often ranging between and degrees F. C.); the low
10
FIRES AND FOREST SUCCESSION IN THE BITTERROOT MOUNTAINS OF NORTHERN IDAHO J. A. LARSEN Iowa State College Foresters have recently begun to seek a more intimate knowledge of the natural, successional stages by which forests regain terrain lost by extensive fires or other pronounced denuding agencies. Studies in this field lead to a closer understanding of the factors which control the distribution, compo- sition and density of the present forest, the silvical requirements of the various species which compose the existing forest and of the soil building or soil deteriorating influences which are operative after large fires. It is the author's privilege in this paper to record certain observations on forest succession in the Bitterroot Mountains in northern Idaho, gathered in the course of several years of forest research in that region. This is the terri- tory lying north of the Salmon river between the crest of the Bitterroot divide and the Columbia river plateau. Perhaps no other region in the United States is visited by forest con- flagrations of similar magnitude, frequency or degree of destruction. The contributing causes are mainly as follows: the moist winter and spring which give rise to a profuse and luxuriant forest vegetation of a highly in- flammable nature; an invariably dry summer with afternoon temperatures often ranging between 95 and ioo degrees F. (350 to 380 C.); the low atmospheric humidity which frequently falls to I5 per cent and lower. These critical conditions are combined with strong, desiccating winds sweeping in from the arid region to the southwest. The fires, therefore, when once under way, travel with great speed and rapidly assume uncontrollable pro- portions. Not infrequently an entire township (approximately 95 sq. km.) of timber is wiped out in the course of one or two days. These large forest fires kill all of the trees and the seedlings. In from ten to twenty years most of this dead timber lies prone, and it then presents a fire hazard of greater magnitude than existed in the green, virgin forest. The second fire, on this account, is even less controllable and more destructive than the first. The return of the forest to the climax composition subsequent to these devastating fires proceeds along certain well defined steps or stages which may be considered parts of the entire cycle of succession, or the natural process of regeneration. The first steps in this succession begin with the appearance of species intolerant of shade, and capable of withstanding con- siderable drought and exposure, and concludes with the establishment of 67 This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain.
Transcript of often ranging between and degrees F. C.); the low
Fires and forest succession in the Bitterroot Mountains of northern
IdahoFIRES AND FOREST SUCCESSION IN THE BITTERROOT MOUNTAINS OF
NORTHERN IDAHO
J. A. LARSEN
Iowa State College
Foresters have recently begun to seek a more intimate knowledge of the natural, successional stages by which forests regain terrain lost by extensive fires or other pronounced denuding agencies. Studies in this field lead to a closer understanding of the factors which control the distribution, compo- sition and density of the present forest, the silvical requirements of the various species which compose the existing forest and of the soil building or soil deteriorating influences which are operative after large fires. It is the author's privilege in this paper to record certain observations on forest succession in the Bitterroot Mountains in northern Idaho, gathered in the course of several years of forest research in that region. This is the terri- tory lying north of the Salmon river between the crest of the Bitterroot divide and the Columbia river plateau.
Perhaps no other region in the United States is visited by forest con- flagrations of similar magnitude, frequency or degree of destruction. The contributing causes are mainly as follows: the moist winter and spring which give rise to a profuse and luxuriant forest vegetation of a highly in- flammable nature; an invariably dry summer with afternoon temperatures often ranging between 95 and ioo degrees F. (350 to 380 C.); the low atmospheric humidity which frequently falls to I5 per cent and lower. These critical conditions are combined with strong, desiccating winds sweeping in from the arid region to the southwest. The fires, therefore, when once under way, travel with great speed and rapidly assume uncontrollable pro- portions. Not infrequently an entire township (approximately 95 sq. km.) of timber is wiped out in the course of one or two days.
These large forest fires kill all of the trees and the seedlings. In from ten to twenty years most of this dead timber lies prone, and it then presents a fire hazard of greater magnitude than existed in the green, virgin forest. The second fire, on this account, is even less controllable and more destructive than the first.
The return of the forest to the climax composition subsequent to these devastating fires proceeds along certain well defined steps or stages which may be considered parts of the entire cycle of succession, or the natural process of regeneration. The first steps in this succession begin with the appearance of species intolerant of shade, and capable of withstanding con- siderable drought and exposure, and concludes with the establishment of
67
This file was created by scanning the printed publication. Errors identified by the software have been corrected;
however, some errors may remain.
68 J. A. LARSEN Ecology, Vol. X, No. i
shade-tolerant trees or species which are both tolerant and moisture loving. The exact number of the steps in the succession, depends, however, in a great measure upon the factors of topography, soil, degree of exposure, the intensity of the fire and the amount of destruction wrought.
It is of considerable interest to note that the subordinate forest vegeta- tion also develops after these fires in regular successional stages in its pro- gression toward the climax subordinate vegetation, and that the final, climax association, or ultimate type, is attained and developed underneath the climax type or association of the forest trees. Those subordinate species of tem- porary occupancy become established immediately after the forest fire and generally simultaneously with the appearance of the temporary arborescent species, while those of permanent occupation follow directly upon the heels of the climax arborescent species.
In composition the subordinate vegetation which follows the fires is also governed by the condition of the habitat, the degree of destruction and the requirements of the individual species, so that in the subordinate flora there is also distinct grouping in accordance with these characters or qualities. However, since the seedlings of the aborescent species develop very slowly during the first three or four years, and much slower than the annuals of the subordinate flora, the latter are in much greater evidence in the beginning. A superficial examination would ordinarily give the impression that there existed nothing but these rapidly growing and very dense annuals during the first few years following the fire.
