Forest vegetation of the Colorado 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 tensively, they have been lumped as Arnica cordiJolia. The cases where Weber's narrow generic concept deviates from the native bluegrass, Poa agassizensis, was lumped with Poa main thrust of present-day North American systematic botany, pratensis. Solidago missouriensis includes some S. canadensis. names were changed to conform with Harrington (1954) and ** Numerous individuals have contributed generously to this Hitchcock & Cronquist (1973). Voucher specimens have been project. Among those to whom I am particularly indebted are B. deposited in the herbarium of Rocky Mountain National Park, Chabot, R. T. Clausen, C. V. Cogbill, J. Douglas, H. G. Gauch, with a few unusual species being deposited in the herbarium of Jr., D. C. Glenn-Lewin, D. Hamilton, K. H. Hildebrandt, D. the University of Colorado, Boulder. Mueller-Dombois, R. L. Peet, D. Stevens, E. L. Stone, J. Vleck, A few species pairs presented consistent problems and their W. A. Weber, T. R. Wentworth, and P. L. Whittaker. 1 treatment as single species was necessary. Garex rossii and C. especially thank R. H, Whittaker for advice and encouragement. brevipes were lumped as Carex rossii. Rosa woodsii and R. Financial support was provided by grants from the National acicularis were lumped as Rosa sp. Cirsium scopulorum and C. Science Foundation, the DuPont Foundation, Cornell Uni- coloradense were lumped as Cirsium coloradense. Extreme versity and the University of North Caroiina Research Council. forms of Arnica cordifolia and A. latifolia are easily distin- The cooperation and support of the National Park Service is guishable, but as these species intergrade and hybridize ex- gratefully acknowledged.

Vegetatio 45, 3 75 (1981). 0042-3106/81/0451-0003/$14.60. © Dr W. Junk Publishers, The Hague. Printed in The Netherlands. 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 tre- muloides. 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 Location Colorado plains, occupies a central position along the east side of the Rocky Mountain massif. The Front Range constitutes the major range of Despite considerable botanical and ecological the southern Rocky Mountain physiographic prov- study, an integrated picture of the composition and ince (Fenneman, 193 I; Thornbury, 1965). Over 300 dynamics of the forest vegetation of these moun- km in length, the Front Range extends from the tains has not yet emerged. Arkansas River on the south, north into Wyoming This monograph represents an attempt to clarify where the Laramie and Medicine Bow Ranges and expand our knowledge of Front Range forests. replace it to form the terminus of the province. Specifically, the vegetation of the Colorado Front Bordered on the east by a foothill belt from 5 to 20 Range has been studied with regard to the composi- km wide, the mountains climb from 1600 m at their tion of the forest vegetation as related to environ7 base on the plains to nearly 4350 m on the highest mental gradients. Stand development following peaks. On the side of these mountains is found a disturbance is interpreted relative to the environ- broad band of forest vegetation bordered both mental gradients and community types described. above (>3 500 m) and below (Fall River Pass, and by the crest of soildistribution and composition is presented based the Mummy Range to the north. on a combination of previously published works (Hanson & Smith, 1928; Johnson & Cline, 1965; Geology Olgeirson, 1974; Retzer, 1974; Smith, 1969) and field reconnaissance (Soil taxonomy follows The study area is underlaid almost entirely by U.S.D.A., 1975). The landscape is divided into four Precambrian granites, gneisses, and schists; the elevation zones for soil description, according to only exception being the easternmost fringe which Johnson & Cline (1965) and Mart (1961), occurs over the Fontain formation, a Pennsylvanian The lower montane zone (I 800 m - 2 450 m) is arkose sandstone and conglomerate (Boos & Boos, dominated by Ustolls and cryoboralfs, As in all the 1934; 1957; kovering & Goddard, 1950). The most elevation zones, frequent areas of lithic orthents important rock is Silver Plume granite, a coarse- occur where bedrock approaches the surface, and grained, commonly prophyritic gran-ite which various alluvial soils are found in the bottoms. makes up the Long's Peak - St. Vrain batbolith Ustolls are the most important soils of the zone, (Peterman et aL, 1967; Boos & Boos, 1934). Second being found on all slopes and aspects at low in importance is the Idaho Springs Formation, a set of Preeambrian metamorphic rocks derived from fine-textured sandstones and shales. Moving from NATIONAl,. -- STUDY AREA FORES'; LARAMIE west to east, Idaho Springs rocks form a sequence ROCKY MT. NATIONAL PK from schist-graphic granite, to biotite-sillimanite schist, to biotite-chlorite schist, to quartz schist. 'WYOMING 41°I Interspersed are areas of gneiss as well as frequent COLORADO pegmatite dikes (Fuller, 1924). Also important in the study area is Mt. Olympus granite, a massive, fine to medium grained, even textured granite ROOSEVELT LARIMER WELD (Boos & Boos, I934). \ NATIONAL • FOREST ) Soils

Soils of the Front Range are mostly immature, heterogeneous, slightly acid and coarse-textured, usually very rocky. Steep topography and frequent ~u-~ ROOSEVELT / NATIONAL occurrence of fire with consequent increased ero- ~S~ FOREST F~I BoutderI ~i 40or, sion have limited the time available for soil matura- i ~ INSTAAR tion on many sites. Most of the more stable soils ARAPAHO ) ' NATIONAL GILPIN above 2 300 m occur on till dating only from the last FO~ST .( glacial advance, though Richmond (1960) has de- scribed localized soil development on isolated tills of greater age. Lithic orthents are found through- out where bedrock approaches the surface, and frequent large rocks and boulders contribute to Fig. 1. Map showing location of study area relative to Rocky marked soil heterogeneity as does variable depth to Mountain National Park and the State of Colorado USA. elevations and primarily on south-facing slopes at shows 'climate diagrams' for Waterdale and the higher elevations. The dense Pseudotsuga menziesii four INSTAAR stations. The Estes Park and forests of north-facing slopes are usually encoun- Allens Park data are consistent with the trends in tered on cryoboralfs. the 1NSTAAR data. In the upper montane zone (2 450 m - 2 850 m), Comparison of the four INSTAAR stations forest vegetation is more fully developed and reveals a gradient in mean annual temperature from cryoboralfs are the most important soil type. At this 8.3°Cat2 195into 3.3 °at3 750m, a lapse rate of elevation ustolls are confined to south-facing slopes 7.5 °C per 1 000 m elevation. With increasing where open Pinus ponderosa forests of the lower elevation the mean daily minimum for January montane extend into the zone. Low grassy meadows drops from 7.8 ° to 16.1° and the daily maximum are primarily on cryaquolls and various histosols. for July drops from 28.3 ° to 19.4 °. The Waterdale Soils of the subalpine zone (2 850 m - 3 500 m) station allows extrapolation to the base of the are more homogeneous, as is the vegetation. The foothills where the mean July daily maximum is widespread Picea engelmannii - Abies lasiocarpa 30.7 ° . Barry (1973) examined the mean daily ranges forests occur predominantly on cryorthods, regard- of temperature and found the greatest variation at less of aspect. Poorly drained sites produce the lowest elevation site (2 195 m) where the maxi- cryaquods. Less dense forest stands of exposed mum range of 15 °C occurs in July. At 3 750 m no sites are frequently on cryoboralfs. At this elevation month has a mean daily range in excess of 8 °C. boggy soils are extensive, with both woody and Average frost-free periods are 125, 104, 56, and 47 sedgy peats common (borohemists and borofi- days for the INSTAAR stations, and are shorter in brists). In the alpine zone (above 3 500 m) valleys subject to cold air drainage. At high eleva- cryumbrepts dominate the alpine turf areas, tions frost can occur at any time during the year cryaquepts are found in most of the meadows and (Griggs, 1956; Ives, 1946). histosols are important in the boggy areas. The climate of the Front Range is strongly continental in character with sudden, extreme changes in weather possible at any time (Marr, Climate 1961; Ives, 1938). Winter weather is the most predictable with cyclonic storms dominating the A relief of 2 600 m coupled with a diverse precipitation patterns, occasionally causing deep topography produces complex climatic conditions. snows. With the northward shift of the prevailing While a detailed meteorological study was beyond westerlies in the summer, cyclonic storms become the scope of the present investigation, the Institute few in number and erratic in behavior. Summer of Arctic and Alpine Research (1NSTAAR), has precipitation is dominated by local, convective collected weather data since 1952 at a series of four valley storms which occur on a regular basis stations 15 km south of the National Park bound- throughout July and early August, diminishing late ary, located at 2 195, 2 591, 3 048 and 3 750 meters in the summer as moisture is lost through stream (Marr, 1961, 1967; Marr et al., 1968, 1968; Barry, flow,(Ives, 1938). These storms can be very violent 1972, 1973). Supplementary data were extracted producing damaging hail and not infrequent snow from the U.S. Weather Bureau records for Estes above 2 400 m. Lightning generated by such storms Park, Aliens Park, and Waterdale. Estes Park is is.an important cause of forest fire. centered on the eastern boundary of the National With the exception of the 3 750 m site, all Park at 2 285 m. Waterdale is located almost due INSTAAR sites show maximum precipitation in east of Estes Park on the edge of the foothills at May with a secondary increase in July and August 1 603 m. Both stations are approx. 40 km north of reflecting convective summer storms. Average total the INSTAA R transect. Allens Park (2 591 m) is on precipitation rises from 532 mm at 2 195 m to 1 050 the eastern boundary of the National Park, only 20 mm at 3 750 m. Ives (1942) has postulated that km north of INSTAAR. certain favorably located subalpine forests, usually Climatological data are summarized using the in sheltered cirque-basins, receive daily convective Klimadiagramm of Walter & Lieth (1967). Figure 2 o b d e INSTAAR (AI) 2195 M Waterdale 1603m 8.8 = 595" f 38.9 (54-77)c " 1 t 37.8 g 50.7 28.3

m

h -9.'] -7.8 i - 35.C -35,9

INSTAAR (BI) 2580 M

34.4 INSTAAR (CI) =,.,050 M , 657 25.0 25.0"

19.4,

-I0.0

-35.6 -I 2.2

-56.6

Fig. 2. An elevational sequence of climate diagrams based on 19.4 data collected at Waterdale and the Institute of Arctic and 12 2 Alpine Research (INSTAAR). The diagrams follow the format proposed by Walter & Lieth ( [ 967). Abscissa in months starting with January, ordinate with one division -- 10 °C or 20 mm precipitation, a = station, b = elevation in m above sea level, c = yrs of temperature and precipitation records, d - mean annual temperature in degrees C, e = mean annual precipitation in mm, f= highest temperature on record, g = mean daily maximum of the warmest month (July), h = mean daily minimum of the coldest month (January), i = lowest temperature on record,j = -16.1 mean monthly precipitation curve, k = mean monthly tempera- -36.E ture curve, I = relative humid season (vertical shading), m = relative period of drought (dotted shading), n = months with mean daily minimum below 0 °C (neutral shading), o = months with absolute minimum below 0 °C (diagonal shading), p = mean duration of the frost-free period in days. rains of considerably greater magnitude, perhaps quent national park establishment was on game 2 500 mm in the course of a summer. Estes Park herd size. While elk (Cervus canadensis) were averages only 405 mm and Aliens Park (2 59t m) originallyabundant, the species was soon extirpated 518 mm, both substantially less than would be by hunting pressure. In 1912 and 1913 elk were expected extrapolating from the INSTAAR data. reintroduced into what is now the national park Waterdale, on the edge of the foothills, receives (Packard, 1947), Settlement, however, limited only 395 mm/yr. available habitat, and in particular winter range Winds can be of major significance for mountain was restricted by cattle grazing. By 1933 the game vegetation along the Front Range where 15 to herds had grown and there was concern for the 25 m/s gales occur almost daily in highest eleva- winter range of deer (Odocoi&us hemionus) as tion forests. Occasional heavy winds in early spring cattle had greatly reduced browse of such preferred blow down large areas of montane and subalpine genera as Purshia, Prunus and Amelanchier forest (major blowdowns within the study area (Thompson, t933). For elk the shortage of winter occurred in 1949 and 1973). range encouraged their habit of stripping Poputus tremuloides bark (DeByle, 1979; Packard, I942). History While this damages trees considerably and increases the rate of Populus mortality during forest succes- Quaternary and recent time has been charac- sion, its presettlement importance is unknown. terized by climatic variability which has played an It is also difficult to determine the extent or important role in shaping vegetation. A climatic impact of early grazing. Cattle were first brought to optimum occurred roughly 7 500 yr ago. This was Estes Park in 1860, and by 1874 there were over followed by three cold or neoglaciat periods alter- I 400 cattle grazing in the basin (Carothers, 195 t). nating with warmer periods. During the neoglacial Much of the area outside the national park is grazed periods small glaciers formed in old cirques, while today. Consequently, most of the low elevation in the intervening periods the ice completely melted study sites must have been grazed at one time. (Richmond, 1972). The most recent neoglacial Within what is now the National Park, it appears lasted from roughly 1 500 to 1 900 with the period that grazing was limited to peripheral, low-eleva- from 1 600 to 1 650 being the coldest 50 yr period tion Pinuspona'erosa forests, and the open, grassy during the last 4 000 yrs (Bray, 1971). As many of parks where hotels maintained horses. With a few the forest trees in the study area became established exceptions, little evidence remains today. under these cooler climatic conditions, the torest Fire frequency has been influenced by post- presently on a site does not necessarily represent the settlement human activities but the extent is un- forest which would become established today under known. Numerous forest fires burned the slopes otherwise similar conditions. near Estes Park in the early post-settlement years of It is uncertain to what extent Indians frequented t860.-1910 (see Clements, 1910). Crandall (1897) the study area before White settlement. Apparently reported a large increase in fire frequency due to Arapahoes, Utes and Cheyennes used the area for human influence throughout the northern Front summer hunting grounds but never lived there year Range during the later part of the nineteenth round (Mussehnan, 1971). As the area was never century. Hanna (1934) suggested that part of the heavily populated, the only impact the indigenous fire increase in the Medicine Bow Mountains might people would likely have had on the natural have been due to vengeful Indians, but this does not communities would have been through periodic appear to have been the case for the present study reduction in game or an increase in fire frequency. area. More recently, fire suppression activities have By the time the region known today as Estes Park resulted in a fire frequency far below natural levels. (In the southern Rocky Mountains broad valleys There has never been extensive logging within and intermountain basins dominated by grassland Rocky Mountain National Park. A saw mill was rather than forest are called parks) was settled in operated for a few years in the Mill Creek region to 1860 the Indians had departed, and there was little salvage trees killed by the Bear Lake fire of t900. evidence of their previous occupation. Elsewhere logging appears not to have been im- An important impact of settlement and subse- portant within the park, and it seems safe to assume that all of the sample sites were on areas where tree insects play important roles. Wind is probably of population structure was the result of strictly greatest importance in areas where overmature natural processes. Some cutting has taken place in trees, those most susceptible to breakage, are the foothills east of the National Park, but no sites plentiful. Many small-scale blowdowns occur in the studied contained evidence of cutting within the last study area each year, and major blowdowns occur twenty years. at irregular intervals. Unfortunately, little is known of their frequency or significance. Forest disturbance Insect outbreaks can be just as devastating. In the 1940's an epidemic of the Englemann spruce beetle The coniferous forests of the Rocky Mountains killed all the Picea engelmannii and most of the can best be described as disturbance phenomona. Abies lasiocarpa over 10 cm dbh (diameter breast Owing to the agencies of fire, wind and insect height) on the White River Plateau in northwestern attack, these forests are periodically destroyed in a Colorado, destroying an estimated I07 m 3 of Picea patchwork manner, resulting in a mosaic of stands alone (Alexander, 1974; Miller, 1970). As might be of differing ages and histories. The complex and expected, outbreaks are usually associated with varied patterns of composition and successional mature and overmature stands (Alexander, 1974). change of these patches constitute an important but Several other insect species can also cause periodic frequently neglected aspect of Rocky Mountain widespread mortality and forest destruction, none forest ecology. of the major tree species being exempt (Amman, Before European settlement in the middle nine- 1977; McKnight, 1968; Roe & Amman, 1970). teenth century, fire was by far the most important The pervasive importance of disturbance in form of disturbance in the forests of the Rocky Rocky Mountain forests dictates that virtually all Mountains (Biswell, 1973; Houston, 1973; Kesse!l, ecological questions be viewed in the perspective of 1976, 1979; Loope & Gruell, 1973; Weaver, 1974). stand development. Plant population structure and Working within the present study area in what is dynamics can be expected to change with stand age, today Rocky Mountain National Park, Clements as can such community and ecosystem properties as (I910) documented a series of Pinus contorta diversity, biomass, productivity, and nutrient flow forests dating from fires which occurred over a (Peet, 1978a, 1981). 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 Methods boreal regions of North America, it is inappropriate to ask whether a particular forest has burned; one Sample definition and placement should ask when it last burned and what the nature of the fire was. In these forests the concept of climax Forest vegetation was broadly defined as vegeta- vegetation is of 0nly limited utility. Instead, forest tion dominated by arborescent species. Specifically, ecology needs to be viewed in the context of stand a st'and had to have either >300 trees and saplings recovery patterns. per ha (>2.5 cm dbh) or basal area (Mueller- Not all fire-modified forests of the Rocky Moun- Dombois & Etlenberg, 1974) of greater than 4 tains have alternating episodes of destruction and m2/ha. These requirements were designed so as to recovery. Open, grassy forests and woodlands of include both open Pinusponderosa woodlands and fire-resistant Pinus ponderosa, and to a lesser subalpine krummholz. Three stands which failed to extent Pseudotsuga menziesii, burn on a regular meet these criteria were included to represent basis, but are usually not destroyed. Rather, fre- transition from woodland to foothill shrubland. quent fires remove accumulated litter, thereby Sampling was preceded by field reconnaissance reducing the danger of holocausts which can result and a literature review aimed at determining the where excessive fuel has accumulated (see Dodge, major trends in vegetational composition. Eleva- 1972; Lunan & Habeck, t973; Wright, 1974). tion, moisture (including topographic position) and Fire is not the only cause of the natural destruc- successional status were suggested to be most tion of Rocky Mountain forests; both wind and important (Bates, 1924; Daubenmire, 1943; Marr, 10

