medicine Bow Moun- tains of southern Wyoming, Oosting & Reed (1952) reported density ratios for Abies over Picea of seven to one in the seedling stratum and four to one in the transgressives, despite a dominance of Picea in the overstory. While similar shifts in dominance from overstory Picea to understory Abies have frequently been reported (e.g., Alexander, 1974; Hansen, 1940; Hobson & Foster, 1910; Langen- heim, 1962; Loope & Gruell, 1973; Marr, 1961; Miller, 1970; Oosting & Reed, 1952; Schmid & Hinds, 1974), their significance is not obvious. Marr (1961), Amundsen (1967) and Oosting & Reed (1952) have all suggested that most Picea, Abies forests are in a steady-state condition. Amundsen, however, allowed that periodic fires occur, while Marr maintained that the forests have almost always been wet enough to inhibit fire. Numerous workers in similar forests have suggested that Picea will gradually be replaced by Abies (Bloomberg, 1950; Daubenmire & Daubenmire, 1968; Hansen, 1940; Horton, 1956; Loope & Gruell, 1973; Moss, 1953). In contrast, Alexander (1974), Miller (1970), and Schmid &.Hinds (1974) expect Picea to increase in dominance because of greater longevity. Fox (1977) has argued that the climax Picea, /tbies forests of the Medicine Bow Moun- tains have a cyclic form of stability with Picea being replaced by Abies which is in turn replaced by Picea. A similar cyclic alternation of species was proposed by Schmid & Hinds (1974), but they argued that insect-induced mortality rather than biological interactions of the two species was the driving force.

While none of the previously described studies

suggested forest structure to be site dependent, examination across elevation and so~l moisture gradients reveals marked variation. Picea appears codominant at steady-state on wet sites, whereas on mesic and dry-mesic sites the species is gradually replaced by Abies. The lower elevation Picea, Abies forests develop even-aged stands after fire with in excess of 500 yr required for the demise of the original cohort. On high elevation sites, even-aged forest structure is largely absent, owing mostly to the low rate of reestablishment after fire.

Marr (1961) and Douglas (1954) have suggested that Pinusflexil is forests are not self-maintaining but are successional to Picea and Abies. Working within the southern portion of the study area, Amundsen (1967) described Pinus flexilis stands which he interpreted to be successional to Picea and Abies. In the present study both successional and compositionally stable Pinus flexilis types were identified, with the successional stands found on somewhat more mesic sites. Within the stable Pinus

flexilis stands, structural development varied from, on the mesie sites, typical even-aged development as described for the Pinus contorta forests to, on the most xeric sites, the low establishment, gradual approach to steady-state pattern observed for high- elevation xeric Picea, Abies forests.

Most Front Range Pinus contorta forests are of post-fire origin and with sufficient time are replaced by Pseudotsuga or Picea and Abies (Clements, 1910; Langenheim, 1962; Marr, 1961; Mason, 1915; Moir, 1969). Amundsen (1967), Moir (1969) and Whipple & Dix (1979), taking exception with earlier workers, suggested the existence of self-maintaining Pinus eontorta forests at intermediate elevations. Moir's conclusion was based on the lack of reproduction by other species in a series of 60 to 100 yr old stands. However, as Pinus contorta was not reproducing eitfier, and Pinus contorta stands in the 60 to 100 yr class are well known for being too dense for significant regeneration (Alexander, 1974; Clements, 1910; Mason, 1915), conclusive evidence was lacking. Amundsen (1967) provided less data for his three alleged stable Pinus contorta stands, but it is evident that virtually no Pinus contorta seedlings were present. While numerous saplings were present, these were most likely suppressed individuals, virtually the same age as the canopy trees. Whipple & Dix (1979), however, presented convincing tree age data supporting the occurrence

51

of climax Pinus contorta forests in the Fraser Experimental Forest southwest of the study area. Recognition of several Pinus contorta types with differing successional and structural characters can clarify this apparent ambiguity. While most Pinus contorta forests are successional to either Pseu- dotsuga or Picea and Abies, there is a limited range of site conditions over which Pinus contorta appears able to form climax stands. Evidence for steady-state Pinus contorta in the study area takes the form of reversed-J diameter distributions in older stands (>250 yr) which suggest continuing regeneration. In addition tree age data are consistent with the interpretation of continuing regeneration. Such stable Pinus contorta stands have also been reported from the central Rocky Moun- tains of Montana and Wyoming (Despain, 1973; Pfister & Daubenmire, 1975; Reed, 1976).

A gradient model o f forest development

It is useful to recognize three primary types of forest development within Front Range forests, remembering that these refer only to arbitrary points in a continuous field of potential variation. These types can be referred to as characteristic of 1) favorable sites, found at middle elevations near the center or on the mesic side of the moisture gradient, 2) severe sites, found at highest elevations or on very dry sites at middle elevations, and 3) episodic sites, found at lower elevations along the transition from forest to grassland. Several charaeteristics of the first two types are illustrated in Fig. 9.

The forest development sequence for favorable sites resembles in part those proposed for northern Rocky Mountain forests by Daubenmire & Dau- benmire (1968) and Day (1972). These workers divided the developmental process into four stages. The first stage is that of initial invasion, during which most species which occur in the region can become established, including the potential climax species. The second stage consists of a dense forest with stagnated growth, dominated by initially rapid- growing, seral species such as Pinus contorta. At this stage the climax species, which became established during the initial invasion phase, are present in the understory, though they are largely suppressed and overtopped by the seral species. During the third phase trees of the seral species die and are replaced by the suppressed individuals of the cli-

52

F a v o r a b l e Si te A

z : ' \ ."< ........ "',, I : : % , " \ ...... \ / . . " - " ' . . . . " . ~

/ - , , 'x ,, ....... 2 x - j ~ ~ > - - ~ / ,, \ x " ' " / / \ . . 2 Y _ .... . . " - - - . _ 2 . . . . . . . . . . . .

,'~,"" 7, TIME

U n f a v o r a b l e S i te B

~.~-~ ~ . , - . ~ ................................. ........:.........._.

~ ; ~ " - ' - \ , TIM E

. . . . . . . . Biomoss . . . . . . . Species Richness

................... Production - - E s t o N i s h m e n t R o t e

Fig. 9. Generalized development patterns for forests of the Colorado Front Range. The favorable site (A) is typical of Pinus contorta, Pseudotsuga menziesii, and Picea engehTmnnii dominated forests. Time scales vary with site, the secondary low in biomass occurring around 275 years for Pinus contorta and around 450 yrs for Picea engehnannii. ]'he unfavorable or severe site (B) is typical of extreme high elevation Picea, Abies forests and xeric PinusJlexilis forests (modified from Peet 1978a).

max species. Late in the third phase the canopy opens sufficiently to allow seedling establishment. The final phase is the true climax or steady-state forest wherein the last of the relics have died and a true all-aged forest has developed. Other students of Rocky Mountain and adjacent forests have reported similar patterns (e.g. Bloomberg, 1950; Horton, 1956; Moss, 1953; Raup, 1946). These stages correspond closely to those recognized in Table 10 for Picea - Abies stands as well as the stages recognized in Tables 4 and 6, except that in the latter two the second stage has been divided into two for a total of five stages.

While the Dauhenmire-Day scheme works well for environmentally favorable sites in the present study area, several additional observations can be made. Toward the end of the first stage, growth of understory species becomes strongly inhibited with diversity dropping to a minimum early in stage 2. On many sites diversity increases again late in stage 3 or early in stage 4 when the canopy is more open and competitive pressures are reduced. Basal area increases well into the third stage, despite natural thinning and decreasing productivity (wood vol- ume increment). With the approach of late stage 3, the remaining trees become increasingly susceptible to fungal infection, insect attack and wind- throw. This marks a period of rapidly declining basal area (and biomass) as the large trees die. At the same time seedling establishment rapidly in-

creases along with annual basal area increment, owing to reduced competitive pressures. Whether the cycle repeats depends largely on the synchrony of mortality and the resulting dispersion of sizes of the newly established trees. If mortality is concentrated during a short period, perhaps a decade or two, a new though damped cycle can be expected. Episodes of severe pine beetle (Dendrocto- nus ponderosae) infestation have been reported to result in two or three different cohorts of Pinus contorta on the same site. The increasing suscep- tibility of Pinus to beetle attack with increasing age can yield a short, 30-40 yr cycle of establishment (Amman, 1977). In contrast, if mortality is gradual, biomass, productivity, diversity and establishment are likely to asymptotically approach steady-state levels. This would be accompanied by development of a steady-state (reversed-J) diameter distribution. Forests dominated by Pinus contorta, Pinus ponderosa, Pinus flexilis, Pseudotsuga, Populus tremuloides, Picea engelmannii and Abies lasiocarpa all exhibit this type of stand development under appropriate site conditions.