FOREST ZONES AND SILVICAL REQUIREMENTS OF THE TREES
In this region, in which the elevations of the mountains vary from 6,ooo to 8,ooo feet (I,800 to 2,400 meters) above sea level, and the valleys from 2,000 to 3,ooo feet (6oo to goo meters), the western yellow pine forests, which are composed mainly of Pinus ponderosa, occupy the lowest zone of tree growth; the western white pine (Pinus monticola), with its associated species of western red cedar (Thuja plicata), western hemlock (Tsuga heterophylla), grand fir (Abies grandis), Douglas fir (Pseudotsuga taxi- folia), and western larch (Larix occidentalis), occupy the slopes of inter- mediate elevation ranging from 2,500 feet to 5,500 feet. Above this middle zone the forest is composed of subalpine species represented by Engelmann spruce (Picea engelmannii), mountain hemlock (Tsuga mertensiana), white bark pine (Pinus albicaulis), alpine fir (Abies lasiocarpa), and lodgepole pine (Pinus murrayana var. contorta). These subalpine species are fre- quently individually present also within the intermediate forest zone. The discussion in this article deals exclusively with the conditions surrounding the succession within the intermediate forest zone.
This middle forest zone is designated by foresters in the region as the western white pine type, chiefly because of the abundance and economic im- portance of the white pine. This tree is not a climax species, however, for
January, i929 FIRES AND FOREST SUCCESSION IN IDAHO 69
hemlock, western red cedar and grand fir are the only true climax repre- sentatives among the trees.
The western white pine and its associated species thrive best on well drained and fertile soil such as is found on the protected easterly or north- erly gradients and on lower slopes, bottoms and benches. In these locations the soil remains moist throughout the entire summer, being sheltered by a broken topography, a very dense crown canopy of the trees, and covered by deep layers of " duff " and humus.
The forests composed chiefly of western white pine are usually even- aged and rather dense. Over large areas the pine is the predominant spe- cies. It maintains a rather rapid and remarkably uniform rate of height growth by which, in most cases and situations, it overtops its competitors. Although both the lodgepole pine and the western larch grow as fast as the pine in most locations, these two are more intolerant of shade and therefore not in a position to arrest its growth seriously or to crowd it out. Mixed with the western white pine there is generally an understory of hemlock, cedar or grand fir wherever the soil is rather moist or the site well pro- tected, especially at the low elevations, while toward the upper margin of the zone and more particularly on the exposed south of west aspects we find more of the Douglas fir and lodgepole pine.
Another characteristic of the western white pine, and one which gives it a considerable advantage over the associated species, is the ability of its seed to lie over in the duff on the forest floor for a period of two or three years and then germinate upon the removal of the overhead canopy. Some of this seed is ordinarily deeply buried by rodents and may escape injury in the course of a forest fire. On account of this prolonged or delayed germi- nation and preservation of the seed there is usually a greater quantity of seed of this species available for germination subsequent to a fire than of any of the other trees in the forest.
All of the forest trees which are found in mixture with the white pine are listed below. The arrangement is such as to show the relative require- ments of the trees for light, moisture and soil fertility, and to bring out their
Tolerance Moisture Soil Permanency
W. yellow pine W. yellow pine Lodgepole pine Lodgepole pine Western larch Lodgepole pine W. yellow pine West. larch Lodgepole pine Douglas fir Douglas fir
Douglas fir West. larch West. larch Douglas fir W. white pine W. white pine Grand fir W. white pine Grand fir Grand fir W. white pine Grand fir
Alpine fir West. hemlock Alpine fir Alpine fir Eng. spruce Alpine fir Eng. spruce W. hemlock W. red cedar W. red cedar W. red cedar Eng. spruce W. hemlock Eng. spruce W. hemlock W. red cedar
70 J. A. LARSEN Ecology, Vol. X, No. I
permanency or lack of permanency in the forest as this develops toward the climax association for a particular site. The least shade-tolerant, the least moisture demanding, and the least fastidious as to soil quality, are placed first on the list.
The effect of these differences in requirements of light, moisture and soil is observed in adjustments of the species in distribution and composition according to local variations in the habitat. Whenever there occur a great variety of the species of trees in the unburned virgin forest there is a dis- tinct grouping in harmony with the above factors and requirements.
FIRST STAGE IN SUCCESSION
It would simplify our position greatly if we could consider first all fires occurring in the climax forest, for from this viewpoint we could more easily trace the influences of the fires and the various steps in the succession. Unfortunately fires do not wait for the completion of the cycle. They are of frequent occurrence, and relatively few stands are permitted to reach the true climax complexion before being overtaken by conflagrations. Nevertheless, whatever the condition of the forest at the time of its destruc- tion, lodgepole pine and western larch become established first on the most severely burned and denuded areas. After small fires the forest ordinarily comes back directly to cedar and hemlock, but after the larger fires the other species may come first, though sometimes with the climax trees in the mix- ture. One of the most noticeable effects of extensive and repeated burns is the appearance of considerable bodies of lodgepole pine and western larch.
These two species usually appear groupwise according to local conditions. The lodgepole pine grows nearly pure on the dry knolls and exposed ridges or otherwise denuded and deteriorated sites, especially toward the upper part of the western white pine zone. Pure stands of lodgepole pine may also be seen on gravelly slopes and benches of relatively low elevation. On north and east exposures, however, where the soil possesses greater mois- ture holding capacity than on south aspects, western larch generally appears in extensive bodies of pure forest in the first stage of the succession. Al- though both of these trees are intolerant of shade, the lodgepole pine seed- lings survive and the larch seedlings succumb on dry warm and exposed sites. On the cool and moist aspects, however, the situation is reversed.
Both of these species possess characteristics which enable them to re- claim burned ground speedily. The lodgepole pine produces seed at an as- tonishingly early age, and the seed is withheld and preserved within the unopened cone until the fires have prepared a suitable germinating bed. It is well known that some of the cones open and shed the seeds shortly after the fire, while numerous others remain firmly closed for an indefinite period. Unopened cones with enclosed seed have been found entirely over- grown with wood. The western larch, on the other hand, is favored by a
January, i929 FIRES AND FOREST SUCCESSION IN IDAHO 7I
remarkably long life and a very high degree of resistance to injury from fire, in that it possesses an unusually thick bark and a foliage of less inflammable nature than the lodgepole pine. On this account very good trees of larch will survive and bridge several critical fires, thereby retaining, after the fire, large seed-trees which become important factors in a speedy restocking with larch-provided only that the soil is not too warm and dry for the rather tender larch seedlings.