1961 ; Whittaker, 1956, 1960). Several other factors tape was stretched in the selected direction defining were seen to be locally significant including soil the center line ofa 0.1 ha sample plot. Ifa subjective depth, soil texture, wind exposure and snow related cheek for homogeneity failed, a new direction was phenomena. selected. Because it is difficult to obtain a representative Forest vegetation covers less of the landscape in sample of vegetation using strictly subjective plot the foothill region than at higher elevations. In location, random sampling is often suggested for addition, the diversity of successional stages char- vegetation studies (Gounot, 1969; Smartt & acteristic of high-elevation forests is not encoun- Grainger, 1974). However, random placement fails tered. For these reasons, it was considered adequate to include many of the unusual and probably more to employ a low intensity sampling scheme for informative types of vegetation, and is extremely forests below 2400 m. 36 sample plots were sub- time-consuming (see Moore et al., 1970). Subjective jectively located in this zone, all to the east of the sampling can yield a much broader coverage of National Park boundary. Samples were scattered vegetational variation in a given amount of time. to represent as much variation in forest composi- For the present study a dual approach was used. tion as possible. The majority of the samples were subjectively All potential sampling locations were inspected chosen (259), while a smaller number of samples for homogeneity; sites with noticeable heterogeneity (46) were randomly selected using a stratified in either the herbaceous or arborescent vegetation technique similar to that of Seely (1961). were excluded. Plots were also inspected for con- Forest vegetation above 2 400 m (almost all tinuous trends of variation from one side to the within the National Park) was sampled intensively other. Plots with a known history of logging were using 269 0. l ha plots. Potential sampling sites were excluded. All sites appearing currently grazed were stratified using seven elevation belts of 150 m width, rejected as were sites showing residual grazing and six topographic-moisture classes. The topo- damage. For lower elevation sites it was not pos- graphic-moisture classes ranged from wet, shel- sible to select only ungrazed sites, these being tered, bottomland sites to exposed ridge tops with virtually nonexistent. Above 2 400 m all sites with a the intermediate classes corresponding roughly to a known history of grazing were excluded. scale of potential direct-beam solar radiation. At least five plots were placed in each of the resultant Sampling procedure 42 categories in such a way as to span the range of successional stages available. For those cells where Tenth hectare quadrats were used for vegetation considerable variation was encountered, additional sampling (see Whittaker, 1960). Once the location plots were sampled. Randomly located plots were of a sample plot was determined, a 50 meter tape sampled before the subjective placement phase was was placed along the center line and the edges of the completed so as to avoid excessive overlap. plot were located 10 m to either side. Within the For random sampling the northern half of the resulting 50 X 20 m rectangle all woody stems study area was divided into six natural drainage greater than 10 cm high were counted and recorded basins. One of these, the Black Canyon area, was by species. Diameters (dbh; 1.37 m above base) randomly selected, Aerial photographs were used were recorded by 2.5 cm (1 inch) classes, with an to delimit homogeneous areas of vegetation. additional size-class for individuals less than I m Twenty-three such areas were located within this high and one for individuals greater than 1 m high 20.62 km 2 watershed. These were subsequently but less than 2.5 cm dbh. The actual size of the plot divided into 100 X 100 m subunits, and two such was considered somewhat flexible and in unusual subunits were randomly selected in each unit with situations was adjusted to compensate for extrem- the center being designated as the sampling point. ely dense or sparse tree populations. Typically Of the 46 subunits so selected, all were sampled adjustments were made if tree (>2.5 cm dbh) except two. For these two completely inaccessible density fell below 20 or above 400 per 0.1 ha (24 sites, random replacements were selected and increases and 56 decreases out of 305 plots sam- sampled. When a random sampling point was pled). In such cages only the area for stern counts reached, a random direction was selected. A 50 m was adjusted and final totals were multiplied by a 11 correction factor to determine the number of stems Gradient analysis expected in 0.1 ha. The herb stratum (leaf area between the ground Approach surface and 1 m) was sampled using a transect of 25 Vegetation is considered as a continuously vary- contiguous 0.5 ?< 2 m subplots running the length of ing, stochastic phenomenon wherein plants respond the 50 m center line. Within each subplot the individualistically to environmental conditions percentage cover of each species was visually esti- (Gleason, 1926; McIntosh, 1967; Whittaker, 1967). mated to the nearest 1%, or above 20% cover to the With vegetation varying continuously in many nearest 5% (maximum of 100% for one species). As dimensions, it follows, as Webb (1954) has sug- sampling was conducted between July I and August gested, that 'plant communities should be classified 20 for sites above 2 500 m (well after snow melt), multifactorially rather than hierarchically'. Gra- and between June 18 and July 1 for sites between dient analysis (Whittaker, 1967) provides both an 1 700 and 2 500 m (before the summer drought), approach for examining patterns of continuous phenology should not have contributed signifi- vegetational variation and a means of multi-fac- cantly to variation in the cover estimates. Data were torial classification. tabulated as per cent cover and frequency for each As visualization of patterns in more than two or species in the plot. All additional herbaceous three dimensions is at best difficult, gradient re- species occurring within the 0.1 ha quadrat, but not presentations are usually based on a few 'master' or encountered within the subplots were recorded as 'complex' gradients (sensu Whittaker, 1956, 1967). present. These gradients are composites of covarying en- Basic site data were recorded for a typical point vironmental factors representing such complexes as near the center of each plot. Included were location, soil-nutrients, elevation, continentality, or soil- elevation, slope, aspect and soil conditions (based moisture. Whether studied in the compound form on a 10 cm soil pit). Four subjective indices were (e.g. Whittaker, 1956, 1960) or synthesized from recorded using a scale of one to five: slope position, components (e.g. Loucks, 1962; Walker & Coup- ranging from valley bottom through concave and land, 1968), the result is a set of'complex gradients' convex slopes to ridge or hilltop; exposure, ranging defining a vegetational response field of small from sheltered draws, through open hillsides to enough dimension that effective visualization and exposed ridgetops; soil drainage from boggy and interpretation is possible. The forests of the Front consistently saturated sites through moist, to dry Range were known to reflect primarily three com- and excessively drained sites with coarse, sandy plex factors: moisture-topography, elevation, and soil; and soil rockiness from no surface rocks (rocks disturbance. These factors were selected for initial >10 cm) to a solid pavement. analysis by gradient analysis methods. The total data set for the vegetation analyses Elevation as an environmental complex gradient consisted of 305 0.1 ha plots. Included were 545 has long been understood to be one of the primary species of vascular plants, 7 575 0.5 X 2 m herb determinants of vegetation in most mountainous plots and approximately 42 000 trees (>2.5 cm regions. Within the forests of the Front Range dbh). elevation appears equally important. Most workers A second data set contained information on tree who have studied the vegetation of these mountains ages. For 28 stands all trees <2.5 cm diameter were have considered it necessary to classify on the basis cored to determine age. Parallel adjacent strip plots of elevation-defined life zones (e.g. Costello, 1954; 10 m wide and of variable length up to 50 m were Daubenmire, 1943; Mart, 1961; Ramaley, 1907, sampled until at least 100 trees were included. 1908; Rydberg, 1916). For the present study, eleva- Increment cores were extracted from below l0 cm tion was quantified using topographic maps and and were angled so as to intersect the center of the altimeter. tree either at or immediately above ground level. Perhaps the most frequent source of variation In the present study these data are used to verify identified in vegetation studies is moisture. A patterns in stand development and age. The details moisture gradient is not, however, easily quanti- of stand age structure will be the subject of a fied, being the product of numerous environmental subsequent paper. factors which vary through the course of the 12

growing season. For the present study no simply single, more predictive index. The exposure and measured index was available or even feasible, moisture scalars were averaged to provide a new given the scope of the project. Instead, a combina- scale of 1 to 5 ranging from boggy, saturated tion of ordination and environmental scalars was bottomlands to dry, exposed ridges. This subjective used to identify a moisture gradient. index was combined with the solar radiation index in a nomogram for determination of the site The moisture scalar moisture scalar (Fig. 3). The nomogram was sub- Whittaker (1956, 1960; Whittaker & Niering, jectively constructed based on field experience but 1965, t968) has used a combination of exposure without reference to vegetation data. For inter- and slope-aspect to provide the basis for a subjec- mediate values of soil moisture and exposure, solar tive index of 'topographic-moisture'. Gradient radiation was considered the dominant factor as analysis studies based on component factors of indicated by the steep slopes of the isopleths. complex gradients have used similar combinations However, for very wet or very dry and exposed of site moisture parameters (e.g. Loueks, t962; sites, solar radiation was less important. Thus, for Minore 1972, Wali & Krajina, 1973; Walker & both high and low soil moisture extremes, the Coupland, 1968; Waring & Major, 1964; Wikum & isopleths are of shallow slope. The result is a Wali, 1974). preliminary indicator of relative site moisture sta- As a preliminary means of quantifying the mois- tus. It should be emphasized that this indicator is ture gradient, slope and aspect were recorded at based only on measurements of slope and aspect each site, These were used to calculate potential combined with subjective estimates of site condi- direct-beam solar radiation, a factor closely related tions. to site micro climate and frequently an excellent predictor of vegetation (e.g.I.oucks, 1962; Minore, 1972; Waring & Major, 1964; Wikum & Wali, Ordination 1974). Potential solar radiation was determined by Ordination is the arrangement of stands in a interpolation from the tables of Frank & Lee (1966) tow-dimensional, abstract space so as to reveal and relativized to a scale of 0 to 10, Additional interrelationships. A useful application is to ordi- aspects of site moisture status were recorded at each nate a set of vegetation samples stratified so site in the form of the previously described sub- that only one major environmental factor influ- jective scales of site exposure and soil drainage. ences the variation, tn this way ordination can be A variation on the method of scalars as applied to used to order stands along a given gradient. This gradient analysis problems by Loucks (t962) and method assures higher compositional continuity Walker & Coupland (1968) provided a convenient than gradients based on arbitrary environmental method for combining the various indicators into a parameters. In addition, the technique tends to

Site Moisture Scalar qn o 5

ca }2

1 O 1 2 3 4 5 6 7 8 9 10 Relative Potential Solar Beam Irradiation

Fig. 3. Nomogramused in constructionof the site moisturescalar from incidentsolar radiationand topographic position. Detailsare explained in the text. 13 include unrecognized sources of variation in the double standardization intrinsic to the algorithm) resultant gradient so as to yield a complex gradient and Bray-Curtis (BC; Bray & Curtis, 1957) ordina- with increased predictive power. This approach was tion. For this test the 2 900 3 100 m elevation selected to derive a moisture gradient. stratum was selected, this being intermediate in For the present study, correspondence analysis elevation and composition. Composite endpoints (reciprocal averaging; Hill, 1973, 1974; Gauch et were used for the BC ordination, each being the al., 1977) was selected as an ordination technique. average of the three dryest or wettest sites. The Tests by Gauch et al. (1977) suggest that while results were evaluated in terms of Spearman rank distortion can present a problem in axis scaling, the correlation with the previously derived site mois- method reproduces a primary axis of variation with ture scalar. Correlations were BC single standardi- a very high degree of reliability and is one of the best zation 0.816, BC double standardized 0.829, CA of the currently available techniques. As data sets single standardized 0.830, and CA double stan- with only one major trend of variation (moisture), dardized 0.843. The results were gratifying in that were desired, the data were stratified into seven the assumed superiority of double standardization subsets with a basal 600 m elevation stratum was supported, as was application of CA as an followed by six 200 m strata. ordination technique. While the BC ordination The ordinations were constructed using under- represented a best effort to define a moisture story quadrat data for three reasons. Analysis using gradient, the correspondence ordination showed trees alone would have been based on only 12 better correlation with the moisture scalar despite species of which only seven were common, whereas the absence of any assumptions about underlying analysis using quadrat data could employ all 545 factors. recorded species. In conifer forests herbaceous All seven elevational strata were ordinated by species are frequently much more sensitive indica- correspondence analysis using cover values doubly tors of site conditions than tree species (Cajander, standardized. For species present with no cover 1926, 1949; Cormack, 1956; Daubenmire, 1976; value, a value of 0.02 (one-half the normal mini- Frey, 1978; Minore, 1972; Mueller-Dombois, 1964; mum) was assigned. The resulting rank correlations Rowe, 1956; Trass & Malmer, 1978; Whittaker, with the moisture scalar are shown in Table 1. 1962). Coniferous forest tree composition is much Ordination results are summarized in Figure 4. more strongly influenced by stand successional history than is the herbaceous stratum. After Gradient scaling disturbance, canopy composition and structural CA provides an ordering of stands along a recovery can take in excess of 500 yr while the gradient but does not necessarily provide an appro- herbaceous vegetation (in terms of species composi- priate scaling. Distortion remains a major problem. tion) usually recovers shortly after canopy closure The spacing of stands along the derived axis can be (Cajander, 1926, 1949; Frey, 1978; Shimwell, 1971; greatly influenced by chance variation in either Trass & Malmer, 1978; Whittaker, 1962). Various forms of data standardization are pos- sible. Standardization by stands prevents unequal Table 1. Spearmanrank correlationcoefficients between ordina- tion axes and the moisture scalar. Stands were stratified by weighting of stands during ordination and to a elevation. Determinationof the moisture scalar is explained in lesser extent compensates for unequal total cover of the text. different successional stages. Species standardiza- tion reduces the influence of the most common Elevational Correlation Numberof Significance species, those likely to be the most widespread and Stratum Coefficient Stands Level ecologically vague in indicator value. Double 1 700-2 300 m 0.839 26 <0.0001% standardization includes both adjustments with 2 300-2 500 m 0.900 27 <0.0001% species standardization occurring first. 2 500-2 700 m 0.831 60 <0.0001% Several approaches were initially tested includ- 2 700 2 900 m 0.805 57 <0.0001% ing singly (standwise) and doubly standardized 2 900 3 100 m 0.815 58 <0.0001% 3 100-3 300 m 0.826 38 <0.0001% data using both correspondence analysis (CA; 3 300 3 500 m 0.533 35 <0.0001% which, in addition, includes a different form of t4 stand composition or distribution of stands along stands on the axes be preserved. Polynomials of up the underlying compositional gradient. Conse- to second degree along with combinations of loga- quently, a mechanism was needed for scaling the rithmic and exponential transformations were seven derived axes so that their units would be evaluated. After rescaling, the ordination position equivalent. The previously derived moisture scalar of each site was calculated on a scale adjusted to a met this need by serving as an independently range of 0 to 100 for each stratum. derived standard. Each ordination axis was scaled Examination of the resulting moisture axes re- against this standard using curve-fitting techniques vealed a few difficulties. Three strata (2 700-2 900, with the constraint that the original ordering of the 2 900-3 100, 3 100-3 300 m) had a single bog stand

oo,,'~° ~o~e"" ~o~,,- v~,,e ~°~'- I L.." ~ V , u- -~'~° v .-~s~°~° ! v ..... 133-35oo m