A frequent observation is that the rate of post- fire seedling establishment is lower at or near treeline than at lower elevations (e.g., Bollinger, 1973; Bunin, 1975; Habeck & Mutch, 1973; Noble & Alexander, 1977; Stahelin, 1943). Stahelin (1943) in particular noted that where grasses and forbs dominate early in the sequence, establishment rates

for tree speies are greatly reduced. Environmentally severe sites are often characterized by such low establishment rates (Fig. 9). Tree seedlings become established so infrequently that the canopy can only very gradually develop to the point where seedling establishment is inhibited by competition from canopy trees. In certain cases establishment is actually enhanced by tree growth owing to the sheltered conditions in the understory. Gradual reestablishment on these sites is paralleled by a gradual increase toward steady-state levels of basal area, annual basal area increment, and diversity. The dramatic oscillations observed on the favorable sites and induced by near synchronous establishment are missing. The size structure approxi- mates the reversed-J shape throughout the developmental process. Diversity typically peaks early in the sequence and then drops gradually to steady- state levels.

Episodic-site forests are similar to those of severe sites in their low seedling establishment rates. They differ in that seedling establishment is episodic and the forests are rarely devastated by fire or other disturbance factors. Rather, fire is a frequent, but low intensity phenomenon which keeps the vegetation open and park~like. Thus, temporal variation in forest structure is almost entirely attributable to establishment events; only during unusually favorable years do significant numbers of seedlings enter the tree population. The resultant size structure is commonly one with bulges resulting from periods of successful regeneration.

The three forms of stand development identified provide reference points in a continuum of structural variation which can be used as a framework for interpreting numerous aspects of forest ecology. Diversity, basal area, biomass, productivity and stability all can be seen as responding to the dynamics of the dominant tree populations, which in turn are responding to local environmental conditions. The implications for future studies should be clear. Investigations of most community and ecosystem properties including diversity, biomass, and productivity need to be conducted within a multidimensional framework including both site condition and stand development. Simi- larly, studies of population change during succession need to be placed in an environmental context. Even within an area as restricted as Rocky Moun- tain National Park, dramatically different patterns

53

can be found for population development under various site conditions. Because of the quantity of data required for studies of this kind, most investigators have failed to relate their results to site or successional variation (Peet, 1981). Of the few studies which have examined biomass or production relative to site factors, none have included a successional component. If future studies of communities and ecosystems are conducted so as to document variation over a range of site and successional conditions, the overall interpretability of the work will be greatly enhanced.

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57

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58

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Accepted 13.10.1980.

Appendix

Community summary tables

Each of the following eight tables summarizes the composition of one community series as described in detail in the text. The series as a whole is summarized first followed by summaries of the component community-types. Summaries are for understory vegetation defined as leaf area below one m in height, Species are separated into three growth forms: trees, shrubs and herbs.

Composition is summarized in terms of prevalent species. The number of prevalent species is equal to the average number of species per stand (0.1 ha) of that type. Prevalent species are those species with highest constancies (percentage of stands where present). The prevalent species of the series as a whole are listed first. These are followed by species which are prevalent in one or more of the community-types, but not in the series as a whole. Constancy values are given for each prevalent species. Fre- quency values (25 0.5 X 2 m subplots) are also given based on those 0. l ha sampling units in which a species was present.

In addition to prevalent species, modal species are indicated. Modal species are those species with their highest constancy in the community type or series under consideration. Each species is modal in one series and one type. Ties were broken on the basis of average cover. Modal species are designated by a line under the constancy value. When a species is prevalent in one type in the series, and modal but not prevalent elsewhere, frequency is omitted from that community list in which the species is not prevalent. Those species which are modal but never prevalent are not listed because of space limitations. An expanded set of summary tables listing all modal species and other supplemental information is available from the author upon request.

In addition to the species lists, a number of summary statistics are presented for each community. These include the number of stands included in the group, Curtis" (1959) index of homotoneity (average constancy of prevalents), average species number, average understory cover (below one m in height), and Hill's (1973) first (1/Simpson's Index) and second (Exp[H']) diversity statistics (see Peer, 1974) calculated for each stand using importance values equal to the average of relative frequency and relative cover.

Com

mun

ity

tabl

e I.

Pin

us p

on

der

osa

woo

dlan

d se

ries

i Pin

us p

onde

rosa

M

esic

Foo

thil

l X

eric

Foo

thil

l M

esic

Mon

tane

X

eric

Mo

nta

ne

Ser

ies

A

Sh

rub

lan

d (

AI)

W

oo

dla

nd

(A

2)

Wo

od

lan

d (

A3)

W

oo

dla

nd

(A

4)

Wo

od

lan

d (

A5)

Num

ber

of s

tand

s 35

9

6 4

6 10

H

omot

enei

ty

.529

,5

80

.645

.6

90

.652

.6

40

Div

ersi

ty:

EX

P(H

')

17.2

16

.6

17.9

16

.6

20.0

15

,8

Div

ersi

ty:

1/h

11.6

10

.6

13.3

11

,3

12.9

10

.7

Spe

cies

per

sta

nd

42.6

44

,8

44.7

40

.3

45.0

38

.8

Und

erst

ory

cove

r %

44

.9

64.0

28

.2

33.8

38

.0

46.4

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

TR

EE

S (

Bel

ow I

m)

Seri

es P

reva

lent

s P

inus

pon

dero

sa

7,1

48.6

4.

0 44

.4

11,2

83

.3

6.0

50.0

4.

0 50

.0

8.0

Pse

udot

suga

men

zies

ii

6.5

31.4

5.

6 83

.3

10.7

50

.0

Add

itio

nal

Pre

vale

nts

Juni

peru

s sc

opul

orum

4.

0 50

.0

Bet

ula

occi

dent

alis

0.

0 25

.0

SH

RU

BS

Se

ries

Pre

vale

nts

Rib

es c

ereu

m

12.5

85

.7

4.0

55.6

12

.0

100.

0 15

.0

100.

0 14

.7

100.

0 15

.1

Pur

shia

tri

dent

ata

22.1

80

.0

20,8

55

.6

20.0

10

0.0

5.3

75.0

17

.6

83.3

32

.4

Rub

us d

elic

iosu

s 3,

6 54

.3

3.2

83.3

7.

0 10

0.0

2.4

Opu

ntia

pol

yaca

ntha

9.

5 51

.4

14,0

88

.9

1.0

66,7

14

.0

50.0

7.

0 R

hus

tril

obat

a 18

.0

34.3

30

.8

77.8

0.

0 66

.7

Yuc

ca g

lauc

a 4.

0 34

.3

4•9

100.

0 1.

3 50

.0

Ros

a sp

. 8.

7 31

.4

4.0

50.0

18

,7

50.0

5.

0

Add

itio

nal

Pre

vale

nts

Cer

coca

rpus

mo

nta

nu

s 25

.7

32.6

77

.8

36.0

50

.0

Ech

inoc

ereu

s vi

ridi

flor

us

11.4

2.

0 44

,4

Jam

esia

am

eric

ana

6.0

66.7

M

ahon

ia r

epen

s 25

.3

50.0

P

runu

s vi

rgin

iana

1.

3 50

.0

8.0

50.0

0.

0 Ju

nipe

rus

com

mu

nis

5.

0 66

.7

Arc

tost

aphy

los

uva-

ursi

6.

7 50

.0

Phy

soca

rpus

mo

no

gy

nu

s 6.

7 50

.0

Sym

phor

icar

pos

oreo

phil

us

5.3

50.0

HE

RB

S

Seri

es P

reva

lent

s A

rtem

isia

lud

ovic

iana

49

.5

94.3

40

.5

88.9

46

.7

100.

0 38

.0

I00.

0 55

.3

10Q

,O

60,4

A

rtem

isia

fri

gida

35

.7

94.3

36

.8

100.

0 28

.7

100.

0 45

.0

100.