On sites suitable for the western white pine, and especially where the pine existed in large numbers before the occurrence on the fire, the white pine usually comes in strongly after the burn and becomes a part of the first stage in the succession.
Much depends also upon the size of the area burned. If the fire which takes place in the climax forest is small, the climax arborescent species will succeed themselves directly without going through the temporary and inter- mediate stages. The same is true of any climax forest anywhere. It may be observed to good advantage in the Beech-Birch-Hard maple forest of the northeast United States.
SECOND STAGE OF SUCCESSION
In the absence of fires in the temporary stands of lodgepole pine and western larch the western white pine and Douglas fir will establish them- selves under and among the former and eventually crowd them out. These intermediate species, when in possession of the ground, represent the second stage in the forest succession. Stands properly belonging to this intermedi- ate step in the succession are on the whole rather abundant. The reason is that the forest of this composition is an excellent soil builder, and because white pine and Douglas fir reproduce unusually well after single fires. Stands of these two species are considered the second stage in forest suc- cession whether they come in under lodgepole pine and larch, or directly after a single fire which has destroyed a climax cedar-hemlock forest or a white pine or Douglas fir forest. In fact the larger percentage of the western white pine stands in Idaho at present have succeeded directly after burns which have destroyed white pine and Douglas fir stands, that is, single fires. It is mainly after double fires that lodgepole pine and larch seed is in abun- dance.
THIRD AND FINAL STAGE IN SUCCESSION
As soon as the western white pine and its associates of intermediate light and moisture requirements reach maturity, begin to decay and thereby pro- duce a broken forest canopy, the western hemlock, western red cedar and grand fir begin underseeding. The forest is then in the early stages of the climax association. These last species of greater shade-tolerance than the former effectually prevent the natural reproduction of the white pine and the Douglas fir. It should be made clear that the grand fir, which is less
72 J. A. LARSEN Ecology, Vol. X, No. i
tolerant than the hemlock and cedar, is often found growing with white pine and Douglas fir in the intermediate stage. The fir is then present only as scattered individuals on northerly aspects, but may be in greater abundance on south slopes, particularly the lower parts of such slopes where the soil is deep and moist.
These climax species respond readily to local differences in the habitat. The western red cedar requires distinctly wet soils but not stagnant condi- tions of water; the hemlock needs a rather moist soil without free water, and the grand fir thrives with much less moisture than either cedar or hem- lock. The result of these different demands is a grouping of the species according to variations in the soil moisture. The cedar occurs in greatest abundance near the streams and on well watered soil. The hemlock is found much oftener on moist flats and on lower north aspects, while the grand fir prefers the rolling lowland and favorable south aspects.
Within the western white pine forest zone one may find considerable Engelmann spruce and alpine fir, but these trees do not properly belong in the intermediate zone. They are at home in the higher zone above the white pine. When found in mixture with the white pine, etc., it is only along the cool streams and in deep sheltered canyons where the soil is moist but cold and where there is comparatively little sunlight.
Of the three climax species mentioned above, the cedar is king. It is by far the largest and the longest lived tree in this region. It attains diameters from six to ten feet and an age up to 6oo years. It is furthermore much less afflicted with fungous and insect attacks than the other trees. The hem- lock and grand fir are in the great majority of cases defective. For these reasons many large, old cedars exist as the only remnants of a former climax forest. These large trees remain chiefly along the stream flats and in deep canyons, where they have escaped destruction by the fires which ravaged the slopes.
The natural tendency, therefore, of repeated forest fires in this forest zone is to limit more and more the numbers and distribution of the tolerant forest trees and relegate them to flats and canyons or lower slopes, while at the same time extending the ranges and increasing the numbers of the in- tolerant species, particularly western larch and lodgepole pine, and, to some extent also, the western white pine.
SUCCESSIONAL STAGES IN THE SUBORDINATE VEGETATION
In the first stage of the succession the subordinate species are mostly those of herbaceous character and of relatively rapid growth, as well as those whose seed is carried to great distances by the wind. The principal mem- bers of this association are:
Antennaria densiflora Nutt. Pearly everlasting Antennaria racemosa Hook. Everlasting
Chaemenarion angustifolium Scop. Fire weed Epilobium palmer Rydb. Willow herb Geranium carolinianum L. Wild geranium Salix scouleriana Barr. Willow Spiraea lucida Dougl. Spiraea Solidago sp. Golden rod Monarda menthaf olia Graham Horsemint Sidalcea neomexecana A Gray Hollyhock Xerophyllum tenax (Pursh) Nutt Pine grass
Among the above species the everlasting, hollyhock, pine grass, golden rod, horsemint and spiraea occupy southerly aspects, and the geranium, fireweed and others prefer the northerly slopes. Practically all of these temporary and shallow-rooted species are beneficial to the site and to the establish- ment of the arborescent species which follow, for, although they do not pre- vent erosion or appreciably increase the quality of the soil and site they throw considerable shade the result of which is a reduction of surface tem- peratures and decreased transpiration of the tender evergreen seedlings underneath. This protection is, of course, most needed on the south and west aspects. On moist and fertile soil the temporary herbaceous vegeta- tion is sometimes altogether too dense for the survival of evergreen seedlings, especially those of larch, lodgepole pine and Douglas fir.