.,ooo'o : oe% °e o,e°'°" oW ~" U~ 1,/ 131-3300 m

I V if" if, V If t,../ , If ,, 1.1" 129-3100 m l ..... I "' I I

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

} ~," .... V 'i'- V V" ~ _ tf _ If I V, 1,7 ~27-2900 m

1 I"/ ,if/ V V I t,/ L/ V ...... ~ L.-" If ~25-2700m

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

v" ...... ~ v' v if ...... If ,, 1,7 i V 117-200o rn ~fWe, k:~ 4~0 6~ 80 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. 15 at the end of the moisture gradient. Bogs are forest and krummholz sequences had consistent compositionally distinct from the other wet stands, positions. which resulted in a shifting of the remaining stands farther to the xeric side than for equi#alent stands Classification in strata without bog sites. These three sites were temporarily removed from the data matrices and Approaches the procedures for ordination and scaling repeated Forest vegetation of the Front Range is a mosaic with the gradient starting at five rather than zero, of patches of differing age, each developing toward the bogs sites each being given a value of zero. but seldom reaching a steady-state dictated by site (Gauch et al., 1977 discuss the effects of extreme or conditions. In attempting to classify such vegeta- 'oddball' stands on CA). tion, a dilemma is faced. Should classification be The 1 700 2 300 m stratum proved to be diag- based on the vegetation present, or on the physical onal, incorporating the effects of both moisture and aspects of the environment which determine the elevation. This stratum was originally of greater patterns of vegetational development? elevation width than the others because of the small Cajander (1926, 1949; see Frey, 1978) was among number of low elevation stands, but the results the first to advocate using the environmental com- necessitated division into two narrower strata. The plex as the basis for classifying vegetation com- ordination and scaling procedures were repeated posed of a spatial-temporal mosaic. He also recog- using 1 700-2 000 m (n = 12)and 2 000-2 300 m(n nized that herbaceous vegetation can be a powerful = 14) strata. The rank correlations with the mois- indicator of site potential. ture scaler were 0.608 and 0.894 resp. The low value Daubenmire (1976; Daubenmire & Daubenmire, for the lowest elevation stratum reflects a failure to 1968), building on this Fenno-Scandian approach, locate and include wet and mesie stands below has advocated using the 'potential climax vegeta- 2 000 m elevation. Virtually all such stands have tion' of a site as the definitive characteristic, an been destroyed for roads, housing, and agriculture. approach now widely applied in western North (Experimentation with correspondence analysis has America (see Daubenmire, 1976; Layser, 1974). As revealed that the power of the technique to repro- Daubenmire (1968) wrote '... it seems best to take duce an assumed ordering of stands from noisy field the philosophical viewpoint that the habitat (soil, data decreases as the length of the gradient is macroclimate, and topography) is the most durable shortened; Gauch et at., 1977). component of the ecosystem, and that the disturbed The 3 300 3 500 m stratum also yielded poor vegetation presents varied appearance owing to results. This was largely a consequence of the differences in the degree to which the ecosystem has combination of forest and krummholz stands with- been thrown out of balance, , .' Conceptually this in the same stratum. Krummholz vegetation is provides an easily visualized, tractable definition. influenced by a large number of site variables Cajander's (1926, 1949; see Frey, 1978)approach including moisture, exposure to wind, snow drift was to use understory species as site indicators. and melt, and soil depth (see Smith, 1969; Willard, While he realized that immediately after distur- 1963). Subsequently, the stratum was split into bance all vegetation appears modified, he found krummholz (n = 10) and forest (n = 25) segments understory species to have excellent indicator value with the extreme mesic and xeric stands included in soon after canopy closure. both. After repeating the ordination and scaling Structurally, the forests of the Front Range are procedure, the forest portion gave a rank correla- very similar to those of Fenno-Scandia studied by tion of 0.714 with the moisture scaler. Because a Cajander, as welt as those studied throughout distinct krummholz moisture gradient failed to western North America by adherents to Dauben- emerge, it was necessary to use weighted-average mire's methodology. Following these workers a species moisture values for species from the highest classification based on the environmental situation elevation forest zone to calculate stand positions on of the site as interpreted using understory vegeta- the krummholz moisture gradient (see Whittaker, tion appears to be the best solution. A gradient 1967). The gradient was then rescaled so that the analytic representation of forest vegetation derived terminal stands which had been used in both the from understory composition offers an effective 16 basis for such a classification. All that is needed is to forests with successional trends from Pinusflexitis dissect the two dimensional representation of forest to Abies lasiocarpa and Picea engelmannii domi- vegetation into forest types. nance were grouped together. Similarly all Pinus contorta stands were grouped together and this Type delimination collection was further divided into groups based on The two dimensional gradient representation whether Abies, Pseudotsuga, or Pinus contorta (Fig. 5) was broken into community-types within would follow the initial post-fire Pinus contorta which the successional patterns of dominant trees forest. As explained in a subsequent section, sue- were reasonably homogeneous. For example, cessional trends were inferred by examination of

NORTHERN COLORADO FRONT RANGE VEGETATION of the EAST SLOPE re~ters :eel 12,00(

3500

ll,OOC 3300

~I00 0,000

:~900 z o 9,000 2700 uJ .J ~J

~5OO 8,000

2300

7,O0O 21OO

1900 6,000

1700 Rovines Shellered Open Slopes Exposed Ridges Slopes NE E SE Wet Mesic N NW W SW S Xeric MOISTURE GRADIENT

Fig. 5, Community mosaic diagram showing the distribution of community series and types relative to gradients of elevation and topographic-moisture. Community 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 community types occupy the same region on the mosaic diagram. 17 tree diameter distribution, and tree ages determined were in turn divided into community-types on the from increment borings. Groups were examined for basis of the successional pattern of the dominants homogeneity of the understory data and a few as well as site characteristics. In these cases species groups were divided. Particularly low homogeneity names separated by dashes indicate successional was encountered at the transition from Pinus trends while commas indicate codominance. 'Mesic ponderosa woodland to Pseudotsuga or Pinus Pinus contorta-Abies, Pieea forest' indicates a type contorta dominated types. This is a consequence of initially forested by Pinus contorta after distur- the important role of soil texture on dry sites. Here bance, but with a climax or steady-state dominated it was necessary to split one unit into two over- by Abies and Picea, in that order of importance. lapping groups, separated by edaphic conditions. Mesic distinguishes this from a type with similar An additional Populus trernuloides dominated canopy characteristics but different understory group was defined. The Populus stands, being composition. Types were assigned codes with a largely successional, did not fall together on the letter indicating the series and a number the type community mosaic like the other dominance-types; within the series. their distribution was more a function of distur- bance and edaphic conditions than elevation and moisture. Otherwise all the dominance-types were Community characterization effectively separated using the two-dimensional gradient representation. Data summarization This approach to community classification ap- pears distinctive in the diverse literature of com- The continuous, highly stochastic variation of munity classification. Pogrebnjak (1930) was one vegetation limits the extent to which community of the first to base a classification on a gradient types can be accurately or precisely characterized. system. Whittaker (1956, 1960; Whittaker & Nier- A few typical stands cannot represent the range of ing, 1965) classified vegetation by the dissection of variation encountered, yet presentation of a suf- gradient analytic diagrams. The approach em- ficiently large number of stands to represent the ployed parallels Aichinger's (1951) recognition of variation present often obscures underlying pat- Vegetationsentwicklungstypen in the combined use terns. In the present study community floristic of dominance-types, successional trends and composition is summarized in tables designed to herbaceous indicator species. However, Aichinger present a maximum amount of information While started by recognizing dominance-types, and used avoiding excessive length. The table format is that herbaceous species only secondarily. The combina- of the author, but several aspects are patterned after tion of gradient analysis based on composition of tables in the works of Curtis (1959) and Dahl the herb-stratum with dominance defined types, as (1956). Tables for the 8 recognized series and 29 well as the combination of mathematical ordina- component types are presented in the Appendix. tion and direct gradient analysis appears new. The Prevalent species are used to characterize com- units so defined come close to the habitat-types of munity composition (Curtis, 1959). For this pur- Daubenmire which are defined on the basis of pose constancy was first calculated for all species potential climax .vegetation, but the gradient ap- encountered in a type (percentage of stands in a proach sets this apart as does inclusion of succes- type in which the species is present). Next, the sional components. For simplicity, the basic units average number of species per stand (d = species recognized are called 'types', and these are com- density of Curtis) was calculated, fractions being bined into larger units called 'series'. rounded to the next highest integer. Prevalent The present study ted to recognition of 8 series species were defined as those d species with highest (Fig. 5) named on the basis of physiognomy and constancy. In case of ties, the order was determined dominant tree species. For example, sites initially by average cover. A list of prevalent species so dominated by Pinus contorta but successional to defined is considered to characterize the composi- Pseudotsuga were placed in the Pinus eontorta tion of the type. Listed for each prevalent species forest series owing ~o the rarity of old-age stands. are the average frequency (in 0.5 X 2.0 m quadrats) Series were also assigned letter codes (A H). Series for those stands in which the species was present 18

(first column), and the constancy (second column). Pinus ponderosa woodlands (A) For each community-type modal species were determined. These were defined following Curtis The dry foothill vegetation of the Front Range is (1"959) as species having their highest constancy in dominated by open Pinus ponderosa woodland, a the type. Again, ties were decided on the basis of formation characterized by scattered trees with less cover values. Modality is indicated for prevalent than 50% cover over a graminoid dominated under- species by underlining the constancy. Modal, non- story. These woodlands are found on the lower prevalent species are listed without frequency slopes over the central portion of the moisture values. While available space precluded listing gradient. They grade into cottonwood (Populus modal species which were never prevalent, these sargentii, P. angustifolia) forests in river bottoms, and other supplementary data are summarized in Pseudotsuga forests on steep, north-facing ravine expanded community tables available from the slopes over the central portion of the moisture author (see Appendix). Certain limitations must be gradient. They grade into cottonwood (Populus placed on the interpretation of modality. In cases of sargentii, P. angustifolia) forests in river bottoms, small numbers of stands defining a community- increasingly confined to the xeric end of the mois- type (3-6), modality can be strongly influenced by ture gradient and are eventually replaced by the stochastic variation in sample composition. Also, Pinusponderosa, Pseudotsuga forests (B). Foothill as this study includes only forest vegetation, mod- forests dominated by Pinus ponderosa and Pseu- ality applies only to forests and not to the region as dotsuga were first described by Vestal (1917) work- a whole. For example, in the present study Do'as ing in the vicinity of Boulder. He recognized 18 octopetala is modal in 'Subalpine Pinus flexilis formations as occurring in the foothills and listed forests' where it has a constancy of only 12.5%, but dominant species. Ramaley (1908, Ramaley & Willard (1963) recognized an alpine community Robbins, 1908) also provided species lists for (Do,asetum octopetalae) where Drvas has a con- several plant communities. stancy of 100%. The lower margin of the foothill woodland Given stands arranged in types, it is desirable to grades into grassland, typically dominated by some have an indicator of the relative uniformity of a combination of Agropyron smithii, Andropogon type, or homotoneity. (Homotoneity refers to be- scoparius, Bouteloua curtipendula, B. gracilis, tween-stand comparison, whereas homogeneity Bromus teetorum and Stipa comata (see Hanson, refers to within stand comparison.) An index 1955; Hanson& Dahl, 1957; Ramaley, 1908; Vestal, defined by Curtis (1959) as the sum of the constancy 1917). On these sites soil texture as determined by values of the prevalents divided by the sum of the parent material (Retzer, 1953) and the erosion constancy values of all the species (or, average deposition cycle appear of central importance in constancy of the prevalents) was calculated for each determining community composition. Slope and forest type. This is similar to indices used by aspect have only limited influence on the composi- Raunkiaer (1934) and Raabe (1952). Itsadvantage tion of these communities, a result consistent with is that it is relatively independent of the number of patterns observed by Hanson & Dahl (1957). As the stands in a community type, since species density present study was confined to granitic substrate, approaches a constant after the first few stands are parent material induced vegetational variation was sampled (Peet, 1974). Curtis obtained values be- largely avoided. However, a shift from grassland to tween 34.5 and 70.3 in his study of Wisconsin woody vegetation occurred with a change from fine vegetation. In the present study using geographi- to coarse textured soils. Cercocarpus montanus cally more restricted, less diverse community-types, and Rhus triloba shrubland dominates on rocky values ranged from 54.5 to 78.5. sites, and grasses dominate on finer-textured Tree composition is summarized using impor- soils. tance values. These were calculated as the average Robbins & Dodds (1908) and Larsen (1930) have of relative density (density standardized to total suggested that at low elevations Pinusponderosa is 100) and relative basal area (also standardized to confined to rocky sites; edges of large rocks or total 100). Where tree density is used, this refers to coarse-textured soils are necessary for seedling stems >7.5 cm (3 inches) dbh. establishment. These observations are consistent 19 with the behavior of Pinusponderosa in the Front mesic sites Purshia tridentata and Agropyron albi- Range. Lower slopes and broad valleys where fine- cans also contribute substantially. textured soils accumulate often have few if any This community-type appears to represent a trees. These are the so-called 'parks' of the southern northern extension and attenuation of theQuercus Rocky Mountains. A similar situation occurs in gambelii chaparral found at the base of the foothills Oregon and Washington where Pinus ponderosa from Denver south (Peet, 1978b). Many of the woodland and shrub-steppe form an intricate species are shared including those with the greatest mosaic in ecotonal areas in response to soil texture cover values. This attenuation continues into (Franklin & Dryness, 1973). These examples all Wyoming where, in the forests of the Wind River conform to a general pattern of semi-arid regions and Teton ranges, both the shrub formation and the where grasses competitively exclude woody plants Pinus ponderosa woodlands are missing, these on fine-textured soils, whereas the deeper moisture being replaced by Artemisia tridentata steppe grad- infiltration and the lack of a continuous sod of ing directly into Pinus contorta forest (Reed, 1952; grasses allows woody plants to succeed on coarse- Reed, 1969). In the Big Horn Mountains to the east textured soils (Waiter, 1970; Wells, 1965). is found a low shrub formation dominated by The Front Range woodlands are rich in species Cercocarpus led(folius and confined almost ex- averaging 43 per 0.1 ha sample. Of these, grasses are clusively to calcareous substrate (Despain, 1973). particularly important accounting for 25% of the prevalent species and 42% of the herbaceous cover. Mesie foothill woodland (A2) Composites are also important accounting for 18% Broadly occupying the center of the moisture of the prevalent species. gradient between 1 900 and 2 200 m, the mesic foothill woodlands are structurally open with be- Pinus ponderosa shrubland (A 1) tween 250 and 500 trees per ha. Pinus ponderosa Comprising the transition from woodland to and Pseudotsuga are the major tree species with Cercocarpus shrubland, the Pinusponderosa shrub- average importance values of 75 and 22 resp. Again, lands are the most xeric of Front Range forest- Juniperus scopulorum is commonly present but types. At lower elevations (<1 700 m) the forma- rarely of significant size or importance. Total basal tion dominates a broad range of habitats on rocky area varies between 10 and 20 m2/ha. slopes. However, at its upper terminus (~2 000 m), Understory cover is low compared with the the formation is largely confined to rocky ridge tops shrubland type, averaging only 28% with grasses and south-facing slopes. The community is open in contributing a third of the total. An average stand appearance with a matrix of shrubs, predominantly includes 45 species making this one of the richer Cercoearpus montanus and Rhus triloba from community-types of the Front Range. Understory which emerge 40 to 275 widely spaced trees per dominance is patchy, varying between the shrubs hectare. Pinus ponderosa dominates the tree stra- Purshia tridentata and Ribes eereum and the tum with an average importance value (I.V.,.aver- graminoids Leueopoa kingii, Carex rossii and age of relative density and relative basal area) of 95. Muhlenbergia montana. Juniperus scopulorum is regularly present but rarely attains a diameter greater than 12 cm dbh. Xeric foothill woodland (A3) Occasionally individuals of Pseudotsuga are en- Between 2 200 and 2 350 m the dry end of the countered. Basal area ranges up. to 12 mZ/ha moisture gradient is occupied by xeric foothill depending on site and disturbance history. woodland. This community, like Pinus ponderosa Despite xeric conditions, understory cover aver- shrubland (A1), has a sparse canopy, tree density ages 64% with grasses providing almost half. Suc- ranging between 75 and 150 per hectare. Basal area culents reach their greatest importance in the Front ranges from 6 to 12 m2/ha. Pinusponderosa with Range forests in this community-type, as do suf- an average IV of 92 is the only important tree frutescents and annuals. Richness is high with an species, though Juniperus scopulorum attains its average of 45 species occurring per plot. Cover is highest importance in this type (averaging 8 but largely dominated by Bromus tectorum, Rhus occasionally reaching 25 on very rocky sites). These triloba and Cercocarpus montanus. On the more stands are the closest approximation in the study 20 area to the Pinyon Juniper (Pinus edulis, Juni- textured soils support woodland. This is consistent perus monosperma, J. scopulorum) woodlands with observations on the influence of soil texture on commonly found south of Denver along the base of competition between grasses and woody species. the Front Range. Nearly half of the total woodland understory cover Due to xeric conditions and shallow soil with of 46% is provided by grasses. frequent exposure of underlying rock, understory Xeric montane woodlands are dominated by cover values are low, averaging only 34%. In xeric Pinus ponderosa which has an average IV of 83, foothill woodland, like xeric montane woodland and Pseudotsuga is frequently present. Total basal (A4), the dominant species is the grass Muhlen- area ranges between 5 and 25 m-~/ha, depending bergia montana which here a~erages 10% cover and ~ again on disturbance history and site conditions. occurs with a constancy of 100%. Though grasses Tree density is low in this woodland type ranging dominate the understory (40% of total cover), small from 40 to 160 trees per hectare, thus allowing a shrubs are again important with Ribes cereum and highly developed herbaceous stratum. However, Rubus deliciosus both having 100% constancy. due to dominance by a few grass species and in Cercocarpus montanus occurs occasionally, sug 7 particular Muhlenbergia montana, the diversity is gesting affinities with the lower-elevation shrub- the lowest found in the woodland types. Again lands. Despite the xeric nature of the community- Purshia and Ribes cereum dominate the shrub type, the large areas of protruding rock contain stratum. frequent seeps and moist cracks. Mesic herbs such as Parietaria pensylvanica and Mimulus glabratus Pinus ponderosa, Pseudotsuga forests (B) occupy these microhabitats. A band of vegetation stretching from mesic Mesic montane woodland (A4) valleys and north-facing slopes at 1 700-2 200 m Mesic montane woodlands suggest a higher ele- across the mosaic diagram (Fig. 5) to xeric south- vation variation of mesic foothill woodland (A2), facing slopes and ridge tops at 2 400 2 800 m, the usually on fine-textured soils. Tree density varies Pinus ponderosa, Pseudotsuga forest series repre- considerably with between 60 and 500 trees per sents the transition from foothill woodland to hectare depending on disturbance history and soil dense, high-elevation forest. Being on the border of texture. Basal area usually varies between 10 and 20 the woodland formation, these forests were origi- m2/ha. Pinus ponderosa is dominant with an nally subject to a high natural fire frequency. average IV of 62, though Pseudotsuga is also Reflecting the varied disturbance history of the important averaging 30. landscape, the present vegetation is composed of a Understory cover values are moderate, averaging mixture of old groves of large trees with dense 38% with grasses comprising a third of.this. Like sapling populations, younger stands with dense, mesic foothill woodlands (A2), this type is rich in even-aged populations of small trees representing species averaging 45 per 0. ! ha. Purshia tridentata post-fire recovery, and stands of mixed age (c.f. and Ribes cereum are the dominants among an Cooper, 1960). Recently fire suppression activities average of six shrub species per stand. The gram- have resulted in unnaturally high sapling densities, inoids Muhlenbergia montana, Carex rossii and as well as increased fuel loads. Sapling densities Leueopoa kingii also contribute greatly to the total have resulted in a decreasing reproductive success cover. of the shade intolerant Pinus ponderosa relative to Pseudotsuga, while high fuel loads have lead to an Xeric montane woodland (A5) increasing probability of devastating forest fires Xeric montane woodland, and xeric Pinus pon- (see Lunan & Habeck, 1973; Weaver, 1974; West, derosa forest (B4) occupy the same position on the 1969). Marr (1961) suggests an alternative explana- community mosaic diagram (Fig. 5), the xeric end tion for the changing forest structure based on of the moisture gradient between 2 450 and 2-850 heavy grazing and consequent reduced competition m. The two types are ecologically differentiated from herbaceous species. While perhaps of local primarily by soil texture. Rocky sites with coarse- importance, such overgrazing appears relatively textured soils are covered by forest while fine- restricted in the present study area. 21