0 21

.6

83.3

43

.1

Car

ex r

ossi

i 36

.0

80.0

16

.8

55.6

47

.3

100.

0 22

.0

100.

0 41

.6

83.3

43

.0

Con

st.

30.0

90.0

90

.0

50.0

40

.0

40,0

40.0

90.0

90

.0

80.0

o~

Co

mm

un

ity

tab

le 1

(co

nt.)

Fre

q.

Con

st.

Fre

q.

Co

nst

. F

req.

C

onst

. F

req.

C

onst

, F

req.

C

onst

. F

req.

C

onst

.

Ger

aniu

m f

rem

onti

i 19

.3

Pen

stem

on v

iren

s 11

.4

Chr

ysop

sis

vill

osa

18.8

P

oten

till

a fi

ssa

12.5

E

riog

onum

um

bell

atum

16

.0

Muh

lenb

ergi

a m

on

tan

a 49

.0

Sti

pa c

omat

a 27

.2

Scu

tell

aria

bri

tton

ii

7.2

Hel

iant

hus

pum

ilus

19

.0

Bro

mus

lan

atip

es

7.4

Sit

anio

n lo

ngif

oliu

m

8.8

Bro

mus

tec

toru

m

42.0

H

arbo

uria

tra

chyp

leur

a 9.

5 A

grop

yron

alb

ican

s 25

.4

Poa

fen

dler

iana

11

.2

Cys

topt

eris

fra

gili

s 5.

5 C

rypt

anth

a vi

rgat

a 6.

0 A

rabi

s d

rum

mo

nd

ii

5.3

Koe

leri

a gr

acil

is

17,6

F

estu

ca s

axim

on

tan

a 13

.0

Ant

enna

ria

parv

ifol

ia

11.2

G

rind

elia

sub

alpi

na

11.2

L

euco

poa

king

ii

32.9

A

stra

galu

s fl

exuo

sus

15.1

S

edum

lan

ceol

atum

22

.3

Pul

sati

lla

pate

ns

16.6

P

hace

lia

hete

roph

ylla

2.

3 S

enec

io f

endl

eri

14.8

A

chil

lea

lanu

losa

7.

4 A

lliu

m c

ernu

um

7.0

Agr

opyr

on t

rach

ycau

lum

24

.6

Cam

pan

ula

rot

undi

foli

a 10

.3

Ad

dit

ion

al

Pre

vale

nts

Bou

telo

ua h

irsu

ta

Cir

sium

un

du

latu

m

Spo

robo

lus

cryp

tand

rus

Ver

basc

um t

haps

us

Bou

telo

ua g

raci

lis

Gut

ierr

ezia

sar

othr

ae

Bou

telo

ua c

urti

pend

ula

~Unk

now

n (d

icot

)

80.0

7.

2 55

.6

12.8

83

.3

24.0

75

.0

26.7

10

0.0

23.1

90

.0

80.0

13

.3

100.

0 10

.0

100.

0 8.

0 83

.3

14.0

10

0.0

71.4

21

.3

66.7

17

.6

83.3

4.

0 75

.0

36.7

50

.0

16.0

80

.0

68.6

8.

0 83

.3

23.0

10

0.0

22.4

83

.3

7.0

80.0

68

.6

16.6

66

.7

20.0

66

.7

16.0

50

.0

17.3

90

.0

65.7

23

.0

66.7

45

.0

100,

0 41

,6

83,3

70

.0

80.0

60

.0

23.5

10

0.0

12.0

33

.3

30.0

33

.3

37.1

70

.0

60.0

10

.0

100.

0 6.

0 10

0.0

5.7

70.0

57

.1

14.6

10

0.0

26.0

50

.0

18.0

33

.3

26.0

60

.0

57.1

5.

6 55

.6

7.0

66.7

10

.0

100.

0 5.

3 50

.0

9~0

40.0

54

.3

14.7

33

.3

16.0

83

.3

5.0

80.0

51

.4

65.5

88

.9

16.0

33

.3

14.0

50

.0

34.4

50

.0

51.4

8.

8 83

,~

12.0

50

.0

9.3

60.0

48

.6

.43.

0 44

.4

16.0

50

.0

8.8

83.3

34

.4

50.0

.4

5.7

8.0

50.0

6.

0 50

.0

12.8

83

.3

12.8

50

.0

45.7

6.

0 66

.7

8.0

100.

0 45

.7

6.7

50.0

8.

6 70

.0

45.7

9.

3 50

.0

5.8

90.0

42

,9

4.0

33.3

16

.7

100.

0 29

.6

50.0

16

.0

33.3

12

.0

100.

0 13

.3

60.0

42

.9

5.3

50.0

16

.7

100.

0 10

.4

50.0

42

.9

9.7

77.8

21

.3

50.0

6.

0 50

.0

40.0

32

.0

83.3

6.

0 50

.0

64.0

66

.7

40.0

8.

0 44

.4

8.0

33.3

23

.4

70.0

40

.0

6.7

50.0

40

.0

83.3

18

.4

50.0

40

.0

9.6

83.3

48

.0

50.0

40

.0

1.0

44.4

2.

0 50

.0

4.0

50.0

37

.1

8.0

83.3

23

.0

66.7

37

.1

8.0

100.

0 8.

8 50

.0

37.1

14

.7

50.0

4.

0 50

.0

6.0

60.0

34

.3

32.0

33

.3

30.0

50

.0

24.8

50

.0

34.3

13

.3

33.3

18

.0

66.7

1.

0 66

.7

28.6

14

.0

66.7

4.

0 50

.0

31.4

9.

3 66

.7

4.0

50.0

20

.0

6.7

66.7

31

.4

4.0

66.7

0.

0 50

.0

25.7

11

.2

55.6

14

.3

12.0

55

.6

14.3

: 7.

2 55

.6

6,4

55.6

Com

mun

ity

tabl

e 1

(con

t.)

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st,

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st.

Unk

now

n 2

(dic

ot)

Cer

asti

um a

rven

se

Am

bros

ia p

silo

stac

hya

Tra

desc

anti

a oc

cide

ntal

is

Ery

sim

um a

sper

um

Tra

gopo

gon

dubi

us

An

dro

po

go

n s

copa

rius

G

ram

inea

e sp

. A

nd

rop

og

on

ger

ardi

i C

hrys

otha

mnu

s vi

scid

iflo

rus

Gal

ium

apa

rine

M

erte

nsia

vir

idis

S

olid

ago

cana

dens

is

Par

onyc

hia

jam

esii

W

oods

ia o

rega

na

Sela

gine

lla m

utic

a L

athy

rus

leuc

anth

us

Sela

gine

lla u

nder

woo

dii

Eri

ogon

um a

latu

m

Par

ieta

ria

pens

ylva

nica

M

imul

us g

labr

atus

B

rick

ellia

gra

ndif

lora

C

hert

opod

ium

tept

ophy

llum

A

ndro

sace

sep

tent

rion

alis

E

upho

rbia

rob

usta

H

acke

lia

flor

ibun

da

Cry

ptan

tha

virg

ata

Sela

gine

lla d

ensa

E

rige

ron

com

posi

tus

The

rmop

sis

diva

rica

rpa

Ant

enna

ria

rose

a P

oa p

rate

nsis

S

olid

ago

mis

sour

iens

is

Pot

enti

lla

hipp

iana

S

olid

ago

spat

hula

ta

Lit

hosp

erm

um m

ulti

flor

um

Car

ex f

oene

a C

onyz

a ca

nade

nsis

14.3

25

~7

11.4

17

.1

31.4

28

.6

8.6

8.6

17.1

11

.4

25.7

11

.4

34.3

25.7

17

.1

8.6

31.4

31.4

11

.4

22.9

14.3

25.7

3.2

19.0

15

.0

14.0

11

.0

7.0

45.3

20

.0

8.0

10.7

10

.7

12.0

55.6

44

.4

44.4

44

.4

44.4

44

.4

33.3

33

.3

33.3

33

.3

33.3

33

.3

36.0

13.0

5.

3 4.

0 2.

7 16

.0

28.0

4.

0

6.7

33.3

66.7

.~b

.o 50

.0

50.0

33

.3

33.3

33

.3

50.0

4.0

20.0

0.0

10.7

9.

3 8.

0 16

.0

6.0

4.0

2.0

0.0

2.0

75.0

100.

0

100.