SECOND STAGE IN THE SUCCESSION OF THE SUBORDINATE SPECIES
The subordinate species listed above give way after two or three years to a second stage of plants composed mainly of perennials, the majority of which possess berry fruits so that the seedling is effected by birds. This seedling evidently begins during the first season after the burning of the forest. In the case of the various species of Ribes information is far from complete. The investigations on the seedling of Ribes carried on in connec- tion with the blister rust control will undoubtedly yield some very interesting and valuable information in this respect. The principal species composing the second set of plants are:
Alnus tenuifolia Nutt. Alder Acer glabrum Torr. Rocky Mountain maple Betula fontinalis Sarg. Mountain birch Ceanothus velutinus Dougl. Mountain balm Cornus canadensis L. Trailing dogwood Cornus stolonifera Michx. Willow dogwood Fragaria sp. Strawberry Holodiscus dumosus Nutt. Ocean spray Lonicera utahensis Wats. Honeysuckle Lonicera involucrata Banks Honeysuckle
Prunus demissa Nutt. Choke cherry Ribes reniforme Rydb., Nutt. Currant Ribes viscossimum Dougl. Currant Rubus parviflorus Nutt. Thimble berry Rubus strigosus Michx. Raspberry Sambucus glauca Nutt. Elderberry Sambucus melanocarpa Gray. Elderberry Symphoricarpos racemosa Michx. Snowberry Sorbus angustifolia Rydb. Mountain maple Vaccinium nembranaceum Dougl. Huckleberry Vaccinium occidentale A. Gray. Huckleberry
Among the above species the Acer, Holodiscus, Ceanothus and Betula may be either wind disseminated or arise by sprouts from earlier specimens.
Lonicera, Ceanothus, Prunus, Holodiscus and Vaccinium are distinctly dry site species, while Betula, Alnus, Sambucus, Rubus parviflorus, Rubus strigosus and Cornus are moisture loving. The remaining species are some- what intermediate in their moisture requirements. After a second burn dry south slopes may support nothing but a scattered growth of Ceanothus velu- tinus or C. sanguineus. Along the water courses where the forest canopy is broken and small patches of fertile soil occurs, one may find impenetrable thickets of Cornus stolonifera, Spiraea tomentosa and Ribes reniforme.
The influence of this group of shrubs is distinctly beneficial, for the rapidly accumulating leaf mold expedites the rehabilitation of the impover- ished soil, the foliage shades and shelters seedlings of the mesophytic forest trees of fir, spruce and white pine, and the roots provide an effective check on soil erosion. This stage of the subordinate vegetation, therefore, becomes an important nurse crop for the white pine and its associated species. After eight or ten years the young white pines emerge from underneath the shrubs, overtop them, and crowd them out entirely. The maple, only, may persist within and underneath the new evergreen forest for a period of fifty to sixty years.
With the elimination of this second set of subordinate species the forest builds its own layers of litter and humus, paving the way for a third set of subordinate vegetation of somewhat uninfluential and unassuming propor- tions composed of the following species:
Adenocaulon bicolor Hook. Silver green Actea rubra Ait Baneberry Adiantum pedatum L. Maiden hair fern Boykinea heucheriforme Rydb. Boykinea Aralia nudicaulis L. Wild sarsaparilla Circea pacifica Asch. and Magn. Circea Claytonia chrysantha Greene Spring beauty
Celernatis occidentalis Homen. Virgins bower Clintonia uniftora (Schult) Kunth Clintonia Felix mas Wood fern Galium triflorum Michx Three-flowered bed straw Galium boreale L. Northern bed straw Limnia spathulata (Dougl.) Heller Miner's lettuce Mertensia spathulata (Dougl.) Howell Mentha canadensis L. Wild mint Mertensia ciliata (Torr) Don. Bluebell Osmorrhiza sp. Sweet cicily Seraphis gigantea Dougl. Hellebore Streptopus amplexifolius L. Trilium petulatum Purch. Trillium Valeriana septemtrionalis Rydb. Valerian Vagneria liliacea (Greene) Rydb. False Solomon seal Valeriana sitchensis Bong. Veronica scutellata L. Veronica Veratrurn Eschscholtzianum (R. & S.) Rydb. Skunk cabbage
Of the above, Boykinea, Seraphis, Streptopus, Montia, Mentha and Veratrum grow only along the streams or on the very wet ground under the broken forest canopy. The remaining species may be found anywhere on moist soil wherever the forest canopy is not dense. The influence of the above species on soil building or in preventing erosion is almost negligible. It is much less important than in any of the preceding associations.
Almost simultaneously with the invasion of this climax formation will appear the climax species in the subordinate forest vegetation. The out- standing species in the subordinate climax associations are the following:
Arctostaphyllos uva-ursi L. Kinnikinnick Asarum caudatum Lindl. Wild ginger Berberis sp. Oregon grape Chimalphila umbellata Nutt. Pipsissiwa or Princess pine Coptis occidentalis Nutt. T. and G. Gold thread Fatsia horrida Benth. and Hook. Devils club Heuchera ovalifolia Nutt. Alumroot Linnea borealis L. Twin flower Lycopodium selago L. Clubmoss Lycopodium sitchensis Rydb. Clubmoss Pachystima myrsinites Raf. Goat brush Pyrola uliginosa Torr. Round shinleaf Claytonia chrysantha Greene Spring beauty Tiarella unifoliata Hook. False miterwort Viola glabella Nutt. Violet
76 J. A. LARSEN Ecology, Vol. X, No. i
In this association, Fatsia and numerous ferns line the water courses; Coptis, Asarum, Pyrola, and Lycopodium are of the most tolerant, for they persist underneath the densest forest. In the intermediate class as to mois- ture and light requirements are Linnaea, Pachystima, Berberis, and among these one finds many species of the foregoing group, especially Tiarella, Claytonia, Galium, Montia and Circea.
The influence of this series upon erosion, soil building or protection to the seedlings is nil.
SUMMARY
When the cedar-hemlock-grand fir forest in the Bitterroot Mountains has been destroyed by fire the forest eventually returns to the climax composi- tion through a series of associations which follow each other in regular suc- cession, beginning in most cases with two intolerant or drought resistant species, western larch and lodgepole pine. This first stage, together with the distinct first stage of the subordinate vegetation improve the site and pave the way for a second forest association composed chiefly of Douglas fir and western white pine with its own peculiar subordinate type of vegetation. This second stage eventually gives way to the climax forest association of cedar, hemlock and grand fir and its own distinct and permanent subordinate vegetation of perennial evergreen species.