In terms of understory cover, shrubs are very dotsuga, Pinus ponderosa forest. This community- important in this community series. On the average type shares many features with the foothill ravine the understory contains over six shrub species per forest (B1). Pseudotsuga and Pinus ponderosa 0.1 ha with Juniperus communis and Ribes cereum share dominance in both, and in both frequent fire having the highest constancies. Among the preva- played an important role before settlement. Juni- lents, Physocarpus monogynus, Juniperus com- perus scopulorum is a frequent associate and Pinus munis. Arctostaphylos uva-ursi and Jamesia ame- flexilis and Pinus contorta occur occasionally on ricana have the highest cover values. Graminoids, the higher-elevation sites. Basal area is variable especially Leucopoa kingii and Carex rossii, also ranging from 8 to 40 m2/ha depending on distur- have high cover, but are much less important than bance history. in the woodland types. Shrubs and graminoids are the principal under- story growth forms with 40% and 22% of the total cover resp. Shrub cover is dominated by Physo- Foothill ravine forest (B1) carpus monogynus, Juniperus communis and Foothill ravine forests are found in cool, moist Arctostaphylos uva-ursi. Understory cover for this draws and on sheltered north-facing foothill slopes. type averages only 19%, reflecting the influence of This forest-type provides the lower elevation limit canopy closure on the competitive ability of tree of many montane and subalpine species. A com- seedlings, shrubs and herbs. Among the herbaceous bination of cold air drainage, low incident solar species Leucopoa kingii has the highest cover. radiation and consequently low potential evapo- Despite low cover values, species richness averages transpiration produces localized conditions favor- 33/0.1 ha with 24 herb species and 7 shrubs. able to species of higher-elevation forests. Pseudotsuga dominates the tree stratum with an average IV of 65 compared with 32 for Pinus Xeric Pinus ponderosa forest (B3) ponderosa, the only other important tree species. Xeric Pinus ponderosa forest occupies the same Examination of average relative seedling, sapling, portion of the community-mosaic as xeric montane and tree density suggests the more shade tolerant woodland (A5), that area between 2 350 and Pseudotsuga to be increasing at the expense of 2 750 m at the xeric extreme of the moisture Pinus ponderosa. The implication is that post- gradient. The primary difference between these settlement suppression has prevented fire from types is edaphic, the forest community being char- keeping these types open, allowing abnormally high acteristic of rockier substrates. seedling establishment and changing forest dom- Pinus ponderosa is the dominant tree species inance. with an average IV of 77. Pseudotsuga is commonly The dense, coniferous canopy and thick litter present but with an average IV of only 18. Seedling layer contribute to the lowest species diversity success differs due to the greater shade tolerance of among the foothill types. The average stand has an Pseudotsuga which has a relative density of 33% understory cover of slightly under 30% with ap- compared to 40% for Pinus ponderosa. proximately 28 species represented. The majority of The combination of xeric conditions, rocky soil, the cover in the understory stratum (62%) is and typically heavy shade from the dense canopy provided by shrub species. Physoearpus mono- results in the lowest average understory cover of gynus is the most characteristic shrub species with any type delimited during the study, 9%. Of this the highest cover and a constancy of 100%. Juni- tot~tl, 53% is contributed by graminoids, and 15% perus communis, Jamesia americana and Areto- by shrubs. The average site has 28 species despite staphylos uva-ursi also contribute substantially to the low cover values. Understory dominants in shrub cover. terms of cover are the graminoids Leucopoa kingii and Carex rossii. The shrub Physocarpus mono- Foothill Pseudotsuga, Pinus ponderosa forest (B2) gynus is also important. The high importance of The central portion of the moisture gradient Purshia tridentata in the shrub stratum underlines between Pinusponderosa woodland (A) and Pinus the affinities with the xeric montane woodland (A5) contorta forest (D) is occupied by foothill Pseu- type. 22

Xeric Pseudotsuga forest (B4) the study area, but by subtleties of environmental On xeric sites between 2 700 and 2 950 m, a variation including drainage, soil aeration, expo- narrow zone of xeric Pseudotsuga forest (B4) sure and cold air drainage. These forests are the separates Pinus ponderosa and Pinus flexilis dom- most diverse of the Front Range in woody species, inated communities. As expected, these two species the average stand containing 5 tree and 8 shrub preempt a portion of the dominance in the periph- species in addition to 28 herb species. eral regions of the type. Like the community-types Due to the pervasive influence of economic immediately below it, xeric Pseudotsuga forests are development at low elevations, foothill cottonwood strongly influenced by edaphic conditions with forests are little known botanically, though they dense stands of usually even-aged Pseudotsuga On probably belong in this series. A combination of the rocky sites, and open stands on the sites with road building, grazing, and agriculture has greatly finer-textured soils. reduced the number of such forests available for Because the tree component consists of two study. Casual observation indicates dominance of edaphic phases, is hard to characterize. In both Populus sargentii below 1 950 m and Poputus phases the dominant tree is Pseudotsuga (average angustifolia at higher elevations, both being asso- IV of 74). Pinusflexil'is and Pinusponderosa which ciated with various Salix species. This appears occasionally share dominance have average IV's of consistent with the brief reports of Marr (1961), 7 and 14. On open sites basal area ranges between Vestal (1917) and Young (1907). Additional obser- 12 and 20 m2/ha and tree density ranges between vations were not made for lack of undisturbed sites 200 and 450/ha. In contrast, on the more densely within the study area. At elevations above 2 200 m forested, rocky sites, basal area ranges between 25 and extending to about 2 800 m, floristically similar and 55 m2/ha and tree density between 1 000 and though nonforested vegetation occurs on very wet, 2 000 stems/ha. usually saturated soils. Combinations of Alnus Thirty-two species occur in the understory of a tenuifolia, Betula occidentalis and various Satix typical stand including 24 herbs and 7 shrubs. species mark the occurrence of these latter com- Altogether, understory cover averages 19%. Shrubs munities which provide the hydric limit of the make up one-third of the cover with Juniperus mixed wet forest (C1) (Young, 1907). comrnunis, Artemisia tridentata and Physocarpus monogynus comprising the largest portion. While Mixed wet forest (C1) Artemisia tridentata has a constancy of only 33%, it Occurring at the wet end of the forest moisture dominates those stands in which it occurs. Among gradient between the foothill cottonwood forests the herbaceous species, Muhtenbergia montana and the start of the high-elevation Picea, Abies usually dominates the ground cover. forests, the mixed wet forests (CI) are composed of a variety of tree species combined in heterogeneous Mesic montane forests (C) assemblages. Two topographic variants can be distinguished, characteristic of cool, sheltered Mesic montane forests make up a heterogeneous ravine bottoms, and warm, moist floodplains. group of stands characteristic of moist, relatively The ravine forest variant is at least partially low elevation sites. At their upper limit they are dominated by Pseudotsuga with scattered Populus dominated by Picea engehnannii and Abies tasio- tremuloides. In wet pockets Betula occidentatis, carpa, usually mixed with Pseudotsuga. On more Populus angustijolia, Alnus tenuifolia and Pieea xeric sites Pinus contorta dominates, frequently pungens are also important. On the dryer, rocky being succeeded by either Abies or Pseudotsuga. microsites Pinus ponderosa and Juniperus scopu- The lower elevational limits are defined by foothill lorum dominate. Basal area ranges from 20 to 30 ravine forest (B 1) and foothill cottonwood forest. m2/ha and tree density is typically between 300 and The central positio n of this group at the intersec- 500 stems per hectare. The floodplain variant is tion of several forest series dominated by different dominated by a combination of species including tree species contributes to its heterogeneous com- Populus angust(folia and Picea pungens with the position. Dominance is determined not primarily former achieving highest dominance on warmer by disturbance as is the case for most forest-types in sites. Both species can attain substantial size, and 23 basal area occasionally reaches as high as 80 m2/ha. dominance and Pinus contorta assumes an early Diversity, understory cover, and tree reproduction successional role. are all reduced in such high basal area stands. In addition to being second only to the wet The average of 60 species per 0.1 ha including 8 montane forest in species richness, this commu- tree species is the highest encountered in the study nity-type has the highest average number of shrub area (see Peet, 1978a). However, the other two species (9.5). Only Rosa has a constancy of 100%, types in the mesic montane series have higher but several shrubs contribute substantially to total shrub richness. Whittaker (1965, 1972; Whittaker & cover including Physocarpus monogynus, Jamesia Niering, 1975) has suggested the Santa Catalina americana, and Juniperus communis. Ribes ce- Mountains of Arizona to have among the most reum and Symphoricarpus oreophilus are also fre- diverse communities in North America, yet the quent. When present, the grasses Calamagrostis richness values he reported are lower than those canadensis and Poa pratensis are among the lead- found in this community-type. ing dominants in understory cover. With its high species diversity, the mixed wet forest type is hard to characterize. Grasses con- Mixed rnesic forest (C3) tribute 30% of the cover with three species primarily Mixed mesic forest occurs at the junction of responsible: Calamagrostis canadensis. Phleum several community-types on the mosaic diagram. pratense and Poa pratensis. Other important spe- After a disturbance such as fire, simultaneous cies include Equisetum arvense, Rosa spec., establishment of Populus tremuloides, Abies lasio- Thermopsis divaricarpa, Geranium richardsonii, carpa, Pinus contorta, Picea engelmannii, Pseu- Arnica cordifolia and Rudbeckia laciniata. dotsuga menziesii, and Alnus tenuifolia can occur, This community-type, in addition to its high thus leading to high tree diversity. This wealth of diversity, has the highest affinity with the flora of tree species precludes description of any simple eastern North America of all the community-types pattern of forest development. The steady-state studied. The marly shared species may represent forest can be composed of Pseudotsuga or Picea, remnants of a more mesic, transcontinental flora, Abies or a mixture depending on the site. Pseu- or may simply be the result of migration along the dotsuga can act as both a pioneer and climax prairie border. The mesic Populus tremuloides species. forests described by Severson & Thilenius (1976) Total understory cover is 50% but 30% of the from the Black Hills of South Dakota are inter- total represents coniferous regeneration and an- mediate between this type and the true eastern other 30% Ericaceous shrubs, mostly Vacciniurn deciduous forests. myrtillus, Shrub diversity is high with an average of 8 species per 0.1 ha. Linnaea borealis is important Montane ravine forest (C2) as are Rosa and Juniperus communis. Of the herbs, On cool ravine slopes and sheltered well-drained only Arnica cordifolia and Haplopappus parryi bottomlands between 2 300 and 2 550 m, montane have a constancy of 100%, though Pyrola secunda ravine forest is dominant. This, like the mixed wet and Goodyera oblongifolia can be considered char- forest, is a heterogeneous forest type, the composi- acteristic. tion of which is greatly influenced by subtleties of site quality and history, On exposed sites Populus Pinus contorta forests (D) tremuloides is the first species to invade after disturbance. The eventual dominants can be Picea Pinus contorta forests are the most central and engehnannii, P. pungens, Pseudotsuga, or Pinus perhaps the most widely distributed forests in the ponderosa depending on moisture and temperature northern Front Range. Occurring between 2 400 conditions. Rocky sites usually have Pinus pon- and 3 200 m and intermediate in moisture require- derosa or Juniperus scopulorum present. On cool, ments, they occupy a central position on the moist sites Picea pungens and Pseudotsuga domi- community mosaic (Fig. 5). Clements (1910) lo- nate with Betula occidentalis and AhTus tenuifolia cated, aged and characterized a series of burns occurring in wet pockets. With increasing elevation dating from 1707. Moir (1969, 1972; Moir & Picea engelmannii and Abies lasiocarpa increase in Francis, 1972) conducted a series of short studies on 24 the Pinus contorta forests near Boulder, south of through the course of succession. Initial, post-fire the study area. Brief descriptions of some of these forests usually consist of a mixture of Pinus con- forests have been published by Marr (1961). More torta and Populus tremuloides. In a typical stand, complete descriptions of stands in the Medicine Pinus quickly overtops the Populus to become the Bow Mountains to the north and the Frasier exclusive dominant. Steady-state composition is Experimental Forest southwest of the study area more variable for while Pinus contorta dominates have been provided by Romme (1977), Whipple the typical site, Picea, Pinus flexilis and Pseu- (1973, 1975) and Wirsing (1973). dotsuga can share dominance. Pinus contorta is a successional species par An average of 19 species is found in the under- excellence. On favorable sites the species seeds in story of a Pinus contorta forest. Juniperus com- quickly after fire forming dense, even-aged stands. munis, Arctostaphylos ura-ursi and Carex rossii On some sites Populus tremuloides shares dom- are the only species with over 70% constancy. Only inance as a successional species. While repeated Juniperus and Arctostaphylos have average cover burning can increase Pinus contorta dominance, values over 2%. Pinus contorta forests usually revert to forests of other, more shade tolerant species when given Mesic and xeric Pinus contorta - Pseudotsuga protection from fire. At~lower elevations succession forests (D2, D3) favors Pseudotsuga. while at higher elevations The lower elevation portion of the Pinus con- Abies and to a lesser extent Picea replace Pinus torta forest series is potentially dominated by contorta. In the central portion of their elevational steady-state Pseudotsuga, though a high natural range, the Pinus contorta forests form a narrow fire frequency causes such stands to be exceedingly band of self-maintaining forests. rare. Post-fire successional stands are dominated Successional variation in Pinus contorta forests by Pinus contorta with varying amounts of Populus makes description of composition difficult. For the tremuloides and Pseudotsuga, depending on seed tree stratum, age sequences must be examined. supply, weather, and site conditions. The overstory Although the herbaceous vegetation is more con- structure and dynamics of these two types are stant through time than is the tree component, similar but the understory species are more respon- cover and frequency values vary tremendously and sive to varying moisture conditions making it must be interpreted with care. desirable to recognize two community-types. The dark, dry understory conditions of succes- An average of 24 species is found in the mesic sional stands (between 30 and 200 years old) result group including 14 herbs and 7 shrubs. Understory in the lowest average number of species per 0.1 ha cover averages 17%. Shrubs are an important and the lowest average understory cover of any component of this type contributing over half of the series. In overall appearance, the understory is understory cover. Both Juniperus communis and nearly devoid of herbaceous growth in most suc- Jamesia americana have high constancy and high cessional stands, and is relatively barren in steady- cover values. Physocarpus monog)'nus occurs in state stands as well. Juniperus communis, Vacci- somewhat over half the stands and has high cover nium myrtillus, Abies, and Arctostaphylos uva-ursi when present. In contrast, herbs are heterogeneous. have the highest understory cover values and are Only Carex rossii has a n average cover greater than the only woody species with constancies over 50%. 0.2%, and only two other species have a constancy No herbaceous species has an average cover of over above 50, Potentilla fissa and Penstemon virens. 1% and Carex rossii alone has a constancy over Twenty-seven species occur in an average stand 5O%. of the xeric Pinus contorta Pseudotsuga forest: 3 trees, 20 herbs and 4 shrubs. In contrast to the Pinus contorta forest (DI) mesic type, only 14% of the total understory cover is Unlike forest-types on environmentally more contributed by erect shrubs, but an additional 23% extreme sites, Pinus contorta forests are almost of the total is in the form of prostrate shrubs, mostly impossible to describe in a generalized way, the Arctostaphylos. Total understory cover is 21%. successional context being critical. Virtually all Juniperus communis has the second highest cover aspects of stand structure and composition change and is present in most stands. 25

