P Jdisturbanceregimes Shortsynthesis 5 07

William Romme, Craig Allen, John Bailey, William Baker, Brandon Bestelmeyer, Peter Brown,Karen Eisenhart, Lisa Floyd-Hanna, David Huffman, Brian Jacobs, Richard Miller, Esteban

Muldavin, Thomas Swetnam, Robin Tausch, Peter Weisberg

May 2007

Historical and ModernDisturbance Regimes

of Piñon-JJuniperVegetation in the

Western U.S.

Citation: Romme, W., C. Allen, J. Bailey, W. Baker, B. Bestelmeyer, P. Brown, K. Eisenhart, L. Floyd-Hanna, D.Huffman, B. Jacobs, R. Miller, E. Muldavin, T. Swetnam, R. Tausch and P. Weisberg. 2007. Historical and ModernDisturbance Regimes of Piñon-Junifer Vegetation in the Western U.S.

Cover photo: Piñon-juniper woodland, Walker Ranch, near Pueblo, Colorado. © Peter McBride

Contents

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Piñon-Juniper: A Variable and Diverse Type of Vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

What We Know About Persistent Woodlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

What We Know About Piñon-Juniper Savannas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

What We Know About Areas of Potential Expansion and Contraction . . . . . . . . . . . . . . . . .11

Tables

1. A suggested key for identifying the three historical types of piñon-juniper vegetation discussed in this paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2. Tips for estimating tree ages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

iii - Contents

Piñon-juniper vegetation covers some100 million acres in the western U.S.,where it provides economic products,ecosystem services, biodiversity, andaesthetic beauty in some of the mostscenic landscapes of North America.There are concerns, however, that theecological dynamics of piñon-juniperwoodlands have changed since Euro-American settlement, that trees aregrowing unnaturally dense, and thatwoodlands are encroaching into formergrasslands and shrublands. Yetsurprisingly little research has beenconducted on historical conditions andecological processes in piñon-junipervegetation, and the research that doesexist demonstrates that piñon-juniperstructure, composition, and disturbanceregimes were very diverse historically aswell as today.

Uncertainties about historical standstructures and disturbance regimes inpiñon-juniper vegetation create a seriousconundrum for land managers and policy-makers who are charged with overseeingthe semi-arid landscapes of the West.Calls for restoration of historical conditions(i.e., those that prevailed before thechanges wrought by Euro-Americansettlers) are being heard with increasingfrequency. However, if those historicalconditions are poorly understood it isdifficult or impossible to set realistictargets for restoration. In the absence ofsite-specific information about historicaldisturbance regimes and landscapedynamics, there is danger that well-meaning “restoration” efforts actually may

move piñon-juniper ecosystems fartherfrom their historical condition.

The purpose of this report is to brieflysummarize our current understanding ofhistorical stand structures, disturbanceregimes, and landscape dynamics inpiñon-juniper vegetation throughout thewestern U.S. The authors gathered for aworkshop in Boulder, CO, on August 22-24, 2006, to develop the informationpresented here. All have conductedresearch in piñon-juniper vegetation, andtogether they have experience with a widediversity of piñon-juniper ecosystems, fromNew Mexico and Colorado to Nevada andOregon. Our summary of currentknowledge is presented as a series ofstatements, each followed by our level ofconfidence in the statement and thegeographical area to which it applies.High-confidence statements generally aresupported by rigorous field studies;statements of moderate confidence arebased on anecdotal evidence or logicaldeductions, but lack the rigorous studiesnecessary to confirm or refute the idea.We intentionally refrain from making anyspecific policy or managementrecommendations in this paper. Instead weprovide the consensus among researchersof what we know (and don’t know) aboutthe science of piñon-juniper vegetation.The authors encourage managers andpractitioners to give us feedback on theutility and shortcomings of this summary.There is much that we do not know aboutpiñon-juniper vegetation, and newsyntheses of this kind will be needed inthe future as new knowledge becomesavailable.