Front Matter
Studies in Animal Aggregations: Natural Aggregations of the Isopod, Asellus Communis [pp. 14-36]
Atometric Rates Read Instantaneously and an Automatic Continuous Recorder for Rate Fluctuation [pp. 37-46]
Comparison of the Physical Factors of Habitats [pp. 47-66]
Fires and Forest Succession in the Bitterroot Mountains of Northern Idaho [pp. 67-76]
Influence of Certain Water Conditions, Especially Dissolved Gasses, on Trout [pp. 77-96]
Fresh Water Studies. I. The Relative Temperature, Oxygen Content, Alkali Reserve, the Carbon Dioxide Tension and pH of the Waters of Certain Mountain Streams at Different Altitudes in the Smoky Mountain National Park [pp. 97-111]
Factors Controlling Forest Types on the Cloquet Forest, Minnesota [pp. 112-125]
Animal Communities of an Illinois Prairie [pp. 126-154]
Reviews
Review: untitled [pp. 157-158]
Review: An Ecological Study of Vegetation on Mt. Desert Island, Maine [pp. 158-160]
Notes and Comment
The Determination of Dissolved Oxygen with the Micro-Winkler Apparatus of Thompson and Miller [pp. 163-164]
Combined Edition of Clements' Plant Succession and Plant Indicators [pp. 164-165]
Back Matter
J. A. LARSEN
Iowa State College
Foresters have recently begun to seek a more intimate knowledge of the natural, successional stages by which forests regain terrain lost by extensive fires or other pronounced denuding agencies. Studies in this field lead to a closer understanding of the factors which control the distribution, compo- sition and density of the present forest, the silvical requirements of the various species which compose the existing forest and of the soil building or soil deteriorating influences which are operative after large fires. It is the author's privilege in this paper to record certain observations on forest succession in the Bitterroot Mountains in northern Idaho, gathered in the course of several years of forest research in that region. This is the terri- tory lying north of the Salmon river between the crest of the Bitterroot divide and the Columbia river plateau.
Perhaps no other region in the United States is visited by forest con- flagrations of similar magnitude, frequency or degree of destruction. The contributing causes are mainly as follows: the moist winter and spring which give rise to a profuse and luxuriant forest vegetation of a highly in- flammable nature; an invariably dry summer with afternoon temperatures often ranging between 95 and ioo degrees F. (350 to 380 C.); the low atmospheric humidity which frequently falls to I5 per cent and lower. These critical conditions are combined with strong, desiccating winds sweeping in from the arid region to the southwest. The fires, therefore, when once under way, travel with great speed and rapidly assume uncontrollable pro- portions. Not infrequently an entire township (approximately 95 sq. km.) of timber is wiped out in the course of one or two days.
These large forest fires kill all of the trees and the seedlings. In from ten to twenty years most of this dead timber lies prone, and it then presents a fire hazard of greater magnitude than existed in the green, virgin forest. The second fire, on this account, is even less controllable and more destructive than the first.
The return of the forest to the climax composition subsequent to these devastating fires proceeds along certain well defined steps or stages which may be considered parts of the entire cycle of succession, or the natural process of regeneration. The first steps in this succession begin with the appearance of species intolerant of shade, and capable of withstanding con- siderable drought and exposure, and concludes with the establishment of
67
This file was created by scanning the printed publication. Errors identified by the software have been corrected;
however, some errors may remain.
68 J. A. LARSEN Ecology, Vol. X, No. i
shade-tolerant trees or species which are both tolerant and moisture loving. The exact number of the steps in the succession, depends, however, in a great measure upon the factors of topography, soil, degree of exposure, the intensity of the fire and the amount of destruction wrought.
It is of considerable interest to note that the subordinate forest vegeta- tion also develops after these fires in regular successional stages in its pro- gression toward the climax subordinate vegetation, and that the final, climax association, or ultimate type, is attained and developed underneath the climax type or association of the forest trees. Those subordinate species of tem- porary occupancy become established immediately after the forest fire and generally simultaneously with the appearance of the temporary arborescent species, while those of permanent occupation follow directly upon the heels of the climax arborescent species.
In composition the subordinate vegetation which follows the fires is also governed by the condition of the habitat, the degree of destruction and the requirements of the individual species, so that in the subordinate flora there is also distinct grouping in accordance with these characters or qualities. However, since the seedlings of the aborescent species develop very slowly during the first three or four years, and much slower than the annuals of the subordinate flora, the latter are in much greater evidence in the beginning. A superficial examination would ordinarily give the impression that there existed nothing but these rapidly growing and very dense annuals during the first few years following the fire.
FOREST ZONES AND SILVICAL REQUIREMENTS OF THE TREES
In this region, in which the elevations of the mountains vary from 6,ooo to 8,ooo feet (I,800 to 2,400 meters) above sea level, and the valleys from 2,000 to 3,ooo feet (6oo to goo meters), the western yellow pine forests, which are composed mainly of Pinus ponderosa, occupy the lowest zone of tree growth; the western white pine (Pinus monticola), with its associated species of western red cedar (Thuja plicata), western hemlock (Tsuga heterophylla), grand fir (Abies grandis), Douglas fir (Pseudotsuga taxi- folia), and western larch (Larix occidentalis), occupy the slopes of inter- mediate elevation ranging from 2,500 feet to 5,500 feet. Above this middle zone the forest is composed of subalpine species represented by Engelmann spruce (Picea engelmannii), mountain hemlock (Tsuga mertensiana), white bark pine (Pinus albicaulis), alpine fir (Abies lasiocarpa), and lodgepole pine (Pinus murrayana var. contorta). These subalpine species are fre- quently individually present also within the intermediate forest zone. The discussion in this article deals exclusively with the conditions surrounding the succession within the intermediate forest zone.
This middle forest zone is designated by foresters in the region as the western white pine type, chiefly because of the abundance and economic im- portance of the white pine. This tree is not a climax species, however, for
January, i929 FIRES AND FOREST SUCCESSION IN IDAHO 69
hemlock, western red cedar and grand fir are the only true climax repre- sentatives among the trees.