Mesic and xeric Pinus contorta - Abies, Picea cause of the high natural fire frequency, stands forests (D4, D5) between 2 900 and 3 100 m on open slopes have These community-types form the upper portion been placed in the Pinus contorta series despite the of the Pinus contorta series. Abies lasioearpa and to dominance of Abies and Picea in old-age, steady- a lesser extent Picea engelmannii replace Pinus state stands. Similarly, stands with Pinus flexilis contorta during succession. Again, tree composi- successional to Picea engelrnannii and Abies lasio- tion and dynamics are similar in these types but carpa have been placed in the Pinusflexilis series. herbaceous vegetation varies sufficiently with mois- The earliest work on the Picea, Abies forests was ture status to warrant recognition of separate types. that of Young (1907) who delimited Pinusflexilis, On mesic sites high cover values of Vaeeinium Pinus eontorta, Pseudotsuga-Picea, and Picea- myrtillus and V. scoparium along with Abies Abies formations and a Populus tremuloides so- contribute to an average understory cover of 42%, ciety. More detailed is the work of Amundsen substantially higher than the 17% average of the (1967) on the subalpine forests of Wild Basin in the Pinus eontorta forest type immediately below on southern portion of the study area. Amundsen the community-mosaic. Havas ( 1971) has suggested recognized four forest types within Wild Basin: the distribution of the green-stemmed, deciduous Pinus contorta successional to Pseudotsuga, Picea- Vaccinium species to be limited by winter snow Abies climax, Pinus flexilis-Pseudotsuga succes- accumulation and persistence; snow being neces- sional to Picea-Abies, and Pinus contorta. Marr sary to prevent desiccation. (196 l) found a similar series of forest types during Species richness is relatively low withan average his studies at INSTAAR. He recognized climax of 25 species per plot including 17 herbs and 5 regions of Picea-Abies and Pinus JTexilis with shrubs. In the shrub group Vaccinium myrtilIus has successional stands of Pinus contorta and Populus an average cover of 8. 1%, 9 times that of the next tremuloides. In the more sheltered valleys he re- highest species, Rosa. The only other shrub species ported thickets of Salix and Betula glandulosa. with high constancy is Juniperus communis. Oosting & Reed (1952) examined the Picea-Abies The most distinctive aspect of the xeric Pinus forests of the Medicine Bow Mountains, studying eontorta Abies. Picea forest is its low diversity. eight stands in detail. They found these forests to be The average stand has only 13.5 species including 4 floristically simple with no significant phytosocio- shrubs and 7 herbs; the lowest average species logical differences with changing site, exposure or richness encountered. Understory cover remains altitude. Subsequently, Romme (1977), Whipple much higher than in the Pinus contorta forest, but (1973, 1975) and Wirsing (1973) have reexamined is composed almost exclusively of Abies seedlings Medicine Bow Picea, Abies forests and have re- and Vaeeinium myrtillus. ported considerable variation corresponding to site In addition to Vaccinium myrtillus, Vaccinium conditions. scoparium occasionally shares understory domi- Picea, Abies forests are remarkably homoge- nance at high elevations, though it has a constancy neous along the length of the Rocky Mountains. of only 35. No herbaceous species contributes Comparisons with stands described from New significantly to cover, the maximum value being Mexico (Dye & Moir, 1977; Peet, 1978b), central 0.35%. The four most frequently encountered herb Colorado (Langenheim, 1962; Whipple, 1975; species are Carex rossii, Epilobiurn angustifolium, Whitfield, 1933), Wyoming (Despain, 1973; Hoff- Pyrola secunda and Arnica cordifolia. man & Alexander, 1976; Oosting & Reed, 1952; Whipple, 1975), Montana (Pfister et al., 1977), Picea, Abies forests (E) Idaho (Daubenmire & Daubenmire, 1968) and Alberta (Horton, 1959; Moss, 1955) suggest a Dominated by Picea engelmannii and Abies uniform structure and a relatively low rate of lasiocarpa, Picea, Abies forests occupy much of species change or turnover with latitude. what has traditionally been called the subalpine zone. This series forms the climax forest vegetation Montane Picea, Abies forest (El) above 3 100 m on all but the most xeric sites, and in Forests on frequently saturated soils in cool, the cool, sheltered valleys down to 2 500 m. Be- sheltered locations between 2 500 and 2 900 m 26

belong to the montane Picea, Abies forest type. The bogs appear as a mosaic of forest and sedge This type differs sharply from the other Picea, meadow, the forest phase being more frequent on Abies forests in herb and shrub composition, and slightly raised portions and Carex aquatilis domi- differs from the lower elevation mixed wet forest in nating the swales. The relative importance of the that the trees are exclusively Picea engehnannii and two phases of the mosaic as well as tree density and A bies lasiocarpa. basal area vary with site conditions. Abies has an Excepting early successional stands, basal area average density roughly twice that of Picea, but ranges between 35 and 55 m2/ha and tree density basal area is nearly equal for the two species between 800 and 1 100 stems/ha. Picea engel- because of the greater size of the Picea. mannii is dominant with an average IV of 68 The understory vegetation is distinctive. Of the compared to 21 for Abies lasiocarpa. Abies appears shrubs, Vaccinium myrtillus has the highest cover to favor the cool conditions of high-elevation sites value, while Gaultheria humifusa and Kalmia pofi- and is less successful at low elevations than Picea. foliaare characteristic, occurringahnost exclusively Comparison of relative seedling, sapling and tree in this type. Carex aquatilis is the most important density of the two species suggests the composition herbaceous species with its average cover of 30%, to be relatively stable with a ratio of roughly 2 to 1. over four times the cover of the next highest species, In terms of basal area, Picea dominates 4 to 1. Caltha teptosepala. The graminoids Eleocharis Cover in the understory averages 56% and spe- pauciftora, Calarnagrostis canadensis and Agrostis cies richness 34/0.1 ha. The shrub laYer with such variabilis also have high cover. Twenty-nine herb prevalent species as Lonicera involucrata, Ribes species occur in the average stand, lacustre and Sambucus racemosa readily distin- Affinities of the Picea, Abies bog forest with the guishes this type from the other high elevation arctic flora (as measured by percent of species in forest-types. Over six shrub species occur in the common) is the highest of any Front Range type. In average stand. comparison with the lower arctic affinities of alpine No less distinctive are the herbaceous species, and timberline vegetation, this suggests the endemic Galium triflorum, Pyrola secunda, Carex disperma character of the alpine vegetation of the southern and Streptopus amplexifofius all have high con- Rocky Mountains and the cosmopolitan character stancies. Particularly important in terms of cover of bog vegetation. are Carex disperma, Streptopus, Equisetum ar- vense, and when present, Calamagrostis canaden- Wet Picea, Abies forest (E3) sis. Prevalents with high indicator value include These forests between 2 900 m and timberline Moneses uniflora, Cinna lat~otia and Gymnocar- with periodically saturated soils are found on pium dryopteris. mineral substrate rather than the organic deposits of the bog forests. Because of the large size of the Pieea, Abies bog forest (E2) dominant trees together with the well-developed Bog forests occasionally develop on flat, poorly herbaceous stratum, this type is among the most drained sites above 2 800 m. These sites are charac- impressive of the Front Range forests. Moist terized by saturated soils with only limited surface conditions result in a particularly long fire-cycle flow of water, and by a thick accumulation of and the sheltered locations in which this forest-type organic material. Boggy sites above 3 300 m usually usually occur serve to reduce the incidence of are not forested. Bog forests are most often fo.und extensive blowdowns. Consequently, many forests behind cirque lakes or in areas of extensive beaver of this type appear to be approaching steady-state (Castor) activity. Because of the rugged topography conditions. of the Front Range, boggy areas are uncommon, The only tree species are the two dominants, and but "the distinctive composition of the vegetation both were present in all stands sampled. The necessitates their recognition as a community-type. normal pattern is for old-growth stands to have Of the 71 species encountered on the three sites many small Abies and a few large Picea. Steady- studied, 40 were modal. Cover averaged 93% with state basal area ranges between 50 and 70 m2/ha, half this attributable to graminoids (including though in some old, even-aged stands basal area in Carices). Mosses also covered much of the ground excess of 85 mZ/ha can be found. surface. 27