1 - Introduction

Introduction

We identify three fundamentally differenttypes of piñon-juniper vegetation, basedprimarily on canopy structure, understorycharacteristics, and historical disturbanceregimes. There is of course great diversitywithin each of the three basic types, butthe classification below represents muchof the variability in piñon-junipervegetation across the western U.S.Research is underway to link thesevegetation types to specific soil types andother environmental characteristics thatwould allow for reliable prediction andmapping across large landscapes, but atpresent we can only identify some verygeneral environmental correlates. Table 1

is a key for distinguishing among thethree types in the field.

(1) Persistent Woodlands are found wheresite conditions (soils and climate) areinherently favorable for piñon and/orjuniper. Canopy and understorycharacteristics vary considerably fromplace to place, from sparse stands ofscattered small trees growing on poorsubstrates (Figure 1) to relatively densestands of large trees on productive sites(Figure 2). Tree density and canopy coveralso may fluctuate in response todisturbance and climatic variability.Nevertheless, these are plant

2 - Piñon-Juniper: A Variable andDiverse Type of Vegetation

Piñon-Juniper: A Variable & DiverseType of Vegetation

Figure 1. Persistent woodland, growing on shallow rocky soils on the mesa tops, with deepercolluvial soils in the canyon bottom. The canopy is composed of Pinus edulis and Juniperusosteosperma. The trees are all-aged, including many old individuals, and the woodlands within thisview contain no evidence of past fire. Colorado National Monument, Colorado, elevation ca. 6,000feet. (Photo by W. Romme, 2006)

communities in which piñon and/orjuniper are dominant species, bothhistorically and currently, unless recentlydisturbed by fire, clearing, or other severedisturbance. Notably, these woodlands donot represent 20th century conversion offormerly non-woodland vegetation typesto woodland.

Persistent Woodlands are typically foundon rugged upland sites with shallow,coarse-textured soils that supportrelatively sparse herbaceous cover even inthe absence of heavy livestock grazing.However, they also occur in a variety ofother settings, such that steep terrain andshallow or rocky substrate are not bythemselves adequate indicators. This typeof piñon-juniper vegetation is found

throughout the West; it appears to beespecially prevalent on portions of theColorado Plateau, where moisture comespredominantly in winter. Indeed, largeexpanses of the Colorado Plateau arecharacterized by ancient, sparse-density,persistent woodlands within spectacularrocky landscapes.

(2) Piñon-juniper savannas are usuallyfound on gentle upland and transitionalvalley locations, where soil conditionsfavor grasses (or other grass-like plants)but can support at least some tree cover.Some savannas apparently have sparsetree cover because of edaphic or climaticlimitations on woody plant growth; manyof these savannas have probably changedlittle during the past century (Figure 3).


Figure 2. Persistent woodland, growing on a moderately productive site with high canopy cover andsparse herbaceous understory. The canopy is composed of Pinus edulis and Juniperus osteosperma;the major understory shrub is Artemisia tridentata. Trees are all-aged, including many individuals >300 years old, and the stand contains no evidence of past fire. Navajo Point, Glen Canyon NationalRecreation Area, Utah, elevation ca. 7,000 feet. (Photo by W. Romme, 2005)

Other savannas, in contrast, have siteconditions that could potentially supportdenser tree cover, but at these locationssavanna structure is maintained byherbaceous competition, recurrent fire,drought, and other disturbances; many ofthese savannas have converted intodenser woodlands during the past centurybecause of release from competitiveeffects on tree seedling establishment,favorable climatic conditions for treesurvival, and lack of fire (Figure 4). Somesavannas and woodlands are found todayin places that were tree-less grasslandsprior to 1900; such sites can sometimesbe recognized by remnant grasslands soilsand species composition.

Piñon-juniper savannas typically are foundon moderately deep, coarse to fine-

textured soils that readily support a varietyof growth forms including trees, grasses,and other herbaceous plants, and inregions that receive reliable summerrainfall that fosters growth of warm-season grasses. This type of piñon-junipervegetation appears to be especiallyprevalent in the basins and foothills ofsouthern New Mexico, where a largeportion of annual precipitation comes inthe summer via monsoon rains. However,it is relatively rare in the Rocky Mountains,northern Colorado Plateau, and the GreatBasin, where precipitation is more winterdominated.