The western white pine and its associated species thrive best on well drained and fertile soil such as is found on the protected easterly or north- erly gradients and on lower slopes, bottoms and benches. In these locations the soil remains moist throughout the entire summer, being sheltered by a broken topography, a very dense crown canopy of the trees, and covered by deep layers of " duff " and humus.
The forests composed chiefly of western white pine are usually even- aged and rather dense. Over large areas the pine is the predominant spe- cies. It maintains a rather rapid and remarkably uniform rate of height growth by which, in most cases and situations, it overtops its competitors. Although both the lodgepole pine and the western larch grow as fast as the pine in most locations, these two are more intolerant of shade and therefore not in a position to arrest its growth seriously or to crowd it out. Mixed with the western white pine there is generally an understory of hemlock, cedar or grand fir wherever the soil is rather moist or the site well pro- tected, especially at the low elevations, while toward the upper margin of the zone and more particularly on the exposed south of west aspects we find more of the Douglas fir and lodgepole pine.
Another characteristic of the western white pine, and one which gives it a considerable advantage over the associated species, is the ability of its seed to lie over in the duff on the forest floor for a period of two or three years and then germinate upon the removal of the overhead canopy. Some of this seed is ordinarily deeply buried by rodents and may escape injury in the course of a forest fire. On account of this prolonged or delayed germi- nation and preservation of the seed there is usually a greater quantity of seed of this species available for germination subsequent to a fire than of any of the other trees in the forest.
All of the forest trees which are found in mixture with the white pine are listed below. The arrangement is such as to show the relative require- ments of the trees for light, moisture and soil fertility, and to bring out their
Tolerance Moisture Soil Permanency
W. yellow pine W. yellow pine Lodgepole pine Lodgepole pine Western larch Lodgepole pine W. yellow pine West. larch Lodgepole pine Douglas fir Douglas fir
Douglas fir West. larch West. larch Douglas fir W. white pine W. white pine Grand fir W. white pine Grand fir Grand fir W. white pine Grand fir
Alpine fir West. hemlock Alpine fir Alpine fir Eng. spruce Alpine fir Eng. spruce W. hemlock W. red cedar W. red cedar W. red cedar Eng. spruce W. hemlock Eng. spruce W. hemlock W. red cedar
70 J. A. LARSEN Ecology, Vol. X, No. I
permanency or lack of permanency in the forest as this develops toward the climax association for a particular site. The least shade-tolerant, the least moisture demanding, and the least fastidious as to soil quality, are placed first on the list.
The effect of these differences in requirements of light, moisture and soil is observed in adjustments of the species in distribution and composition according to local variations in the habitat. Whenever there occur a great variety of the species of trees in the unburned virgin forest there is a dis- tinct grouping in harmony with the above factors and requirements.
FIRST STAGE IN SUCCESSION
It would simplify our position greatly if we could consider first all fires occurring in the climax forest, for from this viewpoint we could more easily trace the influences of the fires and the various steps in the succession. Unfortunately fires do not wait for the completion of the cycle. They are of frequent occurrence, and relatively few stands are permitted to reach the true climax complexion before being overtaken by conflagrations. Nevertheless, whatever the condition of the forest at the time of its destruc- tion, lodgepole pine and western larch become established first on the most severely burned and denuded areas. After small fires the forest ordinarily comes back directly to cedar and hemlock, but after the larger fires the other species may come first, though sometimes with the climax trees in the mix- ture. One of the most noticeable effects of extensive and repeated burns is the appearance of considerable bodies of lodgepole pine and western larch.
These two species usually appear groupwise according to local conditions. The lodgepole pine grows nearly pure on the dry knolls and exposed ridges or otherwise denuded and deteriorated sites, especially toward the upper part of the western white pine zone. Pure stands of lodgepole pine may also be seen on gravelly slopes and benches of relatively low elevation. On north and east exposures, however, where the soil possesses greater mois- ture holding capacity than on south aspects, western larch generally appears in extensive bodies of pure forest in the first stage of the succession. Al- though both of these trees are intolerant of shade, the lodgepole pine seed- lings survive and the larch seedlings succumb on dry warm and exposed sites. On the cool and moist aspects, however, the situation is reversed.
Both of these species possess characteristics which enable them to re- claim burned ground speedily. The lodgepole pine produces seed at an as- tonishingly early age, and the seed is withheld and preserved within the unopened cone until the fires have prepared a suitable germinating bed. It is well known that some of the cones open and shed the seeds shortly after the fire, while numerous others remain firmly closed for an indefinite period. Unopened cones with enclosed seed have been found entirely over- grown with wood. The western larch, on the other hand, is favored by a
January, i929 FIRES AND FOREST SUCCESSION IN IDAHO 7I
remarkably long life and a very high degree of resistance to injury from fire, in that it possesses an unusually thick bark and a foliage of less inflammable nature than the lodgepole pine. On this account very good trees of larch will survive and bridge several critical fires, thereby retaining, after the fire, large seed-trees which become important factors in a speedy restocking with larch-provided only that the soil is not too warm and dry for the rather tender larch seedlings.
On sites suitable for the western white pine, and especially where the pine existed in large numbers before the occurrence on the fire, the white pine usually comes in strongly after the burn and becomes a part of the first stage in the succession.
Much depends also upon the size of the area burned. If the fire which takes place in the climax forest is small, the climax arborescent species will succeed themselves directly without going through the temporary and inter- mediate stages. The same is true of any climax forest anywhere. It may be observed to good advantage in the Beech-Birch-Hard maple forest of the northeast United States.
SECOND STAGE OF SUCCESSION
In the absence of fires in the temporary stands of lodgepole pine and western larch the western white pine and Douglas fir will establish them- selves under and among the former and eventually crowd them out. These intermediate species, when in possession of the ground, represent the second stage in the forest succession. Stands properly belonging to this intermedi- ate step in the succession are on the whole rather abundant. The reason is that the forest of this composition is an excellent soil builder, and because white pine and Douglas fir reproduce unusually well after single fires. Stands of these two species are considered the second stage in forest suc- cession whether they come in under lodgepole pine and larch, or directly after a single fire which has destroyed a climax cedar-hemlock forest or a white pine or Douglas fir forest. In fact the larger percentage of the western white pine stands in Idaho at present have succeeded directly after burns which have destroyed white pine and Douglas fir stands, that is, single fires. It is mainly after double fires that lodgepole pine and larch seed is in abun- dance.