Despite high basal area, the canopy is usually the sequence has an average of only l0 species per sufficiently open for both abundant tree reproduc- 0. I ha with frequently only 3 herbaceous species, tion and vigorous herbaceous growth. The average Pyrola seeunda and Pyrola virens being the only understory cover is 62% with 35 species including herbs regularly encountered. Vaccinium myrtillus 30 herbs. The understory is patchy with soil mois- is the only major shrub species, and it provides the ture, substrate and light intensity strongly influ- majority of the understory cover. Mosses and the encing composition. Senecio triangularis and Mer- lichen Pehigera apthosa are also important in these tensia eiliata from dense patches in very wet but forests. sunny spots, especially near running water. Similar Considering the type as a Whole, over 80% of the locations with more slowly moving water and an understory cover of 51% is from woody species, one accumulation of sediment and organic material half being Vaecinium myrtillus and V. scoparium. frequently support Calamagrostis canadensis and Vaccinium myrtillus is over twice as abundant as V. Carex aquatifis. In equally wet, but shaded loca- seoparium; the reverse of the situation in the xeric tions Saxifraga odontoloma, Trollius taxa and and subalpine Picea, Abies forests. Among the several small Epilobiums are important. Dry hum- herbaceous species, Arnica cordifolia, Pyrola se- mocks with accumulated coniferous litter usually cunda and Epilobium angustifolium are the only support Erigeron peregrinus, Polemonium delica- species with high constancies. Where moist pockets turn and Arnica eordifofia. On shaded, moist occur. Osmorhiza depauperata, Mertensia ciliata, patches of raw humus, Adoxa moschatettina, Ra- Luzula parvijlora and Moneses uniflora are found. nunculus eschscholtzii and Osmorhiza depauperata At high elevations, or on dry sites where the canopy are found. Shrubs are largely confined to/the dryer, is more open, Hieracium gracite and Pedicutaris shadier portions of the understory, especially the racemosa can be important. common Vaccinium myrtillus and V. seopariurn. Ribes montiginum has the highest constancy and Xeric Picea, Abies forest (E5) cover values of the shrub species and is somewhat Open slopes above 3 100 m typically are dom- less specific in its choice of intra-community inated by xeric Picea, Abies forest. Superficially, habitat. this type resembles mesic Picea, Abies forest, both in the codominance of Picea and Abies, and the Mesic Picea, Abies forest (E4) solid understory of Vaccinium. The Vaccinium, Cool, sheltered, well-drained sites above 2 700 m however, is primarily V. scoparium rather than the support mesic Picea, Abies forest. Though rela- V, myrtillus of mesic sites. Also, trees are smaller on tively restricted in distribution at its lower limit, the the xeric type than on the mesic and wet types. Very type occupies an increasingly broad portion of the few Picea were recorded over 50 cm dbh with the moisture gradient with increasing elevation. Be- largest Abies being 43 cm. In contrast, Abies tween 3 100 and 3 300 m this forest and the xeric occasionally reaches 75 cm on the wet and mesic Picea, Abies forest are the two most widespread sites with Picea commonly surpassing 75 cm and forest types. The canopy is dominated exclusively occasionally reaching ll0 cm dbh. Steady-state by Picea engelmannii and Abies lasiocarpa with basal area is only 30 to 40 m2/ha on the xeric type density and basal area varying dramatically with (occasionally reaching 55 in even-aged stands) successional state. Picea dominates initially after versus 40 to 45 m2/ha on mesic and 50 to 70 on wet most fires but Abies tends to dominate in older sites (reaching 85 in even-aged stands). An occa- stands and blowdowns. sional individual of Pinusflexilis or Pinus contorta Mesic Picea, Abies forests, like Pinus contorta can be found on lower, peripheral sites. forests, are near the center of the community- Understory cover averages 51% but in excess of mosaic and have early successional stages with poor 90% of this is provided by woody species. An understory development. The type averages 21 average of three shrub species occurs per 0.1 ha, but species per 0.1 ha including 15 herb species and 4 virtually all of the understory cover is concentrated shrubs. These values can be misinterpreted, how- in the two Vaccinium species. Sixteen herb species ever, as species richness varies dramatically across occur in the average stand, but the composition is the successional sequence. The central portion of not distinctive and the total cover is suppressed by 28 competition from the more abundant shrub group. with Pinus flexilis either sharing dominance or Only Carex rossii has an average cover over 1%, restricted to lower elevations (Peet, 1978b). North of the study area Pinusj7exilis is usually found as a Subalpine Picea, Abies forest (E6) low elevation species. On the east slope of the These forests form the transition from mesic Medicine Bow Mountains Pinus flexilis shares Picea, Abies forest to tundra or mesic krummholz, dominance with Juniperus scoputorum, Pinus depending on exposure. Except for an increase in ponderosa and Pseudotsuga at the transition from diversity caused by an infusion of alpine species, grassland to woodland. Pinus flexitis does not loss of Vaccinium myrtittus from the understory, commonly dominate above the Pinus contorta zone or a more open canopy, the forest generally resem- in the Medicine Bows. Despain (1973) and H offman bles mesic Picea, Abies forest. Except for a rare & Alexander (1976) report Pinusflexilis to share individual of Pinusflexilis, only Picea and Abies dominance at lower timberline with Juniperus occur in the canopy. scopulorum in the Big Horn Mountains of The community appears as a fuzzy mosaic with Wyoming, while still another 5-needle pine domi- open areas resembling alpine meadow and tundra nates the xeric, high-elevation sites, Pinus albi- mixed with denser forest areas dominated by mats caulis. In the Wind River Range of Wyoming, of tree regeneration resulting from layering of Pinus flexilis occasionally forms stands on low Abies. Vaceinium is the most important genus for rocky ridges well below the normal lower tree line, understory cover, being almost exclusively V. sco- while at higher elevations it shares dominance on parium. Ribes montigenum has the second highest the xeric sites with Pinus albicaulis (Reed, 1969, average cover. Among the herbs Polemonium 1976). Further north Pinus albicautis completely delicatum, Penstemon whippleanus, Carex rossii, replaces P. flexitis as the xeric, high-elevation Poa nervosa and Potentilla diversifo#a are most dominant. Weaver & Dale (1974) report Pinus common. In the open phase Artemisia arctica, albicaulis stands in Montana with herbaceous Androsace septentrionalis, Sedum lanceolatum, composition, ecological position and tree popula- Hieracium gracile, Potentitla diversifolia, Festuca tion structure all similar to the Pinus flexitis - brachyphylla and Carex foenea dominate. The Picea, Abies forest of the Front Range. It appears understory averages 34 species/0. I ha. that for the southern and central Rocky Mountains resp., Pinus aristata and Pinus albicaulis are the Pinus flexilis forests (F) dominant xeric, high-elevation pines, and only in their absence in the northern Front Range is Pinus Pinus flexilis dominates forest communities on flexitis comPetitively released to play this ecological exposed, xeric sites above 2 800 m. These sites role. typically have shallow, coarse-textured soils, often PinusJlexilis forests have poorly developed un- with large areas of protruding bedrock, and are derstory vegetation. An average of only 24 species usually located on south-facing slopes, ridges or occurs per stand. Understory cover averages 28% sites otherwise exposed to desiccating winds. At with 71% of this attributable to woody species. Few high elevations (>3 100 m) Pinusflexilis can be species have high constancy.in these stands. Picea either successional to Picea, Abies forest or self engehnannii seedlings are usually present as is maintaining, depending on severity of site condi- Juniperus communis. The herbaceous species with tions. Pinus flexilis, however, is not restricted to highest cover are Saxifraga bronchialis, Calama- these high elevation sites. Occasional individuals grostis purpuraseens, Selaginella densa, Arenaria can be found on xeric, rocky sites anywhere be- fendleri and Carex rossii. tween 2 100 and 3 500 m. The role of Pinus flexilis as a high elevation Montane Pinus flexitis forest (F1) climax species on xeric sites is somewhat atypical Montane Pinus fIexilis forest comprises the for Rocky Mountain forests. From James Peak lower-elevation half of the potentially steady-state south along the Front Range a different 5-needle Pinus flexilis forests. This is distinguished from pine ( Pinus subgenus haploxyton), Pinus aristata, the subalpine Pinus flexilis forest largely by the is usually dominant on xeric, high-elevation sites, absence of alpine species in the understory. While 29 other tree species occasionally share dominance, Subalpine Pinus flexilis forest (F3) PinusJlexilis is the most important species with an As Pinusflexilis does not usually assume stunted, average IV of 75%. On more mesic sites dominance krummholz forms, the subalpine Pinus flexilis is occasionally shared with Pinus contorta, though forest represents the upper limit for forest growth on the driest sites only Pinusflexilis occurs. At its on the most xeric and exposed sites. On somewhat lower elevational limit the type grades into Pseu- more mesic sites the forest grades into xeric dotsuga dominated forest. krummholz (G2). The subalpine Pinusflexilis forest Large numbers of Populus trernuloides sprouts is not greatly different from the montane type, the are present on some sites. On these usually succes- most obvious characteristic feature being the abun- sional sites, Populus rarely exceeds 3 cm dbh but dance of alpine species. As with the montane Pinus can attain densities of over 5 000 stems/ha. In flexilis forest, this community type occurs on very addition, seedlings of Picea, Abies, and Pseudotsu- rocky sites with shallow, coarse-textured soils, ga all occur frequently. usually on ridgetops exposed to strong winds and Montane Pinus flexilis forest characteristically vulnerable to severe erosion. occurs on the rockiest, most exposed sites. Roughly Pinusflexilis is the dominant canopy species of one third of the ground surface is covered by rock this type having an average IV of 61, and on the (>10 cm diameter). Where soil has developed its more extreme sites it is the exclusive dominant. On texture is coarse with large quantities of sand and less extreme sites succession proceeds toward broken rock present. Because of the rocky sub- shared dominance with Picea and Abies. Pinus strate, understory cover is relatively low, only 23%. contorta occasionally occurs on warm, southwest- Three-fourths of the un,derstory cover is made up of facing slopes in the lower-elevation portion of the woody species, and tree species alone constitute type. Basal area appears to reach a steady-state 40%. Juniperus communis, Arctostaphylos uva- maximum of 30 to 35 m2/ha, somewhat lower than ursi and Jamesia americana are the most important the 35 40 m2/ha steady-state value of the montane shrub species. A typical stand contains 23 species Pinus flexilis forest. including 14 herbs. Among the herbs, the highest Understory cover averages 22% with 42% of this cover values are attained by Saxifraga bronchialis being tree species and 30% shrubs. While the and Calamagrostis purpurascens. average stand has 3 shrub species, Juniperus eom- munis is the only one with high constancy. Vacci- Pinus flexilis - Picea, Abiesforest (F2) nium myrtillus and Arctostaphylos uva-ursi occa- Pinus flexilis - Picea, Abies forest occurs from sionally have high cover values. Among the herb- 3 150 to 3 450 m elevation between potentially aceous species, Calamagrostispurpurascens has the steady-state Pinusflexilis and the xeric Picea, Abies highest average cover of the prevalents. forest (E5). Here Pinusflexilis acts as a post-fire successional species with Picea and Abies dominat- Forest - alpine transition (G) ing those rare stands where fire has been absent for a sufficient time for steady-state forest to develop. While most of the forests of the Front Range can Understory cover is higher than in the other be interpreted in terms of three or at most four Pinus flexi6s types, owing to the more mesic complex gradients (moisture, elevation, distur- conditions and the absence of extensive rock out- bance-history, and substrate), timberline commu- croppings. Average understory cover is 37% with nities are more complex. Among other factors, one third from trees species and one third shrubs. wind exposure and patterns, snow drift and melt, The average stand contains 26 species including 2! and temperature must be considered (Willard, herbs. Among the shrub species Juniperus com- 1963; Smith, 1969; Bollinger, 1973; Marr, 1977). It munis and Vaccinium myrtillus have the highest was this complexity which lead to failure of ordina- cover and constancy values. Herbaceous species tion analysis to reveal a simple timberline moisture with high cover values include Arenaria fendleri, gradient. Moreover, none of the above mentioned Carex foenea, Saxifraga bronchialis, Carex rossii factors can be effectively evaluated and quantified and Trifolium dasyphyllum. 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 in the shelter of the lower limbs and a dense mat include only homogeneous vegetation when applied develops. to timberline communities. Islands, or clusters of Understory cover is high averaging 75%. Only A bies or Picea, form where one tree initially under a dense tree crown does the cover appear less becomes established and reproduces by layering. than continuous. Cover by tree species is high in Alternatively, trees can be stunted as krummholz, this lower stratum providing over half of the total, forming ribbons or patches where subtleties of wind though much of this results from layering or low and exposure dictate (see Billings, I969; Marr, krummholz individuals. Of the shrubs, Salix 1977). 1nterdigitating with these various formations brachycarpa and S. planifolia frequently dominate is alpine tundra vegetation. All of these variations edges of krummholz patches. In contrast, Vacci- could fall inside a single 0..1 ha unit despite the nium scopulorum and Ribes montigenum are asso- desirability of dividing them into distinct com- ciated with the understory of tree patches. Of those munity-types. No pretense is made of trying to herbaceous species occurring with high cover, Po- interprete the complex patterns. Rather, the results lemonium delicatum, Penstemon whippleanus and are presented as a preliminary interpretation to Epilobium angustifolium are typically forest spe- facilitate comparison with forest and alpine vegeta- cies, Artemisia arctica, Arenaria obtusiloba, Bis- tion types. Two groups are recognized on the basis torta bistortoides, Potentilla diversifolia and Trifo- of position on a complex gradient incorporating #um dasyphyllum are more characteristic of tundra moisture, soil texture and exposure. In addition communities and occur predominantly between subalpine Picea, Abies forest and the subalpine krummholz patches. Sedum lanceolatum, Selagi- Pinusflexilis forest must be considered as ecotonal nella densa, Antennaria rosea and Trisetum spi- to alpine vegetation. The present series is distin- catum can be considered typical of both phases. A guished from these latter types on the basis of typical stand contains 38 species including 3 shrub frequent wind-trained or krummholz treeforms, and 33 herb species. and the interdigitation of forest and tundra. Alpine vegetation, while not specifically treated Xeric krummholz (G2) in the present monograph, has received consider- Xeric krummholz is the shallow soil, well- able previous attention. Both Kiener (1967) and drained, high-exposure counterpart of the mesic Willard (1963, 1979) have published major mono- krummholz (GI). It too is characterized by an graphs on the alpine of Rocky Mountain National interdigitation of wind-trained tree forms and Park, and Kom~irkov~t (1979; Kom~irkowi & alpine species. However, the alpine element is not Webber, 1978) has examined in detail the alpine of the mesic forb group found in mesic krummholz, Niwot Ridge, an area 15 km south of the study area. but is a mixture of scattered individuals character- istic of alpine fell-fields or, on areas with soil Mesic krummholz (GI) accumulation, dry Kobresia myosuroides, Carex Mesic krummholz takes many forms with the rupestris turf. Cushion forms such as Silene acau- physiognomydetermined by site conditions. At one lis, Arenaria obtusiloba and Trifolium dasyphyl- extreme all trees are less than 2 m high (usually less lum are common. than 1 m) and are strongly shaped by wind-caused Large arborescent individuals of Pieea and Abies ice abrasion. The canopy has a smooth, apparently are mostly absent from this type. Shrub forms of ice-sheared surface, with a varying but continuous Abies are typically more important than Picea, or cover of Abies, Picea and Salix. This vegetation Salix brachycarpa which often defines the edges of often takes the form of ribbons or clumps o1~ krummholz mats. The diversity of the microclimatic vegetation which have formed behind small rocks situations, from dry and exposed rock, to Kobresia or depressions where seedlings, sheltered from the sod, to coniferous understory, allows a rich assem- wind, can become established. On sheltered slopes blage of plants to grow in this type, the average are forests composed of groups of large trees stand containing 40 species. Graminoids are an bordered by small Abies produced through layer- important component of the herbaceous cover in ing. While the initial tree of these clumps is xeric krummholz and include Kobresia myosuro- frequently Picea, Abies usually becomes established ides, Poa lettermannii, and Poa alpina. Where 31 substrate permits, Kobresia myosuroides and Carex produce many small trees (see Patten & Avant, rupestris form solid turf, together with Carex 1970; Steneker, 1974). Frequent and repeated burn- albonigra and C.foenea. Dominant forbs in the turf ing can severely reduce the local conifer popula- areas include Trifolium dasyphyllum, Arenaria tions, while the root-sprouting Populus increases in obtusiloba, Arternisia borealis, Geum rossii, and dominance. Dominance after fire can also depend Hymenoxis acaulis. on seed supply. While there is some question as to the ability of Populus to regenerate from seed in Populus tremuloides forests (14) this region, it seems likely that if the time of year and weather were favorable, many Populus seeds Populus tremuloides stands occur isolated in dry, could blow into a stand and become established. high-elevation grasslands, as successional stands on Pinus seeds are heavier and are not dispersed as both wet and dry sites, and as climax vegetation in readily. Thus, on large burns lack of seed source certain poorly drained areas. This complex pattern could restrict initial Pinus establishment. is complicated further by the apparent ability of The enigmatic gradient pattern of Populus tre- Pinus contorta to grow on many of the same sites muloides can be clarified by viewing its ecological and out compete Populus. Typically, Populus- response in geographic perspective (Peet, 1978b). dominated sites have better soil conditions than South of the range of Pinus contorta in the Rocky Pinus contorta sites (Hoff, 1957; Morgan, 1969; Mountains, Populus trernuloides is the dominant Reed, 1971). When they do occur on xeric sites they post-fire, successional species for almost all middle- are usually on deep and relatively fertile soils, elevation forests. With the northward increase of situations such as talus slopes, moraines and allu- Pinus contorta, the central portion of the range of vium. On shallow soils with underlying bedrock, environmental conditions potentially available to Pinus contorta quickly attains dominance and Populus tremuloides is preempted. In the northern Populus is usually present only as an early succes- Front Range Pinus contorta is widespread and sional species rarely exceeding 3 cm dbh. At middle Populus tremuloides appears restricted to periph- elevations on moist sites (often mixed mesic forest) eral habitats through competitive exclusion. This Populus is frequently successional to coniferous restriction of Populus to varied but relatively species. On these sites Populus attains its greatest extreme sites explains the failure of investigators size locally (>50 cm dbh) but is succeeded by Picea working in only one locality to resolve the com- and Abies. On the edges of meadows and other wet petitive relationships of these two species. sites with saturated soils and well developed herb- The confinement of Populus tremuloides to aceous vegetation, Populus appears to maintain peripheral sites results in a heterogeneous assem- dominance rather than succeeding to conifer forest. blage of stands. However, these stands do have the Root sprouting allows continued establishment unifying characteristic of rich herbaceous vegeta- and spread in dense herbaceous undergrowth, a tion in relation to adjacent coniferous forests. This mechanism not available to the local coniferous in turn makes Populus stands even more distinctive species. Similar stable Populus stands have been and precludes effective inclusion in environmentally reported by Marr (1961) and others (Reed, 1971; similar coniferous types. However, on those sites Wirsing, 1973). where a clear successional pattern had developed, While none of the stands included in the Populus the stand was placed in the type containing the group show strong successional trends toward presumed steady-state. Only where Populus was coniferous dominance, all appear relatively young the dominant and no clear successional pattern was and slow conifer invasion remains likely. A few yet apparent was the stand placed in the undif- individuals of Picea, Abies, Pinus contorta and ferentiated Populus group. In effect, the stands Pinusflexilis were found in many of the stands. The grouped as Populus tremuloides forests constitute a relative dominance of Populus varies from site to heterogeneous group of misfits. These stands occur site in response to edaphic factors, and competition between 2 650 and 3 150 m elevation and cover a from the rich herbaceous layer is also important. broad range of moisture conditions. On the com- A few small Populus stems are present in most munity mosaic they overlap most of the Pinus forest-types and after fire these root sprout to contorta series and limited portions of adjacent series. 32