0 ~5

70

75.0

75

.0

50,0

50

.0

50.0

50

.0

50.0

50.0

26.0

11

.0

4.0

22.7

4.

0 13

.3

10.7

8.

0 6.

6 16

.0

66.7

66

.7

66.7

50

.0

50.0

50

.0.

50.0

50

.0

50 ..._2

_0 33

,3

6.9

6.7

8.6

16.0

15.2

70.0

60.0

70.0

40.0

50.0

o~

62

Communit,s table 2. Pinus ponderosa - Pseudotsuga Forest Series,

Series B Foothill Ravine Forest (Bl)

Foothill Pinus ponderosa - Pseudotsuga Forest (B2)

Xeric Xeric Pinus Pseudotsuga ponderosa Forest (B3) Forest (B4)

Number of stands 40 Homoteneity ,530 Diversity: EXP(H’) 11.3 Diversity: 1 /A 8.0 Species per stand 30.6 Understory cover % 18.2

Freq. TREES (Below 1 m)

Series Prevalenrs Pseudotsuga menziesii Juniperus scopulorum

12.5 80.0 15.4 100.0 12.5 87.5 8.0 45.5 4.2 40.0 7.2 71.4 3.2 31.3 3.2 45.5

Additional Prevalents Betula occidentalis Populus tremuloides Pinus flexilis

SHR CBS Series Prevalents Juniperus communis Ribes cereum Arctostaphylos uva-ursi Purshia tridentata Physocarpos monogynus Jamesia americana Acer glabrum

9.5 90.0 6.9 80.0

14.0 60.0 4.0 57.5

28.2 55.0 17.1 55.0

3.3 42.5

Additional Prevalents Symphoricarpus oreophilus Rubus deliciosus Artemisia tridentata

HERBS Series Prevalents Carex rossii Potentilla fissa Penstemon virens Geranium fremontii Leucopoa kingii Artemisia ludoviciana Senecio fendleri Sedum lanceolatum Pulsatilla patens Poa fendleriana Antennaria rosea Frasera speciosa Koeleria gracilis Antennaria parvifolia Artemisia frigida Muhlenbergia montana Heuchera bracteata Cystopteris fragilis Arabis drummondii

29.2 14.6 10.1 71.0 24.6 22.3

1.3 14.7 10.1 10.0

6.2 2.7 5.2 5.6 1.7

17.3 4.6 5.7 5.1

Poa pratensis 4.6

Const

40.0

95.0 80.0 72.5

70.0 67.5 65.0

60.0 52.5 47.5 45.0 45.0 45.0

42.5 42.5 40.0 37.5 35.0 35.0 35.0 32.5

7 ,601

9.9 6.9

28.0 29.5

Freq. Const. Freq. Const. Freq. Const.

0.0 14.3

15.3 85.7 16.0 42.9

23.0 57.1

34.3 36.0

7.0

100.0 85.7 57.1

8.8 71.4

8.7 85.7 13.7 100.0

7.2 71.4 1.0 57.1

33.0 57.1

1.3

2.7 0.8

85.7

42.9 71.4

10.7 42.9 17.0 57.1

16 I1 ,552 ,579

12.1 9.3 8.4 6.6

33.0 28.3 19.2 9.0

11.3 37.5

12.3 93.8 7.1 83.1

18.5 68.8 0.4 56.3

30.2 56.3 10.2 68.8

2.5 50.0

1.3 81.8 9.3 100.0 6.4 90.9 2.7 1oo.o 4.8 45.5 4.0 66.7 7.5 81.8 5.3 50.0

15.0 36.4 24.0 33.3

2.0 36.4

29.6 93.8 20.0 81.3 10.4 62.5

6.5 68.8 25.3 75.0 20.9 56.3

6.0 62.5 15.6 68.8 10.3 43.8 10.0 50.0

9.8 56.3 3.0 50.0

37.8 100.0 5.3 54.5 6.7 81.8 4.8 90.9

22.5 72.7 20.8 90.9

8.9 81.8 14.0 54.5 12.0 27.3

5.6 45.5

4.4

3.4 1.5

56.3 4.5 72.7

10.0 36.4 10.3 63.6 12.0 63.6

43.8 50.0

4.0 37.5 4.0 63.6 8.0 27.3

6 .596

14.8 11.0 31.8 19.3

Freq. Const.

12.6 100.0

10.0 33.3

1.3 50.0

20.0 33.3

32.7 100.0 13.3 100.0 18.4 83.3 25.3 50.0 16.0 50.0 28.8 83.3

7.2 83.3 9.3 50.0

25.3 50.0 18.7 50.0

4.0 50.0 6.7 50.0 4.0 50.0 4.0 66.7 9.0 66.7

21.3 50.0

2.7 50.0

Community table 2 (cont.)

Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq.

63

Const.

Harbouria trachypleura Achillea lanulosa Sitanion longifolium

Additional Prevalents Mertensia viridis Erigeron speciosus Fragaria vesca Galium boreale

Heuchera parvifolia Clematis cotumbiana Arabis fendleri Polypodium vulgare Selaginella underwoodii Saxifraga bronchialis Festuca saximontana Solidago spathulata Carex foenea Erigeron compositus Androsace septentrionalis Solidago multiradiata Selaginella densa Helianthus pumilus Chrysopsis villosa Eriogonum umbellatum Thermopsis divaricarpa

3.7 9.0 4.0

32.5 30.0 30.0

4.0 57.1 29.3 42.9 17.3 42.9 10.7 42.9

14.7 42.9 2.7 42.9

17.5 1.3 42.9 12.5 1.3 42.9 10.0 18.0 28.6

2.9 10.4

7.2

5.1 5.1 5,7

21.3 6,0 5,6

43.8 3t.5

31.3

43.8 43.8 43.8 37.5 37.5 31,3

3.0 1.6

4.0

17.6 7.2 2.0

36.4 45.5

36.4

45.5 45.5 36.4

2.7

4.0

30.0

4.0

t7,3 17.3 26.0

50.0

50.0

33.3

50.0

50.0 50.0 33.3

64

Community table 3. Mesic Montane forest series,

Mixed Wet Montane Ravine Mixed Mesic Serles C Forest (CI) Forest (C2) Forest (C3)

Number of stands Homoteneity Diversity: EXP(H') Diversity: 1/), Species per stand Undevstory cover %

28 4 9 15 .518 .652 .595 .622

16.8 26.7 18.9 13.0 11.9 20.0 t3.1 9.0 40.9 60.3 46.8 32.2 54,1 80.2 49.8 49.7

Freq. Const. Freq. Const. Freq. Const. Freq, TREES (Below 1 m)

Series Prevalents Populus tremuloides 21.5 67.9 22.0 Picea engetmannii 18.2 64.3 Abies lasiocarpa 35,5 60,7 Pseudotsuga menziesii 9.1 50.0

Additional Prevalents Juniperus scoputorum 5.3 Betula occidentalis 10.7 6.0 Alnus tenuifolia 32. t 8.0

SHR UBS Series Prevalents Rosa sp. 29.3 96~4 30,0 J uniperus communis 13,4 71.4 Physocarpus monogynus 20.2 67.9 12.0 Jamesia americana 22,7 57.1 Mahonia repens 19.0 57.1 ,~ Acer glabrum I0.5 57.1 10.7 Ribes tacustre 10.3 50.0 20.0 Vaecinium myrtillus 52.3 46.4 Lonicera involuerata 8,7 42.9 16.0

=

Additional Prevalems Ribes inerme 24.0 Symphoriearpos oreophilus 25.3 Cornus stolonifera 10,7 14.0 Prunus virginiana 8.0 Ribes cereum Rubus idaeus 42..__.~9 Linnaea borealis 35,7

HERBS Series Prevalents Arnica cordifolia 28,5 89,3 28.0 Fragaria vesta 16.2 71.4 6.0 Pyrola seeunda t8.8 7114 .... 2.0 Osmorhiza depauperata 16.6 71,4 34.7 Clematis columbiana 12,8 ,67.9 Haplopappus parryi 15.8 57.1 Galium boreale 29.6 53.6 32.0 Carex rossii I 1.7 50.0 Pyrota virens 7.4 50.0 Epilobiam angustifolium 8.9 50.0 Calamagrostis canadensis 20.9 46.4 ..... 44.0 Erigeron speciosus 17.5 46.4 Smilacina stetlata 19.7 46.4 36.0 Taraxacum officinale 19,0 46.4 27.0

Const,

t00.0 30.0 88.9 t 1.4 46.7 21.3 33,3 17.6 1130.0

38.7 t00.0 11.3 66,7

75.0 1.0 44.4 50,0 50._..L0

I00.0 35.t I00.0 25.4 93.3 12.0 77.8 14,2 73.3

50.0 34.9 77.8 11.6 66.7 26.0 66,7 23.0 53.3 13.0 44.4 20.4 73.3

7!,0 I5.3 66.7 6,3 46.7 50.0 5.0 44.4 t0.5 53.3

52.3 86.7 50,0

75.0 75,0 18.3 77.8 50.0 50.0

7.5 88.9 4.6 7 7 . 8

53.2 66.7

75.0 29.1 77.8 28.3 100.0 50.0 21.8 100.0 12.9 60.0 50.0 15.0 44.4 22.3 93.3 75,0 18.3 77.8 t0,0 66.7

9.5 88.9 l&3 100.0

100,0 27.5 88.9 12,0 44.4 11.6 66.7 7.0 44.4 6.2 60.0

10,9 77.8 7.3 40.0 75,00 22.0 66.7

18.0 44,4 18,5 53.3 !00.0 12.0 77.8

16.0 77.8


Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq, Const.