(3) Areas of potential woodlandexpansion & contraction are found wheresite conditions are only intermittentlysuitable for piñon and/or juniper


Figure 3. Juniper savanna, growing in fine-textured soils in a region of low precipitation where lowtree density probably is a result of competition for moisture rather than fire or other disturbances.Trees are predominantly Juniperus osteosperma, with occasional Juniperus monosperma andPinus edulis. Many of the trees appear to be relatively old (>150 years), i.e., crowns are flattenedand branches are large and gnarly (Table 2). Evidence of past fire is scarce. San Juan Basin,northwestern New Mexico, elevation ca. 6,800 feet. (Photo by B. Jacobs, 2006)

establishment and survival over long timescales. Trees may become more abundantduring moist climatic periods or duringlong disturbance-free intervals, butsubsequently die back during drought andassociated insect outbreaks or followingfire. Many woodlands of this type have ashrub-dominated understory (Figure 5).Some such areas may have a distinctivehistorical fire regime, and therefore couldbe classified as a separate piñon-juniper-shrub type if such a distinction is usefulfor management. This vegetation typeoverlaps somewhat with disturbance-maintained savannas, but refers primarilyto areas that fluctuate between shrublandor grassland structure and treedominance, as well as other sites where

soils or climate are only occasionallysuitable for tree growth and survival.

Areas of potential woodland expansionand contraction are found on a widevariety of substrates and climaticconditions. Such areas sometimes arelocated adjacent to persistent woodlandsor savannas, where soil conditionsgradually change from favorable tomarginal for tree growth. However, precisebioclimatic conditions associated withareas of potential piñon-juniper expansionand contraction are poorly defined at thistime. This type of piñon-juniper vegetationis found throughout the West; it appearsto be especially prevalent in the GreatBasin.


Figure 4. Piñon-juniper savanna, growing in relatively deep soils on gentle terrain, in a region ofsummer-dominated precipitation. Trees are predominantly Juniperus monosperma with occasionalPinus edulis. Most trees are less than 150 years old, but there is at least one older tree per acre.Blue grama (Bouteloua gracilis) is the dominant grass; cholla cactus (Opuntia imbricata) is alsopresent. With a well-developed herbaceous stratum within a relatively productive environment, lowtree density at this site may have been maintained historically by periodic fire. However, fire historystudies have not been conducted in this area to confirm or reject this hypothesis. NearMountainair, New Mexico, elevation ca. 6,400 feet. (Photo by W. Romme, 2006)


Figure 5. An area of potential expansion and contraction, on fine-textured soils in a region ofmoderate, winter-dominated precipitation. The canopy is composed of Pinus edulis, Juniperusosteosperma, and Juniperus monosperma. The prominent shrub is Artemisia tridentata. The treesare all-aged. Note the relatively old-appearing (>150 years, Table 2) piñon trees in thebackground, with flattened crowns and large branches near the top; and the younger-appearingpiñon and juniper trees in the foreground, with spire-shaped crowns. The younger trees areevidence of expansion during the past century, but contraction also has occurred during a severeregional piñon mortality event within the past decade (see Figure 7). Evidence of past fire isscarce. San Juan Basin, northwestern New Mexico, elevation ca. 6,800 feet. (Photo by B. Jacobs,2006)

1. Spreading, low-intensity surface fireshad a very limited role in moldingstand structure and dynamics ofpersistent piñon-juniper woodlands inthe historical landscape. In very sparsewoodlands, especially on rocky terrain,fires typically burned individual treesbut did not spread extensively becauseof lack of surface fuels. Surface spreadwas more likely to occur in higher-density woodlands growing on moreproductive sites, but even here firespread was typically from crown to

crown, especially when wind-driven.Regardless of tree density, thedominant fire effect was to kill most orall trees and to top-kill most or allshrubs (Figure 6). This statement alsois true of most fires today. HIGH CONFIDENCE … APPLIES TOPERSISTENT WOODLANDSTHROUGHOUT THE WEST