Average understory cover in Populus tremulo- related to both moisture and elevation gradients. ides forests is 57% with leafy herbs and graminoids Grasslands and shrublands reach highest elevations dominant. This is indicative of the lush understory on the driest sites. Bordering these open communi- of the Populus forests. An average of 34 species ties, the Pinus ponderosa zone extends from shel- occurs per stand. Shrubs are well represented with tered slopes at 1 700 m to dry ridges and south- an average of six species per stand. Dominant slopes at 2 700 m. The traditional Pseudotsuga among these in terms of cover are Arctostaphylos zone forms a diagonal from ravine forests at 1 700 uva-ursi and Juniperus communis, though Rosa m to xeric ridges at 2 800 m. The Picea, Abies zone leads in frequency and constancy. The numerous includes a pronounced extension down to 2 500 m Populusroot suckers also contribute a large portion in the cool ravines but is rarely found below 2 900 m of the understory cover. N.o herbaceous species on open slopes. Between the Pseudotsuga and could be considered characteristic, a result which Picea, Abies zones is a heterogeneous diagonal reflects the diversity of environmental conditions group of vegetation types including the mesic represented in the group. Highest cover is attained montane, Pinus contorta, and Pinus flexilis series. by Thermopsis divaricarpa, Arnica cordifolia, and Species successional positions vary with site but Haplopappus paro,i. can be clarified if viewed in relation to elevation and moisture gradients. Pinus contorta dominates the central portion of the community mosaic and is Gradient patterns successional to Pseudotsuga at low elevations, to Abies and Picea at high elevations, and is stable on Forest zones certain intermediate sites. The successional posi- tion of Pseudotsuga is equally complex. At low Rocky Mountain vegetation has traditionally elevations it is a climax species, replacing Pinus been described in terms of elevational belts or life ponderosa. At higher elevations Pseudotsuga is a zones. Ramaley (1907, 1908) and Rydberg (1916) climax species replacing Pinus contorta. On xeric were among the first to propose such classifica- sites the species often occurs alone. In some mesic tions. The various early schemes have been re- montane forests Pseudotsuga is a post-fire, pioneer viewed and revised by Daubenmire (1938, 1943) species giving way to climax stands of Picea and who suggested use of six vegetation-based zones: Abies. However, on other mesic sites, Pseudotsuga alpine, spruce-fir (Picea, Abies), Douglas fir (Pseu- is both a pioneer and a climax species. Similarly, dotsuga), Ponderosa pine (Pinus ponderosa), Abies plays a variable successional role. Abies is juniper-pinyon (Juniperus, Pinus edulis) and oak- often the climax species of stands initially colonized mountain mahagony (Quercus gambelii, Cercocar- by Pinus contorta or Pieea engelmannii after fire. pus montanus). Similar zones have been described However, under certain conditions of seed rain, specifically for Colorado by Costello (1954), though Abies can be the first species to dominate a site after he combined the Pinusponderosa and Pseudotsuga fire, and is with time partially replaced by Picea. zones. Moir (1969) proposed addition of a Pinus Species successional roles, like patterns in diversity, contorta zone, arguing that this species forms a need to be studied in a multidimensional perspec- climax forest type between the Pseudotsuga and tive. Picea-Abies forests. Despite numerous attempts to refine life-zone Understorv vegetation classifications, no concensus exists or is likely to exist (Peet, 1978b). As with any form of vegetation" The small number of tree species occurring in classification, no particular set of zones can be Front Range forests allowed communities to be considered to be a priori correct. Shortcomings will readily characterized using dominant trees. Like always be evident as the complex vegetation of a trees, herb and shrub species vary continuously and mountainous region can never be accurately repre- independently in importance with respect to the sented by a simple set of classes. elevation and moisture gradients, but their much Examination of the gradient model presented in greater number (>530) precludes a simple, gra- Figure 5 shows the traditional elevation zones to be dient-based mosaic representation such as was used 33 for tree composition. As an alternative, represen- defined on the basis of having high levels of incident tative species can be examined individually and solar radiation, they are also warm sites. In contrast common patterns can be sought. Table 2 illustrates ravine slopes at the moist end of the gradient are for 26 selected species variation in the weighted cool, being sheltered from direct sunlight and often average position on the final ordination-based being subject to cold air drainage. Thus, the moisture gradient across a series of 7 elevation belts decrease in temperature resulting from increased (the 1 700-2 000 m elevation belt was not included elevation is compensated for by a shift in position because of the small number of samples). along the topographic-moisture gradient. The species listed in Table 2 have been divided The second group contains species which are into two groups. The first group is composed of largely constant in their moisture gradient positions species which show a strong temperature-correlated with changing elevation. Apparently these species distribution. Specifically, with increasing elevation respond much more strongly to moisture related the species are found farther toward the xeric end of aspects of the environment than to temperature. the topographic-moisture gradient. For example, Species like Calamagrostis canadensis, Carex aqua- Jamesia americana and Physoearpus monogynus tilis and Equisetum arvense are virtually restricted are both largely confined to cool, north-facing to saturated soils and appearently cannot tolerate ravine slopes at 2 000 m, but at 3 000 m are limited even short periods of drought. A number of other to exposed south-facing slopes at the xeric end of species are limited to xeric sites, probably in part the moisture gradient. As the xeric sites are largely because of a requirement 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. species in two of the most important groups, the lncluded in this group are species like Selaginella shrub and graminoid (Poaceae, Cvperaceae) guilds, densa, Muhlenbergia montana and Artemisia fri- are examined. gida which are found over a broad range of Several characteristic groups of shrub species are elevations, but only on dry, exposed sites. evident in Rocky Mountain forests, the groups An alternative approach to the study of under- reflecting similar responses to the macrogradients story vegetation is to recognize groups of similar, of moisture and elevation. The distributions of the potentially competing species, or guilds (sensu most important members of these groups are Root, 1967; Schimper, 1903). Growth forms pro- illustrated in Figure 6. vide a useful basis for such a classification. Fol- The major shrubs of dry, low-elevation wood- lowing this approach the distributions of the major lands are Purshia tridentata on the sunny micro-

Melers Jo/ix planifoha V myrtillis I Gaulthena hurmlus Vocc/nturni scoparium V scoparium Salix brachycaroa Ribes montiqenum~ 3500

lRJbes I ,~o~.~e"/ /Z Vnccinium m~ rnontigen urn scopor~urn / 3300 ]-L ...... i" . oio,om on,0.;o,o .... ,, " I Vaccmium rnyrtil lus I I soaper,urn L ] ~ scoporium I Vacciniurn V. myrtillus~ ~l~ r~ ~.... I myrtillus ] ~ I I V. scoparmm ~"~ ~ ~Tumperus 3100 I ! RJbes lacustre mlum myrtillus ~Tamesio I I 3-uniperus Arctostophylos I I I / 1 Z 2900 n /d'umperus communis W /Lonicera I "~ ~ ~" Arctostaphylos ~ O e involucrata [ ium myrhlhs / ~ ..~""~ < Ribes 1/ Rosa sp. / ~"~ i 3'~4 / lacustre ,~ / 11 / Arctostophylo 2700 ~ ~ /.Tuniperus J/., Z HJgmperu s j communis/ cornmunis~lY 0 • Linnaea, Rosa • Juniperos / • J'amesia • Physocarpus 18~ / IIII I I_ ~°~a°~° -I I ,~F Purshia tridentata Ribes cereum 2500 / w /

I Phy-socarous rnonogynuss.~ ~ ~ ~ ~ -- Ribes cereum ! I Ribes cereum 2300 I Arct°st°phyt°s~ r / Cercocarpus

/ I Physocarpus /Purs~ia tridentota ~I f i I 2100 J-uniperus communis Ribes cereum i_ I O'amesia americana / Arctostaphylos / Ribes cereum j f / J 1900 / Rhus trilobo / / / / / 1700 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. 35

sites with fine-textured soils and Ribes cereum of volucrata, Ribes lacustre, Sambucus racemosa and the cooler, rockier sites. The most xeric sites at the Linnaea borealis to produce the greatest shrub lower woodland border are often dominated by two diversity in the region, an average of 11 species per larger shrubs, Cercocarpus montanus and Rhus 0.1 ha. How these species are differentiated with trilobata. In ravines and on cool, north slopes respect to microhabitat factors is not yet clear, but where true forest replaces woodland, a well-devel- certainly all are broadly overlapping in their distri- oped shrub group contains Acer glabrum, Jamesia butions. americana, Mahonia repens, Physocarpus mono- The central portion of the mosaic diagram domi- gynus, Ribes inerme, Rosa, Shepherdia canadensis nated by Pseudotsuga and Pinus contorta forest is and Symphoricarpus oreophilus. At higher eleva- low in shrub diversity. Here the only species tions, near 2 500 m, the ravine guild overlaps with a consistently important are Juniperus communis, cool, moist forest guild containing Lonicera in- Arctostaphylos uva-ursi and Salix scouleriana.

Meters

5500

Ca~bx aquat~lis Calamagrostis IC. rossii,"~ldP". 9o° ~c. ~°~.,.,,.I /" I~~b. rossft ~. ~300 - Iconadens~s I ~...... ~ -iI \ E eochoms• I I / I pouciflora I I I 5100 I I I / i i I t I / t- r...... -] z 2900 / LI.J / Carex rossli / n," £.9 2700 Z 0 ! / ,II w / 2500 O$1Is / ILl ~reoSl s I

, 2500 t-,pht

~o I ... / 2100 I ~o I / --

\" ~" t f 0 O0

1900 o, ,..;<5,(o>,:o ./

1700 1 Wet 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 pestris, Carex albonigra, Poa nervosa and many Pseudotsuga as potential climax dominants, and other species. there is a parallel shift in the shrub synusia toward Analysis of vegetation in terms of component overwhelming dominance by Vaccinium. Vacci- guilds such as graminoids and shrubs appears to nium myrtillus essentially carpets the understory of have much promise for improving our understand- all the lower elevation Picea, Abies forests, but with ing of community structure. Future studies might increasing elevation or dryness it is replaced by V. profitably focus on the interactions and differentia- scoparium. At the transition from forest to alpine tion of similar species occurring over a portion of tundra and meadow, shrub diversity again in- the mosaic diagram such as the rich shrub com- creases, probably in response to higher light levels. munities of the mesic montane forests, or the At this transition the edges of krummholz mats are graminoids of the alpine transition. often dominated by Salix brachycarpa and Ribes montigenum, with Vaccinium remaining important Species richness throughout. Graminoids present different patterns than In Figure 8 species richness, defined as the those shown by shrubs (Fig. 7). The central portion number of species per 0.1 ha sample, is plotted of the mosaic diagram is again characterized by low simultaneously against the elevation and topo- diversity with few if any species other than the graphic-moisture gradients. Appearing as a bowl- ubiquitous Carex rossii being found in the dense shaped surface, species richness is lowest in the shade of the Pinus contorta or Picea and Abies central portion of the figure corresponding to forests. However, on the periphery of the diagram, areas dominated primarily by dense, even-aged on moist sites or where the canopy is open, forests. In contrast to the predictions of Terborgh numerous species are found. Primarily grasses of (1973), the most centrally located (and probably the the plains and low foothills dominate on the low most widespread) forest types appear the least rich elevation, exposed slopes of the Cercocarpus shrub- in species. Highest richness is found in the relatively lands. These include Stipa comata, Agropyron restricted mesic montane forests where moisture is albicans, Andropogon seoparius and on more abundant and temperatures are moderate. In addi- disturbed sites the smaller Bromus tectorum and tion, moderately high richness can be found across Bouteloua hirsuta. In the woodlands of the Pinus ponderosa Pseudotsuga zone, Muhlenbergia 5500m ...... "..Alpine (~Trensitioi'i~ ;....Alpine montana is the dominant species of open, dry sites, • '.,+%+..+.. "' +. and Leucopoa kingii is equally important on some- what higher and cooler woodland sites. l-- Picea, Abies Forests ..Forests z Middle elevation wet sites are often dominated LI2 by Calamagrostis canadensis and the weedy ad- ventives, Poa pratensis and Phleum pratense. At QZ (.9 o ...... higher elevations (>2 700 m) Calamagrostis con- Z O tinues to dominate the understory on saturated ~2600rn soils but with codominance by Carex aquatilis. :> L.d ...... _1 Carex disperma also codominates on wet sites UJ Forests )...~.... / ...... / between 2 700 and 2 900 m, and Eleocharis parvi- above 2 900 m in the bog forests. ."7 / p( rosa+ ,.." p ...... flora ~//Pseudotsug'a Woodlands ...... The transition to alpine is rich in graminoids. . !I/Forests ,.'" / ...... Carexfoenea is almost as abundant as Carex rossii ,~I / /.." ." //~040 ...... Shrub onds in the wooded areas while Calamagrostispurpuras- ...... acl~ "~'-'~.' ~rasslan~- "" uh~ tens and Trisetum spieatum are the most impor- 1700m tant species of the dryer sites. With continued Wel Xeric increase in elevation, a sod of alpine species can be TOPOGRAPHIC- MOISTURE GRADIENT found interdigitating with forest. Here dominant Fig. 8. Species richness (number of species per O. 1 ha) plotted species include Kobresia myosuroides, Carex ru- against gradients of elevation and site moisture• 37 the entire moisture gradient at both the high and J-shaped diameter distributions (Meyer, 1952; low elevation transitions from forest to grass or Leak, 1964, 1965). If the probability of an arbitrary shrub dominated communities. individual dying is constant for all size-classes, and Various authors have reported richness to be the birth rate is constant, the diameter distribution maximal at low elevations (e.g. Reed, 1969; Whit- can be described by a negative exponential distribu- taker, 1956, 1960; Yoda, 1967) and at middle tion which has this reversed-J shape. Leak (1965, elevations (e.g. Daubenmire & Daubenmire, 1968; 1969) considered this distribution typical of the Whittaker & Niering, 1965, 1975). Richness has deciduous forests he studied in New England. been reported to peak at the mesic end of a moisture Similarly, if the probability of dying decreases at a gradient (e.g. Daubenmire & Daubenmire, 1968; constant rate relative to size, the resultant distribu- Glenn-Lewin, 1975) as well as in the central portion tion is a power function which is a more concave ofa mesic to xeric moisture gradient (e.g. Auclair & reversed-J. Hett (1971; also Hett & Loucks, 1971) Goff, 1971; Whittaker, 1956; Whittaker & Niering, suggested that for Acer saccharum (and probably 1965). All these patterns, and several others, can be most other forest tree species) early mortality is found in Front Range forests. Here the effects of high because of low light levels and intense under- elevation and moisture interact in such a way that is story competition. She further suggested that mor- is essential to consider responses to both factor- tality decreases with increasing age until senescence complexes simultaneously when examining species is reached. Goff & West (1975) concurred, adding richness (Poet, 1978a). Species richness patterns are that if a senescence phase is included, sigmoidal further complicated by the interaction of succession diameter distribution curves should be expected. with the environmental gradients as will be dis- Regardless of which generating model is ac- cussed in the following section. cepted 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 pres- Community dynamics ence of many more small trees than large. Diameter distributions of successional stands are Structural analysis more variable in form. When disturbance removes a major portion of the original tree population, a Harper (1967, 1977) considered plasticity of large number of young trees can usually invade. plant response to be one of the major factors With growth, these young trees soon preempt delaying development of a theory of plant popula- critical resources and inhibit additional regenera- tion dynamics, largely because plasticity limits use tion. Skewed bell-shaped diameter distributions of age-based theory. He concluded size rather than appear most characteristic of such stands (see age to be the most important aspect of plant Bailey & Dell, 1973; Bliss & Reinker, 1964; Day, population structure. Rabotnov (1969; see Harper 1972; Ilvessalo, 1937; Nelson, 1964), the heights and & White, 1974) and his co-workers have used an breadths of the curves being influenced by initial approach based on life-stages as an alternative to density and synchrony of seedlings establishment. age in studies of plant population structure. To The essential pattern is that the more favorable the apply this approach to trees, life-stages need only be site and the more intense the competition, the assigned on the basis of uniform diameter-classes greater the initial preemption of resources. This (e.g. Hartshorn, 1975). Because size relates to leads to suppression of the smaller trees and canopy position and survival probability more elimination of regeneration until such time as strongly than does age, examination of diameter mortality of established trees reduces competitive classes in preference to age-classes appears to be a pressures. Based on these considerations, a stand productive approach for examining forest dynam- with a reversed-J diameter distribution for each of ics. its species can be considered to be near the steady- A stable or steady-state forest should be charac- state condition. Deviation can be interpreted as terized by balanced birth and death rates. Such evidence of reduced reproductive success and prior forests which typically have many small trees and a disturbance (e.g., Jackson & Faller, t973; Johnson few large ones are referred to as having reversed & Bell, 1975; Schmelz & Lindsey, 1965). 38

The gradient-based classification was used to comparison, any conclusions reached using this provide sets of stands of variable age but with method would be suspect. Given the limitations of relatively constant site conditions and develop- the method, the patterns reported must be con- mental potential within which progressive changes sidered somewhat qualitative; only average condi- in tree population structure could be examined. tions and suggestion of the range of possible Bell-shaped curves were considered indicative of variation should be inferred. even-aged stands with the breadth of the bell a consequence of the length of the establishment Patterns of forest development period, the initial density and site quality. Negative exponential distributions (reversed-J) were consid- Pinus contorta forests ered indicative of sustained replacement and thus of Typical diameter distributions for an age se- approach to steady-state conditions. All stands quence of stands of the Pinus contorta forest type within a community-type were examined together are shown in Table 3. The developmental pattern and arranged in what appeared to be plausible suggested by these stands conforms to that nor- successional sequences. Because variation in site mally reported for even-aged conifer forests. A bell- condition, seed rain, and disturbance history can all shaped diameter distribution appears early in stand influence the rate of.succession, arrangement of development and, given absence of further forest stands based on diameter distributions should serve perturbation, remains for the lifetime of the initial to make stands more readily comparable than cohort, usually 250-300 yr. As the stand develops, arrangement on the basis of stand age alone. both mean tree diameter and the variance in tree Ordering forest stands grown under similar site diameter increase. conditions into sequences of increasing age can The bell-shaped curves and increasing mean tree provide a valuable means of examining forest size are in large part a direct result of reproductive development. However, even with the large data failure. Low competitive pressures immediately base employed, an element of subjectivity is un- after disturbance allow a high rate of tree seedling avoidable in the construction of such develop- establishment. Because of the resulting high density mental sequences. The highly stochastic nature of of seedlings, canopy closure is followed by a period tree establishment and growth as well as variation of intense intraspecific competition with low aver- in seed rain and climate lead to considerable age growth rates and high mortality. Virtually no interstand variation, thus obscuring underlying new seedlings become established after initial patterns. Without a large number of stands for canopy closure. The length of time until canopy