65

Achillea lanulosa Galium triflorum Oryzopsis asperifolia Goodyera oblongifolia Fragaria virginiana Actaea rubra Equisetum arvense Heracleum lanatum Chimaphila umbellata Carex geyeri Cystopteris fragilis Thalictrum fendleri Smilacina racemosa Antennaria rosea Geranium richardsonii

Additional Prevalents Ligusticum porteri Thermopsis divaricarpa Dodecatheon pulchellum Trifolium repens Clematis columbiana Hydrophyllum fendleri Monarda fistulosa Phleum pratense Poa pratensis Rudbeckia laciniata Aralia nudicaulis Viola canadensis Carex canescens Carex microptera Carex foenea Mertensia ciliata Pteridum aquilinum Montia chamissoi Aconitum columbianum Unknown Cirsium centaureae Pseudocymopterus montanus Aquilegia caerulea Rudbeckia hirta Urtica dioica Frasera speciosa Geum macrophyllum Potentilla fissa Geranium fremontii Campanula rotundifolia Artemisia ludoviciana Bromus richardsonii Pedicularis racemosa Calypso bulbosa Hieracium albiflorum Saxifraga bronchialis Streptopus amplexifolius

31.0 13.7 14.7 15.3 29.8 4.0

17.2 I1.2 20.8 10.4 10.0 4.4 9.2 5.2

32.9

42.9 42.9 42.9 42.9 39.3 39.3 35.7 35.7 35.7 35.7 35.7 35.7 35.7 3517 32.1

28.6

17,9 14.3

25.0 14.3

14.3

14.3 10.7" 10.7

21.4 7.1

14.3

28.6 i0 . / 7.1

10.7

28.6

28.6

30.0 26.7

24.0 4.0

30,0 10.0

20.0 4.0 5.0

12.0 2.0

49.3

16.0 32.0 30.7 21.3 17.3 13.3 6.7

54.0 28.0 52.0 24.0 28.0 14.0 20.0 18.0 20.0

8.0 6.0

10.0 4.0

10.0 6.0 6.0 6,0 8,0 2.0 4.0 4.0

100.0 75.0

75.0 75.0

lO0.O

50.0 50.0

iOO.O 50.0 50.0 75.0

100.0 75.0 75.0 75.0 75.0 75.0 75.0 50.0 50.0 50,0 50.0 50.0 50.q ..... 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.O 50.0 50.0 50.0 50.0 50.0 50,0 50.0

31.3 12.0 21.3

30.4 4.0 2.7

12.8

10.7 14.0 1.6

8.0 20.8

7.0

27.0

6.7

4.0

5.0 21.6 7.2 0.0

14.7

66.7 66.7 33.3

55.6 44.4 33.3 55.6

33.3 66.7 55.6

33.3 55.6

44.4

44.4

33.3

33.3

44.4 55.6 55.6 44.4 33.3

11.5 16.4

20.8 10.4

4.7

14.5

22.7 2.7 3.3 8.0 4.0

53.3 73.3

66.7

40.0

53,3

40.0 40.0

,40.0. 33.3 33.3

Co

mm

un

ity

tabt

e 4.

Pin

us

con

tort

a f

ore

st s

erie

s.

Ser

ies

D

Pin

us

con

tort

a F

ore

st (

D1

)

Mes

ic

Pin

us

con

tort

a-

Pse

ud

ots

ug

a F

ore

st (

D2

)

Xer

ic

Pin

us

con

tort

a-

Pse

ud

ots

ug

a F

ore

st (

D3

)

Mes

ic P

inu

s co

nto

rta-

Ab

ies,

P

icea

Fo

rest

(D

4)

Xer

ic P

inu

s co

nto

rta-

Ab

ies,

P

icea

Fo

rest

(D

5)

Nu

mb

er o

f st

and

s 84

H

om

ote

nei

ty

.448

D

iver

sity

: E

XP

(H')

7.

8 D

iver

sity

: 1/

~.

5.7

Spe

cies

per

sta

nd

19

.5

Un

der

sto

ry c

ov

er %

27

.5

Fre

q.

23 .5

26

7.6

5.6

18.7

17

.9

Co

nst

. F

req

.

12 .5

20

7.2 5.

0 23

.8

17.3

Co

nst

. F

req

.

8 .5

71

11.4

8.

0 26

.9

21.3

Co

nst

. F

req

.

13 .5

47

t0.8

8.

0 25

.3

42.0

Co

nst

. F

req

.

TR

EE

S (

Bel

ow 1

m)

Seri

es P

reva

lent

s A

bies

las

ioca

rpa

36.9

52

.4

51.6

P

icea

en

gel

man

nii

15

.0

46.4

14

.7

39.1

26

.7

Po

pu

lus

trem

ulo

ides

14

.9

40.5

23

.3

52.2

4.

7 50

.0

17.0

Ad

dit

ion

al

Pre

vale

nts

Pin

us c

on

tort

a 21

.6

65.2

43

.0

50.0

P

seu

do

tsu

ga

men

zies

ii

11.1

75

.0

Jun

iper

us

sco

pu

loru

m

0.0

50.0

P

inus

fle

xili

s 12

.0

75.0

SH

R U

BS

Se

ries

Pre

vale

nts

Jun

iper

us

com

mu

nis

12

.9

86.9

14

.3

91.3

10

.2

91.7

17

.7

87.5

5.

6 V

acci

nium

myr

till

us

52.3

57

.1

15.2

43

.5

52.7

A

rcto

stap

hy

los

uv

a-u

rsi

17.5

51

.2

16.8

82

.6

7.4

58.3

36

.5

100.

0 R

osa

sp.

20

.1

46.4

19

.2

43.5

4.

6 58

.3

2.0

25.0

34

.2

Jam

esia

am

eric

ana

11.6

44

.0

7.7

60.9

22

.9

91.7

7.

0 50

.0

Ad

dit

ion

al

Pre

vale

nts

Ph

yso

carp

us

mo

no

gy

nu

s 24

.0

58.3

A

cer

gla

bru

m

2.3

58.3

R

ibes

cer

eum

2.

7 50

.0

1.3

37.5

M

aho

nia

rep

ens

28.0

L

onic

era

inv

olu

crat

a 12

.0

Ru

bu

s id

aeus

9.

0 V

acci

niu

m s

cop

ariu

m

HE

R B

S

Seri

es P

reva

lent

s C

arex

ros

sii

19.6

73

.8

19.8

91

.3

19.6

83

.3

37.1

87

.5

14.0

P

yrol

a v

iren

s 8.

5 45

.2

6.0

60.9

5.

0 33

.3

14.5

Co

nst

.

69.2

69

.2

61.5

76.9

92

.3

84.6

46.2

38

.5

30.8

46.2

61

.5

28 .5

21

5.8

4.4

13.6

34

.9

Fre

q.

42.5

10

.3

8.4

14.5

66

.3

20.0

36.8

14.4

9.

1

Co

nst

.

85.7

75

.0

32.1

85.7

92

.9

32.1

35.7

64.3

39

.3

Co

mm

un

ity

tab

le 4

(co

nt.)

F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

.