2. Historical fires in persistent piñon-juniper woodlands generally did not“thin from below,” i.e., they did not kill

7 - What We Know About Persistent Woodlands

What We Know About PersistentWoodlands

Figure 6. A recent stand-replacing burn in persistent woodlands, the ca. 25,000-acre MustangFire of 2002, photographed in fall of 2006. Note the sharp boundaries between burned andunburned woodlands, and the nearly complete canopy mortality within the burn perimeter —typical characteristics of large fires in persistent piñon-juniper woodlands, both historically and atpresent. Trees are Pinus edulis and Juniperus osteosperma. Near Flaming Gorge Reservoir,northeastern Utah, elevation ca. 6,200 feet. (Photo by C. D. Allen, 2006)

predominantly small trees. Instead,they tended to kill all or most of thetrees within the places that burnedregardless of tree size. This statementalso is true of most fires today. HIGH CONFIDENCE … APPLIES TOPERSISTENT WOODLANDSTHROUGHOUT THE WEST

3. Historical fire rotations (i.e., the timerequired for the cumulative areaburned to equal the size of the entirearea of interest) varied from place toplace in persistent piñon-juniperwoodlands, but generally were verylong (e.g., two to six centuries). Thisstatement also is true of mostpersistent woodlands today. HIGH CONFIDENCE … APPLIES TO

PERSISTENT WOODLANDSTHROUGHOUT THE WEST

4. Some persistent woodlands are stablefor hundreds or even thousands ofyears without fire, other than isolatedlightning ignitions that burn only asingle tree and produce no significantchange in stand structure. Manywoodlands today show no evidence ofpast fire, though they may haveburned in the very remote past (manyhundreds or thousands of years ago).This may be true especially of verysparse woodlands in rocky areas, butis also true of many higher-densitywoodlands growing on moreproductive sites. HIGH CONFIDENCE … BUT PRECISEGEOGRAPHIC APPLICABILITY NOTADEQUATELY KNOWN

5. In some persistent woodlands, standdynamics are driven more by climatic

fluctuation, insects, and disease thanby fire. For example, a massive piñonmortality event occurred recently inthe Four Corners region as a result ofdrought, high temperatures, and barkbeetle outbreaks (Figure 7).HIGH CONFIDENCE … BUT PRECISEGEOGRAPHIC APPLICABILITY NOTADEQUATELY KNOWN

6. Tree density and canopy coverage haveincreased in many or most persistentwoodlands during the 20th century. HIGH CONFIDENCE … BUT PRECISEMAGNITUDE OF INCREASE, CAUSES,AND GEOGRAPHIC APPLICABILITYNOT ADEQUATELY KNOWN

7. The gradual increase in tree densityand canopy cover during the 20thcentury in most persistent PJwoodlands is not due entirely, or evenprimarily, to fire exclusion. Effects ofpast livestock grazing and/or favorableclimatic conditions for tree growthduring most of the 20th centuryprobably are more importantmechanisms in most areas. However,we do not yet fully understand howgrazing, climate, and fire exclusioninteract to promote increased treedensity, nor can we say withconfidence which of thesemechanisms is most important in anyspecific location. In addition, somestands that resemble persistentwoodlands — i.e., with dense treecanopies, shallow rocky soils, andsparse herbaceous cover — actuallymay represent an eroded anddegraded state of formerly lower-density woodlands that had grassyunderstories and could have supported


frequent fire. However, research tocritically test this interpretation is lackingin most areas.MODERATE CONFIDENCE … BUTGEOGRAPHIC APPLICABILITY AND SITESPECIFIC CAUSES NOT ADEQUATELYKNOWN

8. Recent large, severe fires in persistentpiñon-juniper woodlands probably arenormal kinds of fires, for the most part,because similar fires occurred before1900 (as documented, e.g., at Mesa Verdeand on the Kaiparowitts Plateau).However, the frequency of large severefires appears to have increasedthroughout much of the West during thepast 20 years, in piñon-juniper and also in

other vegetation types. This increased fireactivity may be a consequence of warmertemperatures and longer fire seasons,greater fuel continuity developing fromincreasing tree cover, invasion by highlyflammable annual grasses (e.g.,cheatgrass, Bromus tectorum), or acombination of all these causes. However,with the very long fire rotations thatcharacterize persistent piñon-juniperwoodlands, we cannot yet determine ifthis recent apparent trend represents real,directional change or simply a temporaryepisode of increased fire activity, similarto comparable episodes in the past.MODERATE CONFIDENCE … APPLIESTO MOST PERSISTENT WOODLANDSTHROUGHOUT THE WEST