THIRD AND FINAL STAGE IN SUCCESSION
As soon as the western white pine and its associates of intermediate light and moisture requirements reach maturity, begin to decay and thereby pro- duce a broken forest canopy, the western hemlock, western red cedar and grand fir begin underseeding. The forest is then in the early stages of the climax association. These last species of greater shade-tolerance than the former effectually prevent the natural reproduction of the white pine and the Douglas fir. It should be made clear that the grand fir, which is less
72 J. A. LARSEN Ecology, Vol. X, No. i
tolerant than the hemlock and cedar, is often found growing with white pine and Douglas fir in the intermediate stage. The fir is then present only as scattered individuals on northerly aspects, but may be in greater abundance on south slopes, particularly the lower parts of such slopes where the soil is deep and moist.
These climax species respond readily to local differences in the habitat. The western red cedar requires distinctly wet soils but not stagnant condi- tions of water; the hemlock needs a rather moist soil without free water, and the grand fir thrives with much less moisture than either cedar or hem- lock. The result of these different demands is a grouping of the species according to variations in the soil moisture. The cedar occurs in greatest abundance near the streams and on well watered soil. The hemlock is found much oftener on moist flats and on lower north aspects, while the grand fir prefers the rolling lowland and favorable south aspects.
Within the western white pine forest zone one may find considerable Engelmann spruce and alpine fir, but these trees do not properly belong in the intermediate zone. They are at home in the higher zone above the white pine. When found in mixture with the white pine, etc., it is only along the cool streams and in deep sheltered canyons where the soil is moist but cold and where there is comparatively little sunlight.
Of the three climax species mentioned above, the cedar is king. It is by far the largest and the longest lived tree in this region. It attains diameters from six to ten feet and an age up to 6oo years. It is furthermore much less afflicted with fungous and insect attacks than the other trees. The hem- lock and grand fir are in the great majority of cases defective. For these reasons many large, old cedars exist as the only remnants of a former climax forest. These large trees remain chiefly along the stream flats and in deep canyons, where they have escaped destruction by the fires which ravaged the slopes.
The natural tendency, therefore, of repeated forest fires in this forest zone is to limit more and more the numbers and distribution of the tolerant forest trees and relegate them to flats and canyons or lower slopes, while at the same time extending the ranges and increasing the numbers of the in- tolerant species, particularly western larch and lodgepole pine, and, to some extent also, the western white pine.
SUCCESSIONAL STAGES IN THE SUBORDINATE VEGETATION
In the first stage of the succession the subordinate species are mostly those of herbaceous character and of relatively rapid growth, as well as those whose seed is carried to great distances by the wind. The principal mem- bers of this association are:
Antennaria densiflora Nutt. Pearly everlasting Antennaria racemosa Hook. Everlasting
Chaemenarion angustifolium Scop. Fire weed Epilobium palmer Rydb. Willow herb Geranium carolinianum L. Wild geranium Salix scouleriana Barr. Willow Spiraea lucida Dougl. Spiraea Solidago sp. Golden rod Monarda menthaf olia Graham Horsemint Sidalcea neomexecana A Gray Hollyhock Xerophyllum tenax (Pursh) Nutt Pine grass
Among the above species the everlasting, hollyhock, pine grass, golden rod, horsemint and spiraea occupy southerly aspects, and the geranium, fireweed and others prefer the northerly slopes. Practically all of these temporary and shallow-rooted species are beneficial to the site and to the establish- ment of the arborescent species which follow, for, although they do not pre- vent erosion or appreciably increase the quality of the soil and site they throw considerable shade the result of which is a reduction of surface tem- peratures and decreased transpiration of the tender evergreen seedlings underneath. This protection is, of course, most needed on the south and west aspects. On moist and fertile soil the temporary herbaceous vegeta- tion is sometimes altogether too dense for the survival of evergreen seedlings, especially those of larch, lodgepole pine and Douglas fir.
SECOND STAGE IN THE SUCCESSION OF THE SUBORDINATE SPECIES
The subordinate species listed above give way after two or three years to a second stage of plants composed mainly of perennials, the majority of which possess berry fruits so that the seedling is effected by birds. This seedling evidently begins during the first season after the burning of the forest. In the case of the various species of Ribes information is far from complete. The investigations on the seedling of Ribes carried on in connec- tion with the blister rust control will undoubtedly yield some very interesting and valuable information in this respect. The principal species composing the second set of plants are:
Alnus tenuifolia Nutt. Alder Acer glabrum Torr. Rocky Mountain maple Betula fontinalis Sarg. Mountain birch Ceanothus velutinus Dougl. Mountain balm Cornus canadensis L. Trailing dogwood Cornus stolonifera Michx. Willow dogwood Fragaria sp. Strawberry Holodiscus dumosus Nutt. Ocean spray Lonicera utahensis Wats. Honeysuckle Lonicera involucrata Banks Honeysuckle
Prunus demissa Nutt. Choke cherry Ribes reniforme Rydb., Nutt. Currant Ribes viscossimum Dougl. Currant Rubus parviflorus Nutt. Thimble berry Rubus strigosus Michx. Raspberry Sambucus glauca Nutt. Elderberry Sambucus melanocarpa Gray. Elderberry Symphoricarpos racemosa Michx. Snowberry Sorbus angustifolia Rydb. Mountain maple Vaccinium nembranaceum Dougl. Huckleberry Vaccinium occidentale A. Gray. Huckleberry
Among the above species the Acer, Holodiscus, Ceanothus and Betula may be either wind disseminated or arise by sprouts from earlier specimens.