Epi

lobi

um a

ngus

tifo

lium

A

rnic

a co

rdif

olia

P

yrol

a se

cund

a H

aplo

pap

pu

s pa

rryi

P

oten

till

a fi

ssa

Sed

um l

ance

olat

um

Pen

stem

on

vir

ens

The

rmop

sis

diva

rica

rpa

Sol

idag

o sp

athu

lata

S

enec

io f

endl

eri

mnt

enna

ria

rose

a

Ad

dit

ion

al

Pre

vale

nts

Sax

ifra

ga b

ronc

hial

is

Fra

gari

a ve

sca

Cle

mat

is c

olum

bian

a S

elag

inel

la d

ensa

S

olid

ago

mis

sour

iens

is

Ant

enna

ria

parv

ifol

ia

Gal

ium

bor

eale

A

rtem

isa

ludo

vici

ana

Leu

copo

a ki

ngii

A

chil

lea

lanu

losa

F

estu

ca s

axim

on

tan

a F

rase

ra s

peci

osa

Ara

bis

dru

mm

on

dii

A

stra

galu

s pa

rryi

O

xytr

opis

lam

bert

ii

Art

emis

ia f

rigi

da

Eri

gero

n co

mp

osi

tus

Har

bo

uri

a tr

achy

pleu

ra

Car

ex f

oene

a E

rige

ron

spec

iosu

s O

smor

hiza

dep

aup

erat

a C

amp

anu

la r

otun

difo

lia

Fra

gari

a vi

rgin

iana

C

alam

agro

stis

can

aden

sis

Pyr

ola

asar

ifol

ia

Chi

map

hita

um

bell

ata

Cys

topt

eris

fra

gili

s P

enst

emo

n w

hipp

lean

us

10.7

21

.3

13.3

22

.5

11.8

11

.4

12.4

29

.5

18.9

5.

1 12

.2

44.0

42

,9

42.9

40

.5

40.5

36

.9

33.3

32

.1

31.0

29

.8

23.8

8.3

2.4

14.9

20.5

11

.3

6.9

12.0

39

.6

19.6

4.

4

8.0

43.5

30

,4

34.8

52

.2

60.9

39

.1

43.5

39

.1

39.1

39.1

2.0

4.8

8.0

7.5

6.4

1.6

12.8

6.

4 11

.0

9.0

3.0

4.0

2.0

33.3

41

.7

66.7

66.7

41.7

41.7

41.7

41

.7

33.3

33

.3

33.3

33

.3

33.3

13.3

20

.0

11.4

45

.0

10.0

9.

5 7.

0

32.0

5.6

16.,0

11

.0

12.0

5.

0 5.

0 26

.7

18.7

17

.3

18.7

18

.7

14,7

75.0

10

0.0

87.5

50

.0

50.0

10

0.0

50.0

37.5

62.5

50.0

50

.0

50.0

50

.0

50.0

37

.5

37.5

37

.5

37.5

37

.5

37.5

18.8

32

.0

18.9

34

.5

6.4

76.9

10

0.0

84.6

84

.6

38.5

12.0

16

.9

14.3

21

.8

53.6

50

.0

50.0

32

.1

28.0

30

.8

17.3

21

.4

34.4

10.7

38.5

46.2

53.8

53

.8

38.5

30

.8

30.8

30

.8

30.8

30

.8

45.7

22

.3

7.2

73.0

27

.0

17.0

4.

0 0.

0 3.

6 32

.1

",M

Co

mm

un

ity

tabl

e 5.

Pic

ea.

Ab

ies

fore

st s

erie

s.

Ser

ies

E

Wet

Mo

nta

ne

For

est

(El)

B

og F

ores

t (E

2)

Wet

Pic

ea,

Abi

es F

ores

t (E

3)

Mes

ic P

icea

, A

bies

For

est

(E4)

Xer

ic P

icea

, A

bies

For

est

(E5)

Su

bal

pin

e P

icea

, A

bies

For

est

(E6)

Nu

mb

er o

f st

ands

68

8

3 14

17

17

9

Hom

oten

eity

.4

35

.642

.6

84

.583

.5

35

.652

.6

20

Div

ersi

ty:

EX

P(H

')

11.7

16

.5

15.4

16

.0

8.1

7.3

14.4

D

iver

sity

: 1/

h 8.

2 12

.1

11.1

11

.5

5.8

4.8

9.7

Spe

cies

per

sta

nd

28.2

34

.1

37.0

35

.1

21.5

21

.4

34.8

U

nder

stor

y co

ver

%

54.9

56

.1

93.1

62

.1

50.7

50

.9

45.5

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

Con

st.

Fre

q.

TR

EE

S (

Bel

ow 1

m)

Seri

es P

reva

lent

s

Abi

es l

asio

carp

a 39

.5

97.1

24

.6

87.5

38

.7

100.

0 28

.9

92.9

56

.0

100.

0 44

.7

100.

0 25

.8

100.

0 P

ieea

eng

elm

anni

i 28

.8

91.2

17

.3

75.0

25

.3

100.

0 19

.7

I00.

0 24

.5

88.2

21

.5

94.1

30

.5

88.9

S

orbu

s se

opul

ina

3.2

7.4

4.0

33.3

Ad

dit

ion

al

Pre

vale

nts

Aln

us t

enui

foli

a 16

.0

25.0

P

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us t

rem

uloi

des

8.0

11.8

SH

R U

BS

Se

ries

Pre

vale

nts

Vac

cini

um m

yrti

llus

49

.1

69.1

18

.7

37.5

46

.7

100.

0 32

.6

50.0

62

.3

94.1

56

.8

88.2

V

acci

nium

sco

par

ium

44

.6

66.2

14

.0

66.7

'

20.5

57

.I

34.9

64

.7

67.0

94

.1

50.9

77

.8

Rib

es m

on

tig

enu

m

17.1

38

.2

22.7

64

.3

2.7

35.3

23

.4

77.8

R

ibes

col

orad

ense

6.

1 33

.8

4.0

57.1

3.

0 44

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Juni

peru

s co

mm

un

is

8.0

27.9

4.

0 33

.3

11.2

58

.8

4.0

44.4

Ad

dit

ion

al

Pre

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n ts

Lon

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a in

volu

crat

a 15

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100.

0 5.

7 41

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a sp

. 13

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tre

1.5

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62

.5

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bu

cus

race

mos

a ~

14.0

50

.0

4.9

52.9

L

inna

ea b

orea

lis

12.0

37

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Gau

lthe

ria

hu

mif

usa

5.

9 22

.0

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S

alix

pla

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lia

1.5

24.0

66

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mia

pol

ifol

ia

2.9

14.0

66

.7

HE

RB

S

Seri

es P

reva

lent

s

Epi

lobi

um a

ngus

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lium

19

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17

.3

75.0

22

.7

100.

0 26

.4

71.4

19

.3

64.7

16

.6

76.5

18

.0

66.7

A

rnic

a co

rdif

olia

34

.9

70.6

22

.7

75.0

0.

0 66

.7

35.0

85

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51.7

82

.4

27.6

64

.7

30.7

33

.3

Po

lem

on

ium

del

icat

um

23.9

69

.1

26.8

92

.9

22.4

58

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10.1

88

.2

44.4

10

0.0

Con

st.

Co

mm

un

ity

tab

le 5

(co

nt.)

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

Car

ex r

ossi

i P

yrol

a se

cund

a M

erte

nsia

cil

iata

E

rige

ron

pere

grin

us

Pen

stem

on w

hipp

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us

Sen

ecio

tri

angu

lari

s L

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a pa

rvif

lora

O

smor

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dep

aupe

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S

trep

topu

s am

plex

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ius

Poa

ner

vosa

A

chil

lea

lanu

losa

T

rise

tum

spi

catu

m

Hie

raci

um g

raci

le

Mit

ella

pen

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ra

Sed

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Cys

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fra

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s P

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Ang

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a gr

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Cas

till

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rhex

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ia

Sel

agin

ella

den

sa

Ad

dit

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al

Pre

vale

nts

Gal

ium

tri

flor

um

Car

ex d

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Equ

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se

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acle

um l

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um

Lig

usti

cum

por

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min

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C

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la p

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ns

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ex a

quat

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C

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is g

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sca

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is c

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hyp

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co

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nu

m

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ho

dan

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m

Sax

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dont

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a

23.0

23

.0

26.5

22

.4

11.2

31

.8

14.2

20

.3

29.0

20

.0

13.3

4.