Figure 7. Grey piñon skeletons illustrate the severe mortality event that occurred in Pinus edulisthroughout much of the Four Corners region between 2002 and 2004. Tens of millions of piñontrees died from interactions among drought stress, bark beetle outbreaks, and anomalous hightemperatures. Mortality was most severe in northern New Mexico, northern Arizona, southernColorado, and southern Utah. Bandelier National Monument and Los Alamos National Laboratory,New Mexico, elevation 6,500 feet. (Photo by C. D. Allen, 2006)

9. In many regions where a large fractionof annual precipitation comes in thesummer, savannas and grasslandswere more extensive historically thanthey are today. During the 20thcentury, many former savannas andgrasslands in these regions have beenconverted to juniper or piñon-juniperwoodlands of moderate to highcanopy coverage, especially in parts ofNew Mexcio, Arizona, and perhapsalso in southeastern Colorado. HIGH CONFIDENCE … BUT PRECISEGEOGRAPHIC APPLICABILITY NOTADEQUATELY KNOWN

10. Livestock grazing and fire exclusionsince the late 1800s are importantmechanisms driving the conversion ofsavanna to piñon-juniper woodland inat least some areas, but probably notall. Twentieth-century climatic

conditions also have played a role inat least some areas, but how thesepotential causes interact is not wellunderstood. On some sites withrelatively high effective moisture fortrees, tree cover may have becomegreat enough to suppress herbaceousgrowth, thereby reducing the potentialfor subsequent surface fires andcreating a new fire regime moresimilar to that of persistent woodlandsthan to the former savanna fireregime. Disentangling the mechanismsdriving tree expansion in formergrasslands and savannas is a high-priority research topic.MODERATE CONFIDENCE … BUTPRECISE MAGNITUDE OF INCREASE,CAUSES, AND GEOGRAPHICAPPLICABILITY NOT ADEQUATELYKNOWN

10 - What We Know About Piñon-Juniper Savannas

What We Know About Piñon-JuniperSavannas

11. Expansion of piñon and/or juniper intopreviously non-wooded areas(especially former shrublands andgrasslands) is occurring extensively insome regions, notably the Great Basinand parts of Arizona and New Mexico,but is of comparatively limited extent inother areas, notably western Colorado.An important consequence of thisprocess is that formerly heterogeneouslandscape mosaics — intermingledpatches of woodland, shrubland, andgrassland — are becoming morehomogeneous as trees becomeestablished within the formershrubland and grassland patches. HIGH CONFIDENCE … BUT PRECISEGEOGRAPHIC APPLICABILITY NOTADEQUATELY KNOWN

12. Presence of young piñon-juniperwoodlands does not necessarily implyexpansion into formerly non-woodlandvegetation. The young trees mayinstead represent recovery ofpersistent woodlands from previousdisturbance (e.g., clearing or fire).Thus, knowledge of local history isimportant. HIGH CONFIDENCE … BUT PRECISEGEOGRAPHIC APPLICABILITY NOTADEQUATELY KNOWN

13. Near the current altitudinal andgeographical limits of piñon andjuniper, expansion may representnatural, long-term change in thespecies' range (e.g., piñon expansionin the northern Front Range ofColorado and near Flaming Gorge

Reservoir in northeastern Utah duringthe past several centuries) HIGH CONFIDENCE … BUT PRECISEGEOGRAPHIC APPLICABILITY NOTADEQUATELY KNOWN

14. Expansion of piñon and/or juniper intopreviously non-wooded areas occurredprior to Euro-American settlement onat least some sites; it is not strictly a20th century phenomenon, and is notnecessarily an abnormal processcaused by past land use or fireexclusion. Loss of piñon and/or juniperfrom marginal sites also occurredhistorically, and has occurred recentlyin some areas, e.g., the Four Cornersregion (Figure 7).HIGH CONFIDENCE … BUT PRECISEGEOGRAPHIC APPLICABILITY NOTADEQUATELY KNOWN