Lonicera, Ceanothus, Prunus, Holodiscus and Vaccinium are distinctly dry site species, while Betula, Alnus, Sambucus, Rubus parviflorus, Rubus strigosus and Cornus are moisture loving. The remaining species are some- what intermediate in their moisture requirements. After a second burn dry south slopes may support nothing but a scattered growth of Ceanothus velu- tinus or C. sanguineus. Along the water courses where the forest canopy is broken and small patches of fertile soil occurs, one may find impenetrable thickets of Cornus stolonifera, Spiraea tomentosa and Ribes reniforme.
The influence of this group of shrubs is distinctly beneficial, for the rapidly accumulating leaf mold expedites the rehabilitation of the impover- ished soil, the foliage shades and shelters seedlings of the mesophytic forest trees of fir, spruce and white pine, and the roots provide an effective check on soil erosion. This stage of the subordinate vegetation, therefore, becomes an important nurse crop for the white pine and its associated species. After eight or ten years the young white pines emerge from underneath the shrubs, overtop them, and crowd them out entirely. The maple, only, may persist within and underneath the new evergreen forest for a period of fifty to sixty years.
With the elimination of this second set of subordinate species the forest builds its own layers of litter and humus, paving the way for a third set of subordinate vegetation of somewhat uninfluential and unassuming propor- tions composed of the following species:
Adenocaulon bicolor Hook. Silver green Actea rubra Ait Baneberry Adiantum pedatum L. Maiden hair fern Boykinea heucheriforme Rydb. Boykinea Aralia nudicaulis L. Wild sarsaparilla Circea pacifica Asch. and Magn. Circea Claytonia chrysantha Greene Spring beauty
Celernatis occidentalis Homen. Virgins bower Clintonia uniftora (Schult) Kunth Clintonia Felix mas Wood fern Galium triflorum Michx Three-flowered bed straw Galium boreale L. Northern bed straw Limnia spathulata (Dougl.) Heller Miner's lettuce Mertensia spathulata (Dougl.) Howell Mentha canadensis L. Wild mint Mertensia ciliata (Torr) Don. Bluebell Osmorrhiza sp. Sweet cicily Seraphis gigantea Dougl. Hellebore Streptopus amplexifolius L. Trilium petulatum Purch. Trillium Valeriana septemtrionalis Rydb. Valerian Vagneria liliacea (Greene) Rydb. False Solomon seal Valeriana sitchensis Bong. Veronica scutellata L. Veronica Veratrurn Eschscholtzianum (R. & S.) Rydb. Skunk cabbage
Of the above, Boykinea, Seraphis, Streptopus, Montia, Mentha and Veratrum grow only along the streams or on the very wet ground under the broken forest canopy. The remaining species may be found anywhere on moist soil wherever the forest canopy is not dense. The influence of the above species on soil building or in preventing erosion is almost negligible. It is much less important than in any of the preceding associations.
Almost simultaneously with the invasion of this climax formation will appear the climax species in the subordinate forest vegetation. The out- standing species in the subordinate climax associations are the following:
Arctostaphyllos uva-ursi L. Kinnikinnick Asarum caudatum Lindl. Wild ginger Berberis sp. Oregon grape Chimalphila umbellata Nutt. Pipsissiwa or Princess pine Coptis occidentalis Nutt. T. and G. Gold thread Fatsia horrida Benth. and Hook. Devils club Heuchera ovalifolia Nutt. Alumroot Linnea borealis L. Twin flower Lycopodium selago L. Clubmoss Lycopodium sitchensis Rydb. Clubmoss Pachystima myrsinites Raf. Goat brush Pyrola uliginosa Torr. Round shinleaf Claytonia chrysantha Greene Spring beauty Tiarella unifoliata Hook. False miterwort Viola glabella Nutt. Violet
76 J. A. LARSEN Ecology, Vol. X, No. i
In this association, Fatsia and numerous ferns line the water courses; Coptis, Asarum, Pyrola, and Lycopodium are of the most tolerant, for they persist underneath the densest forest. In the intermediate class as to mois- ture and light requirements are Linnaea, Pachystima, Berberis, and among these one finds many species of the foregoing group, especially Tiarella, Claytonia, Galium, Montia and Circea.
The influence of this series upon erosion, soil building or protection to the seedlings is nil.
SUMMARY
When the cedar-hemlock-grand fir forest in the Bitterroot Mountains has been destroyed by fire the forest eventually returns to the climax composi- tion through a series of associations which follow each other in regular suc- cession, beginning in most cases with two intolerant or drought resistant species, western larch and lodgepole pine. This first stage, together with the distinct first stage of the subordinate vegetation improve the site and pave the way for a second forest association composed chiefly of Douglas fir and western white pine with its own peculiar subordinate type of vegetation. This second stage eventually gives way to the climax forest association of cedar, hemlock and grand fir and its own distinct and permanent subordinate vegetation of perennial evergreen species.
Front Matter
Studies in Animal Aggregations: Natural Aggregations of the Isopod, Asellus Communis [pp. 14-36]
Atometric Rates Read Instantaneously and an Automatic Continuous Recorder for Rate Fluctuation [pp. 37-46]
Comparison of the Physical Factors of Habitats [pp. 47-66]
Fires and Forest Succession in the Bitterroot Mountains of Northern Idaho [pp. 67-76]
Influence of Certain Water Conditions, Especially Dissolved Gasses, on Trout [pp. 77-96]
Fresh Water Studies. I. The Relative Temperature, Oxygen Content, Alkali Reserve, the Carbon Dioxide Tension and pH of the Waters of Certain Mountain Streams at Different Altitudes in the Smoky Mountain National Park [pp. 97-111]
Factors Controlling Forest Types on the Cloquet Forest, Minnesota [pp. 112-125]
Animal Communities of an Illinois Prairie [pp. 126-154]
Reviews
Review: untitled [pp. 157-158]
Review: An Ecological Study of Vegetation on Mt. Desert Island, Maine [pp. 158-160]
Notes and Comment
The Determination of Dissolved Oxygen with the Micro-Winkler Apparatus of Thompson and Miller [pp. 163-164]
Combined Edition of Clements' Plant Succession and Plant Indicators [pp. 164-165]
Back Matter