2 14

.2

21.5

6.

7 5.

6 17

.7

16.9

3.

5 15

.0

54.4

54

.4

48.5

48

.5

44.1

38

.2

38.2

36

.8

35.3

32

.4

32.4

32

.4

29.4

27

.9

27.9

26

.5

25.0

25

.0

25.0

23

.5

10.3

16.2

22

.1

23.5

5.9

16.2

14

.7

8.8

4.4

13.2

22

.1

31.0

23

.3

45.0

5.

6 5.

3 49

.1

37.6

14.4

35.3

34

.3

54.0

24

.7

10.0

21

.6

16.8

4.

8 31

.0

24.0

10

.0

11.0

73

.3

34.7

16

.0

10.7

100.

0 75

.0

50.0

62

.5

75.0

87

.5

62.5

62.5

100.

0 87

.5

75.O

75

.0

75.0

62

.5

62.5

62

.5

50.0

50

.0

50.0

50.0

37

.5

37.5

37

.5

.

37.5

33.3

38.7

29.3

2.0

14.0

48.0

6.0

40.0

89

,3

61.3

38

.7

34.0

22.7

0.

0

100.

0

100.

0

100.

0

66.7

66.7

66.7

66.7

66.7

10

0.0

100.

0 10

0.0

66.7

100.

0 10

0.0

10.4

47.7

20

.3

36.0

29

.7

30.7

25

.7

23.5

27,1

5.

3

47.2

20.8

37

.1

65.7

13.6

34

.5

35.7

92.9

92

.9

100.

0 50

.0

64.3

50

.0

57.1

64.3

42

.9

35.7

35.7

50

.0

50.0

35.7

78

.6

15.4

28

.6

4.0

42.3

14.0

21

.8

12.0

8.0

23.2

2.9

41.2

30

.5

94.1

25

.0

88.9

82

.4

14.7

52

.9

47.1

19

.0

44.4

41

.2

5.7

41.2

11

.2

88.2

17

.8

100.

0

47,1

52

.9

41.2

29.4

29.4

41.2

15.3

64

.7

32.0

77

.8

16.9

52

.9

16.8

55

.6

6.0

47.1

5.

0 44

.4

11.4

41

.2

22.4

55

.6

6.2

64.7

8.

6 77

.8

17.1

41

.2

1.0

44.4

17

.5

64.7

I 1

.0

44.4

Co

mm

un

ity

tab

le 5

(co

nt.)

-.

a F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

. F

req.

C

onst

.

Ele

ocha

ris

pauc

iflo

ra

Epi

lobi

um a

naga

llid

ifol

ium

C

arex

ill

ota

Junc

us d

rum

mo

nd

ii

Pod

agro

stis

hum

ilis

A

gros

tis

vari

abli

s C

alam

agro

stis

scr

ibne

ri

Junc

us m

erte

nsia

nus

Vio

la a

dunc

a E

pilo

bium

sp.

U

nkno

wn

Car

ex c

anes

cens

C

arex

occ

iden

tali

s T

roll

ius

laxu

s B

isto

rta

bist

orto

ides

R

anun

culu

s es

chsc

holt

zii

Ado

xa m

osch

atel

lina

E

pilo

bium

hor

nem

anni

i V

eron

ica

wor

msk

jold

ii

Ped

icul

aris

bra

cteo

sa

Luz

ula

spic

ata

Del

phin

ium

bar

beyi

C

arex

nov

a P

yrol

a vi

rens

H

aplo

papp

us p

arry

i S

olid

ago

spat

hula

ta

Ara

bis

dru

mm

on

dii

C

arex

foe

nea

Pot

enti

lla

dive

rsif

olia

A

nten

nari

a ro

sea

And

rosa

ce s

epte

ntri

onal

is

Art

emis

ia a

rcti

ca

Fes

tuca

bra

chyp

hyll

a M

erte

nsia

vir

idis

D

raba

str

epto

carp

a S

ibba

tdia

pro

cum

bens

G

eum

ros

sii

Eri

gero

n si

mpl

ex

Car

ex e

lyno

ides

D

raba

aur

ea

Sax

ifra

ga d

ebil

is

Tri

foli

um p

arry

i

2.9

4.4

2.9

17.6

2.

9

2.9

4.4

19.1

14.7

19

.1

16.2

13

.2

7.4

5.9

14.7

13

.2

30.0

40

.0

16.0

8.

0 0.

0 76

.0

44.0

40

.0

80.0

40

.0

12.0

20

.0

4.0

66.7

66

.7

66.7

66

.7

66.7

33

.3

33.3

33

.3

33.3

33

.3

33.3

33

.3

33.3

33.3

9.7

28.0

14

.0

14.3

18

.3

17.3

17

.3

12.0

34

.4

15.0

9.

0

50.0

64.3

57

.1

50.0

50

.0

42.9

42

.9

42.9

35

.7

28.6

28

.6

5.0

4.0

47.1

35

.3

10.5

4.

5 4.

0

12.0

66

.7

64.7

11

.2

55.6

47

.1

35.3

21

.3

66.7

14

.3

77.8

8.

6 77

.8

7.4

77.8

39

.3

66.7

17

.3

66.7

2.

4 55

.6

3.2

55.6

25

.0

44.4

14

.0

44.4

20

.0

44.4

4.

0 44

.4

8.0

44.4

6.

0 44

.4

26.7

33

.3

Communi O' table 6. Pinus flexilis forest series.

71

Series F

Montane Pinus flexilis Forest (F 1)

Pinus flexilis- Picea, Abies Forest (F2)

Subalpine Pinus flexilis Forest (F3)

Number of stands 26 9 9 8 Homoteneity .532 .606 .603 .538 Diversity: EXP(H') 9.8 9.9 10.7 8,8 Diversity: I i ~. 7,1 7.4 7.4 6.4 Species per stand 23.5 22.9 26, ! 21.4 Understory cover % 27,8 22,8 37.5 22, 5

Freq, Const. Freq. Const. Freq. Const. Freq. Const. TREES (Below | m)

Series Prevalents Picea engelmannii 16.7 84.6 t3.3 66.7 t7.8 100.0 18.3 87.5 Pinus flexilis 21,4 76.9 20.0 100.0 18.0 66,7 28.0 62.5 Abies lasiocarpa 13.7 46.2 24.8 55,6 3.0 50.0

Additional Prevatents Populus tremuloides 16.7 66.7 Pinus contorta 10.0 44.4

SHRUBS Series Prevalents Juniperus eommunis 22,4 88.5 20.5 88.9 22.9 77.8 24.0 100.0 Vaccinium myrtillus 25.3 46.2 8.0 44.4 39.2 55.6 25,3 37.5 Jamesia americana 10.0 30.8 15.2 55.6 Arctostaphylos uva-ursi 24.0 26.9 30,0 44.4 16.0 37.5

A dditional Prevalents Satix seouleriana 5.3 33,3 Ribes montigenum 2.0 44.4 Vaccinium scoparium 14.7 33,3

HERBS Series Prevalents

Carex rossii 17.I 80.8 16.7 66.7 18~3 77,8 16.5 I00~0 Saxifraga bronchialis 28.6 76.9 38.9 77.8 18,9 77.8 28.0 75.0 Calamagrostis purpurascens 24,4 76.9 36,0 100.0 9.6 55.6 19.3 75.0 Selaginella densa 17.8 69.2 12.7 66.'7' 24.7 66.7 16.0 75.0 Solidago spathulata 21.3 69.2 21, I 77,8 3t.3 66,7 9.6 62.5 Sedum 1anceolatum 18.2 69.-"'~ 6.0 44.4 25.3 I00.0 15,2 62.5 Erigeron compositus 12.3 61.5 9.0 88.9 6.0 44.4 25,0 50.0 Antennaria rosea 16,0 50.0 16.0 55,6 14.7 66,7 Epilobium angustifolium 15. I 50.0 8.0 44.4 26.0 66.7 2.7 37.5 Arenaria fendleri 28.0 46.2 31.3 66.7 24.0 50.0 Penstemon whippleanus 13.8 42.3 t8.5 88~9 f.3 37.5 Trisetum spicatum t6.0 38,5 t4,7 33~3 23.2 55.6 Draba streptocarpa 5,6 38.5 6.0 44.4 9.3 3Z5 Frasera speciosa 22.7 34.6 25,0 44.4 30,0 25,0 Heuchera bracteata 3,1 34.6 4,0 44.4,, Potentilla fissa 27.5 30.8 32,0 66.7 Achiltea lanulosa 26,5 30.8 28.8 55.6 Poa nervosa t 1,5 30,8 14.4 55,6 6.7 37.5

Additional Prevalents Senecio fendleri 7,2 55,6 Penstemon virens 16.0 44.4 Carex foenea 33.6 55.6

72


Freq, Const. Freq. Const. Freq. Const. Freq. Const

Polemonium delicatum Festuca brachyphylla Arabis drummondii Trifolium dasyphyllum Pedicularis parryi Pyrola secunda Thermopsis divaricarpa

3.2 7.0 8.0

38.7 36.0

55.6 44.4 44.4 33.3 33.3

0.0

26.7

1.3 24.0

37.5

37.5

37.5 25.0

73

Community table 7. Forest-Alpine transition series.