15. Expansion of piñon and/or juniper intopreviously non-wooded areas mayhave been more extensive in the 20thcentury than in the previous fewcenturies, at least in some regions.Grazing and fire exclusion probably aremajor causes of expansion of piñonand/or juniper into some savannas,grasslands, and shrublands, but not all.Climate change probably also plays animportant role. However, we do nothave a good understanding of howthese potential causes interact. MODERATE CONFIDENCE … BUTPRECISE MAGNITUDE OF INCREASE,CAUSES, AND GEOGRAPHICAPPLICABILITY NOT ADEQUATELYKNOWN

11 - What We Know About Areas of Potential Expansion & Contraction

What We Know About Areas ofPotential Expansion & Contraction

12

TTaabbllee 11.. A suggested key for identifying the three historical types of piñon-junipervegetation discussed in this paper. Note: This key has not yet been extensively fieldtested. The authors will appreciate feedback on how well it works (or does notwork).

1a. Total tree canopy cover (piñon and juniper combined) < 10 % …… 21b. Total tree canopy cover (piñon and juniper combined) > 10 % …… 6

2a. At least one old tree (over 150 years old*) per acre …… 3 2b. Old trees (over 150 years old*) fewer than one per acre …… 4

3a. Understory dominated by grassland species, often on deep soils and gentle topography ……Savanna (relatively stable)

3b. Understory dominated by shrubs or other species not associated with grassland, often onshallow soils and rugged or rocky topography …… Persistent Woodland (a very sparseform of persistent woodland)

4a. Large dead wood (> 10 inches diameter, standing or fallen), conspicuously present,showing evidence of past fire, woodcutting, or other severe disturbance …… PersistentWoodland (recovering from disturbance)

4b. Large dead wood (> 10 inches diameter, standing or fallen), conspicuously absent …… 5

5a. Understory dominated by grassland species, often on deep soils and gentle topography ……former Grassland being converted to Savanna

5b. Understory dominated by shrubs or other species not associated with grassland, often onshallow soils and rugged or rocky topography …… Area of Potential Expansion andContraction (with recent episode of expansion)

6a. At least 10% of canopy trees are over 150 years old* …… Persistent Woodland6b. Fewer than 10% of canopy trees are over 150 years old* …… 7

7a. Large dead wood (> 10 inches diameter, standing or fallen), conspicuously present,showing evidence of past fire, woodcutting, or other severe disturbance …… PersistentWoodland (recovering from disturbance)

7b. Large dead wood (> 10 inches diameter, standing or fallen), conspicuously absent …… 8

8a. Understory dominated by grassland species, often on deep soils and gentle topography ……former Grassland or Savanna being converted to Woodland

8b. Understory dominated by shrubs or other species not associated with grassland, often onshallow soils and rugged or rocky topography …… Area of Potential Expansion andContraction (with recent episode of expansion)

* See Table 2 for assistance in estimating tree ages.

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TTaabbllee 22.. Tips for estimating tree ages. Tree size is often an unreliable indicator oftree age, and the oldest trees on a site are sometimes among the smallerindividuals. The best way to determine tree age is by extracting an increment coreand cross-dating the rings. Piñons usually can be dated accurately if the wood issound, but junipers generally are difficult or impossible to cross-date. Wherecoring and cross-dating are not feasible, other qualitative features can be used todistinguish between relatively old trees vs. relatively young trees with a moderatedegree of confidence:

Characteristic Relatively Young Trees Relatively Old Trees

Crown shapeConical, with pointed tipand lower branches stillliving

Flattened top, with lower branches dead orpreviously shed

Branch structureBranches becomeprogressively smaller frombottom to top of tree

Large, gnarly branches throughout the livingportion of the crown

Dead wood

Little dead wood in thebole, few dead branches,little or no sign of past woodcutting

Large dead branches, often broken off or cutby wood gatherers long ago; bark missingfrom portions of the bole and weatheredwood exposed in these places

BarkSomewhat smooth, withrelatively shallow furrows

Often rough and shaggy (junipers only),with relatively deep furrows (piñons andjunipers)

P Jdisturbanceregimes Shortsynthesis 5 07

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