Series G

Mesic Krummholz (G1)

Xeric Krummholz (G2)

Number of stands 9 6 3 Homoteneity .658 .646 .772 Diversity: EXP(H') 12.5 11.9 13.8 Diversity: 1 / ?t 7.0 7.1 6.8 Species per stand 39 38.3 40.3 Understory cover % 77.8 74.1 85.3

Freq. Const. TREES (Below 1 m)

Series Prevalents Picea engelmannii 16.0 100.0 Abies lasiocarpa 53.5

SHRUBS Series Prevalents Salix brachycarpa 40.0 66.7 Vaccinium scoparium 27.3 66'13 Ribes montigenum 12,8 55.6

Additional Prevalent Vaccinium myrtillus

HERBS Series Prevalents Arenaria obtusiloba 16.0 t00.0 Sedum lanceolatum 10.7 !OOrO Potentilla diversifolia 6.2 100.0 Trifolium dasyphyllum 13.0 88.9~ Geum rossii 17.0 88.9 Polemonium delicatum 19.0 ~8K9' Artemisia arctica 135 -g'ffT".'ff Selaginella densa 7.5 88.9 Bistorta bistortoides 15.5 88.9 Antennaria rosea 6.5 88.9 Penstemon whippleanus 8.6 77.8 Epilobium angustifolium 9.1 77.8 Trisetum spicatum 3.4 77.8 Artemisia borealis 28.0 66.7 Arenaria fendleri 10.0 66.'f Castilleja occidentalis 3.3 ....... 66.7

, , ,0

Hymenoxys grandiflora 0.7 66.7 "Carex foenea 21.6 55.6 Silene acaulis 10.4 55.6 Mertensia viridis 10.4 55.6 ~ Bistorta vivipara 12.0 5 5 £ Mertensia ciliata 3.2 55.6 Saxifraga rhomboidea 7.2 55.6 Carex albonigra 9.0 44.4 Anemone narcissiflora 19.0 44.4 Festuca brachyphylla 13.0 44.4 ~ Pedicularis parryi 6.0 " - ~ Erigeron simplex 10.0 44.4 Draba streptocarpa 6.0 44.4 Poa alpina 4.0 44.4

Freq. Const. Freq,

12.7 100.0 22.7 59.2 83.3 44.0

34.0 66.7 52.0 28.8 83.3 20.0 13.0 66.7 12.0

48.0 16.7

9.3 100,0 29.3 8.7 100.0 14.7 5.3 100,0 8.0 7.2 83.3 22.7

16.8 83.3 17.3 22.7 100.0 8.0 t5.3 100.0 8.0 8.0 83.3 6.7

19.3 100.0 4,0 8.8 83.3 2,7 7.2 83.3 t2.0 8.8 83.3 10.0 3.2 83.3 4.0

, , , ,,, ,,,,,,,,

32.0 50.0 24.0 1.3 50.0 18.7 0,0 50.0 6.7 1.3 50.0 0.0

21.3 50.0 22.0 14.7

9.3 50.0 12.0 13.3

4.0 66,7 10.7 50.0 2.0

t8.0 25.3 50.0 14.7 50.0

1.3 50.0 20.0 12.0 50.0

6.0 2.0

Const.

100.0 100.0

66.7 33.3 33.3

100.0 lOO.O I00.0 lOO,O 100.0 66.7

100.0 66.7

100.0 66.7 66.7 66.7

100.0 100.0 100.0 100.0 66.7'

I00.0 , , , , , , ,

66,7 100.0

66.7 66.7

33.3

66.7 66.7

74 Community table 7 (cont.)

Freq. Const, Freq. Const. Freq. Const.

Campanula rotundifolia 1.0 44.4 Carex chalciolepis 1.0 44.4 K obresia myosuroides 14.7 33.3 Poa fendleriana 1K7 33.3 Carex rossii 18,7 33,3

Additional Prevalents Poa nervosa 33._.~3 Polemonium viscosum 33,3 Solidago spathulata Trifolium parryi 22.2 Gentiana algidd 22.2 Achillea lanulosa Juncus parryi 22._2 Oreoxis alpina 33,3 Senecio taraxacoides 33.--'~ Poa lettermanii 22.2 Calamagrostis purpurascens - Hymenoxys acaulis 22.2 Androsace septentrionalis 33,J Carex rupestris 22.) '

26.0 33.3 18.7 50.0

22.7 50.0 5.3 50,0 0.0 50.0

26.0 50.0 16.0 33.3 10.0 33.3 10.0 33.3

2.0 66.7 2.0 66~_..27

16.0 66 7

9.3 100.0 4.0 100.0

14.0 66.7 10.0 66.7 8.0 66.7

10.0 647 28.0 33.3

Community table 8. Populus forest.

Populus Forest Series H (H 1)

75

Number of stands t 1 11 Homoteneity .540 .540 Diversity: EXP(H') 15.5 15~5 Diversity:I / X 11.0 t 1,0 Species per stand 34,4 34,4 Understory cover % 57.9 57.9

Freq, Const. Freq, Const, TREES (Below 1 m)

Series Prevalents Populus tremuloides 51.3 I00,0 51.3 I00.0 Pinus contorta 8.8 45.5 8.8 45.5 Picea engelmannii 5.0 ~ 5.0 36.4

SHR UBS Series Prevatents Rosa sp. 47.2 90.9 47.2 90.9 Arctostaphylos uva-ursi 26.0 72.7 26.0 72.7

, , , ,~ . . . . . . . .

Juniperus communis 12,0 72.7 I2.0 72.7 Mahonia repens 44.8 45.5 44.8 45.5 Vaccinium myrtillus 23.2 45.5 23.2 45.5 Salix scouleriana 2.0 36.4 2.0 36.4 Jamesia americana 9.3 27.3 9,3 27.3

HERBS Series Prevalents Achillea lanulosa 27.6 90,9 27,6 90.9 Epilobium angustifotium 26.0 90.9 26.0 90.9 Arnica cordifolia 24.4 81.8 24.4 81.8 Thermopsis divaricarpa 84.0 72.7 84.0 72.7 Haplopappus parryi 42.0 '72.7 42,0 72.7 Potentilla fissa 9.5 72.7 9,5 72.7 Carex rossii 29.7 63.6 29.7 63.6 Carex foenea 29.7 63.6 29.7 63.6

, , , , ,

Taraxacum officinale 24.6 63.6 24,6 63.6 Bromus lanatipes 61.3 54,5 61.3 54,5 Erigeron speciosus 58.0 54,5 58.0 54.5, Fragaria vesca 32.7 54.5 32,7 54.5 Poa pratensis 26.0 54.5 26.0 54.5 Osmorhiza depauperata 30.0 5~15 30,0 54.5 Arabis drummondii 10.0 54.5 10.0 54.5 Sedum lanceolatum 7.3 5415 7.3 54.5 Campanula rotundifolia 22.4 45.5 22.4 45.5 Artemisia ludovieiana 23.0 36.4 23.0 36.4 Penstemon'whippleanus 17.0 36.4 17.0 36.4 Antennaria rosea 7.0 36,4 7.0 36.4 Etymus glaucus 33.3 27.3 33,3 27.3 Lupinus argenteus 76,0 27.3 76.0 27.3 Galium boreale 84.0 27.3 84.0 27.3 Phleum pratense 18.7 27.3 18.7 27.3 Heracleum lanatum 21.3 27.3 21.3 27.3 Poa nervosa 20.0 27.3 20.0 27.3

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