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 4 3 0 1 Stand 108: Stage 4, --225 years old Pinus contorta 47 2 3 l 6 7 3 6 5 13 13 9 6 7 1 1 0 Stand304: Stage 5, --3 l0 years old Pinus contorta 83 64 18 23 14 6 3 3 4 5 8 6 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. 39

closure and the associated reproductive failure mature, even-aged stands, including both those varies. Under extremely favorable conditions 10 yr stands with a bimodal distribution suggesting re- is adequate, while in some cases 50 or more yr may newed regeneration and those with unimodat dis- be required. Regardless of the time period, all tribution with mean diameters greater than 22.5 stands with this developmental pattern are referred cm. Class 5 contains stands approaching an all-size to initially as 'even-aged'. In stand 183 shown in diameter distribution. The average characteristics Table 3, none of the individuals >10 cm tall were of the stands belonging to each of these classes are less than 24 yr old while the oldest individual was summarized in 'Fable 4. only 42. The smaller individuals, although com- Across the developmental sequence suggested by petitively suppressed, were close to the same age as Table 4, basal area increases steadily to a site the canopy dominants. Even-age stand structure specific maximum (--32 ma/ha), and then remains cannot persist indefinitely. Eventually a sufficient relatively constant for roughly 150 yr. This latter number of trees will have died so that openings period is one of stand stagnation with little net which appear in the canopy cannot be filled by production. Diameter increase is often less than 2 suppressed individuals from the understory. When cm in 100 yr for all except the dominant canopy competitive pressures have declined sufficiently, trees. While basal area remains reasonably con- regeneration will resume. Stand 108 in Table 3 stant, large, compensatory changes occur in tree shows the start of this process with an increase in density and size. Average diameter increases from seedling density. Stand 304 represents a transition 11.3 to 22.25 cm while the density of Pinus contorta phase with relict members of the original popula- (>7.5 cm) drops from 1 578 to 315 per ha. tion persisting during the establishment of a new Despite a high variance in initial stockage, most but less even-aged cohort of future canopy domi- stands converge toward a site specific basal area nants. maximum within 100 yrs. Density, however, is Each of the 23 stands of Pinus contorta forest much more variable as is suggested by the standard sampled during the study was classified as belong- deviations in Table 4. Weather patterns, proximity ing to one of five developmental classes. Class 1 of seed sources, time of disturbance, intensity of fire consists of stands with a unimodal diameter distri- and numerous other factors can strongly influence bution and mean diameter between 2.5 and 7.5 initial stocking levels. Because Pinus contorta is cm. Class 2 contains similar stands with a mean tolerant of extreme crowding and capable of pro- diameter between 7.5 cm and 15 cm. Class 3 is longed suppression (Alexander, 1974; Clements, composed of unimodal stands with mean diameters 1910; Horton, 1956; Mason, 1915), effects of initial between 15 and 22.5 cm. Class 4 contains over- 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 more open and to have lower biomass and canopy trees. Nonetheless, there is a pattern of continually cover than the even-aged Pinus contorta forests decreasing density due to natural thinning. which predominate in the study area. The basal Because of the combined effects of canopy tree area of 17.5 m2/ha shown for near steady-state mortality and reproductive failure, the opening of (Class 5) stands in Table 4 is probably abnormally the canopy is inevitable with this usually taking low, owing to the small number of new generation place when the stand is between 250 and 325 yr old. trees which had reached maturity in the stands Old trees are frequently infected with heart rot and studied. More likely, steady-state basal area is become increasingly susceptible to wind breakage. around 24 m2/ha. The actual value is a question of Mistletoe and insect infestation (Amman, 1977) are only academic interest, however, as the natural fire also common in older trees. These factors com- frequency is sufficiently high that second and third bined with the greater exposure of the remaining generation forests are rarely if ever encountered. trees to storm damage can concentrate canopy Just as the high density of trees near the middle of breakup in a relatively brief interval, often less than the age sequence precludes seedling growth, so too 30 yr. In such cases an abrupt drop in basal area is the remainder of the understory stratum sup- occurs. Similar dramatic transitions have been pressed. Cover of herbaceous vegetation (Table 4) reported for the conifer forests of Fenno-Scandia drops from 25% in the early stages of establishment (ilvessalo, 1937; Sir6n, 1955), and Alberta (Ploch- to under 1%. Similarly, the number of species in the mann, 1956). sequence (Table 4) drops from an early high of 24 to The structure of a forest following death of the a low near 14, only to increase to 27 again during first generation of dominants will be strongly the breakup and subsequent recovery phases of influenced by the synchrony of mortality in the stand development. Although the results shown in previous stand. If mortality is rapid, due perhaps to Table 4 suggest this variation in diversity to be a wind storm or a severe insect outbreak, the new attributable almost exclusively to changes in the generation will tend to be even-aged and struc- success of herbaceous species, it is not clear why the turally similar to the previous forest. If, however, competitive impact should be of less importance for mortality is gradual, a mixed-age stand will develop woody species, unless they simply have greater which can be expected to more closely approximate longevity. a steady-state, all-sized structure. A continuing se- Structural development in other forest types of quence of oscillations in basal area and cover could the Pinus contorta series appears not greatly unlike result (Plochmann, 1956) or, more likely, natural that in the Pinus contorta type. In Pinus contorta- damping could lead to a steady-state structure Pseudotsuga forest initial composition can be soon after the demise of the first generation trees. dominated by either Pinus contorta or Pseudotsuga Because Pinus contorta seedlings and saplings have depending on seed supply and site conditions. A the capability of growing to maturity only in open frequently encountered pattern is for both Pinus forests, steady-state forests can be expected to be 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 3 l 0 1 Pseudotsuga 15 7 5 1 3 2 1 I 0 1 0 0 1 1 2 0 0 1 I 0 0 1 41

for Pinus to overtop the Pseudotsuga, leaving it Pinus contorta stands. Basal area is typically higher suppressed in the understory. In addition, Pseu- than in the Pinus contorta forest, with the site dotsuga establishment probably continues some- specific maximum close to 36 m2/ha compared to what longer than does that of Pinus due to greater 32, and a steady-state value near 32 m2/ha com- shade tolerance. This pattern is illustrated by stand pared to 24 in the Pinus contorta forest. 53 (Table 5) which has bell-shaped diameter distri- The primary difference between Pinus contorta butions for both Pinus contorta and Pseudotsuga. forest and Pinus contorla - Abies, Picea forest is The plants of the two species have roughly the same that a successional shift in species composition mean age but have mean diameters of 11.2 and 5.8 occurs in the latter type. Pinus contorta dominates cm respectively. initially though other species are often present. Mean diameter increases with time, but the size With growth of the Pinus, most of the other species distribution retains its belt-shaped form and are crowded out, though some Abies and Picea will Pseudotsuga remains confined to the understory remain suppressed in the understory. With increas- during most of the stand development period (e.g., ing stand age Abies and Pieea become established stand 261). Pinus contorta typically fails to repro- in the understory and become increasingly impor- duce on these sites, thus allowing the longer-lived tant as is indicated by the shift in importance values Pseudotsuga to increase in dominance whenever in Table 6. openings appear in the canopy. Stand 247 is an The diameter sequences shown in Table 7 illus- example where many of the canopy Pinus and trate the later stages of stand development. Stands Pseudotsuga have died and in which Pseudotsuga is 29,239, and 71 can be viewed as a developmental becoming increasingly dominant. sequence on xeric sites. After canopy break-up, Initial post-fire forest development in Pinus which occurs around 300 yrs, Abies assumes domi- contorta - Abies, Picea forest is again similar to nance. Note the lack of advanced Abies regenera- that encountered in the Pinus contorta forest type. tion in stand 29 where the onset of canopy break-up Table 6 summarizes a developmental sequence is suggested by the large number of standing dead based on 24 stands classified using the same 5 stages Pinus contorta. Here Abies is present only in the employed for the Pinus contorta forest. As in the smallest size class. Because stand 239 represents a Pinus eontorta forest, basal area increases steadily xeric extreme of the type, it has less advanced to a site specific maximum and then stagnates. In regeneration than most stands in the type. Stand the latter stages of stand development basal area 107 (Table 7) is typical of more mesic conditions again drops, though not as dramatically as in the with considerable advanced regeneration. In such

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

Relative Importance~ Number of Typical Basal Area Diversityz Stage Samples Ages (m•/ha) Pinus contorta Abieslasiocarpa Piceaengelmannii Herbaceous Total l 3 20-70 [ 1.8 72.1 4.0 20.9 21.7 30.7 (11.4) (47.9) (6.6) (36.2) (1.5) (4.2) 2 3 70-125 34,9 97.5 0.1 0.2 6.0 12.7 (9.1) (3.6) (9.2) (0.2) (4.6) (6.4) 3 7 125-175 35.9 55.3 30.0 8.6 4.0 10.1 (13.2) (32.7) (31.0) (7.0) (3.6) (5.5) 4 7 t75-250 38.6 42.5 19.5 20.9 5.1 11.0 (7.2) (30.8) (18.3) (11.6) (2.9) (4.2) 5 4 250-350 32.3 13.3 32.1 53.9 5.2 9.7 (1.3) (12.8) (9.9) (7.9) (3.5) (4.0)

Standard deviations shown in parentheses. Relative importance is the average of the relative density (>7.5 cm dbh) and the relative basal area. ~' Species per 0.1 ha. 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 I Picea engetmannii 19 5 4 4 8 0 5 0 3 4 2 I 1 I 0 1 0 1 0 Pinus contorta 11 3 3 1 3 4 1 1 0 0 1 1 0 t 0 0 0 0 1 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

mesic stands Abies and to a lesser extent Picea contorta and Populus tremutoides were co-domi- become established early and remain suppressed in nants, Populus was consistently overtopped and the understory. It is common to find individuals showed evidence of substantial recent mortality. well over 100 yr old and only I m tall. Large Species diversity is initially high, around 31/0.1 numbers of seedlings and small saplings are present ha, in the Pinus contorta - Abies, Picea forest, but by the time canopy break-up starts with the con- drops off rapidly with canopy closure to around sequence that very little Pinus contorta regenera- 10/0.1 ha (Table 6), a pattern similar to that of the tion occurs and the drop in basal area and cover is Pinus eontorta forest. This forest differs from not as dramatic as that described for the other Pinus contorta forest, however, in that little if any Pinus contorta types. recovery can be seen in the latter stages of stand Stand 239 (Table 7) represents an older phase of development. A combination of a well-developed the forest type represented by stand 29, but with groundcover of Vaccinium myrtillus and dense larger and more abundant regeneration. The final reproduction of Abies and Picea provides a highly stand in this sequence, number 107, represents near competitive environment even after death of the steady-state conditions with all three species having original canopy trees. Again, the change in diversity roughly negative exponential diameter distribu- appears to reflect only a change in the herbaceous tions. component. Pinus contorta frequently must compete with Populus tremuloides for dominance after fire. Picea, Abies forests While the precise nature of the ecological relation- Located above the Pinus contorta forests on the ship between these species remains uncertain, stand elevation gradient and occurring on all but the structural comparisons reveal strong similarities. driest sites, Picea, Abies forests show considerable All of the Populus diameter distributions examined variation in structure despite the presence of only showed evidence of the stands being even-aged, two major tree species. Tables 8 and 9 illustrate none appearing structurally stable. In most cases some of this structural richness in the form of the size distribution was bell-shaped with the mode representative diameter distributions. less than 18 cm dbh, but with a large number of ~,s a means of examining stand development, all seedling class individuals produced from root samples from wet, mesic, and xeric Picea, Abies sprouts~ In stands over 30 yrs of age where Pinus forests were classified as belonging to one of four 43

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

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 Populustremuloides ? 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 tion, though occasionally a few large, relictual with bell-shaped diameter distributions (indicative individuals are included, Average characteristics of even-age structure) with average diameters of of stands in these four stages are summarized in Picea less than or equal to 12.5 cm. Stage two Table 10, and representative diameter distributions contains similar stands with average diameter of for Picea, Abies forests are shown in Table 8 and 9. Picea between 12.5 and 25 cm. Stage three includes Summary tables were not constructed for the other the remaining even-aged stands, those with mean forest types in the Picea, Abies series because of the diameters >25 cm. The final stage is composed smaller numbers of available samples. of all stands close to an all-size diameter distribu- 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 dence of either past or present even-aged struc- forest) are largely free from the otherwise all ture. Stand 235 (Table 8) is a successional stand pervasive influence of fire. Wind, however, plays an with apparent residual effects of disturbance, both important role here as a disturbance factor. For bog in an extremely high basal area (84 m2/ha) and a forests in particular, an open canopy and shallow somewhat bell-shaped diameter distribution of rooting can lead to a high incidence of wind throw. Picea. Such a stand can be expected to eventually The largest tree found in the bog forests had a undergo a dramatic decline in basal area and cover diameter of only 44 cm, while in the more sheltered similar to that which characterizes Pinus contorta but closely related wet Picea, Abies forest, in- forests during death of the canopy dominants. dividuals with a diameter near 100 cm dbh were not Stand 226 represents a still older stage where only a uncommon. In the bog and montane Picea, Abies small number of the original trees persist and where forests studied, diameter distributions were mostly basal area has dropped to only 57.5 m2/ha. With near the steady-state, reversed-J shape, though approach to near steady-state conditions, average often with irregularities suggestive of prior canopy basal area drops still further to around 50 m2/ha disruption. In both stands 27 and 300 shown in (Table I0). Table 8, Picea appears to be retaining constant In mesic Picea, Abies forests the developmental dominance relative to Abies. pattern after fire is not unlike that in Pinus contorta Wet Picea, Abies forest is generally more pro- forests. The first stage in forest development is ductive than bog forest and often exhibits evi- establishment of Picea and Abies seedlings. Es-

Table 10. Picea, Abies forest developmental sequences.

Number of Basal Area Diversity2 Stage Samples (m2/ha) Rel IV Picea ~ Rel IV Abies Herbaceous Total

Wet Picea, Abies forest l 2 9.8 27.6 72.4 42.5 49.0 (1.1) (29.7) (29.7) (13.4) (15.6) 2 2 73.9 66.9 33.1 20.5 24.5 (14.5) (27.6) (27.6) (4.9) (4.9) 3 5 51.9 65.8 34.2 30.0 34.2 (12.0) (10.7) (10.7) (8.9) (9.9) 4 5 49.7 59.5 40.5 28.4 34.8 (18.0) (8.1) (8.1) (3.6) (3.8) Mesic Picea, Abies forest 1 3 12.2 37.0 67.3 16.7 23.7 (1.0) (37.1) 38.3) (3.5) (5.1) 2 3 41.2 76.5 22.5 5.0 10,3 (7.9) (14.0) (13.8) (3.5) (5.8) 3 4 53.8 63.1 34.0 11.0 15.7 (15.5) (14.1) (12.8) (5.5) (6.2) 4 7 40.5 56.6 40.8 20.7 28.7 (8.2) (17.2) (17.5) (9.7) (12.2) Xeric Picea, Abies forest 1 2 12.2 60.4 39. 29,0 35.0 (7.2) (28.4) (28.4 (1.4) (4.2) 2 6 29.7 52.1 36.9 17.0 22.3 (3.7) (17.8) (18.8 (4.6) (5.2) 3 6 45.3 59.2 39.7 14.0 8.7 (10.6) (10.5) (13.0 (5.2) (5.0) 4 3 37.5 56.1 32.5 10.7 15.7 (4.3) (21.4) (11.2 (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. 45 tablishment rates are usually lower than in the stand with Abies far ahead in tree density. It is Pinus contorta forests with the result that diameter reasonable to expect this trend to continue with the distributions are broader and not infrequently virtual elimination of Picea after perhaps three negatively skewed (see Table 9, stands 199, 185). generations. Of course, the natural fire-cycle would Here even-age typically means a 20 to 70 year normally prevent such a process from reaching period of establishment, a situation similar to that conclusion. Even in the unlikely event that a small reported by Franklin & Hemstrom (1981) for Pseu- piece of forest managed to remain unburned for a dotsuga in Oregon. Low elevation stands, however, millennium, adjacent forests would surely burn were observed to have diameter distributions vir- providing the necessary continued seed source for tually identical to those found in the Pinus contorta Picea to maintain at least a small role in the - Abies, Picea forest suggesting rapid establishment structure of the unburned forest. rates under favorable environmental conditions. Large blowdowns provide another, not uncom- Establishment rates also vary with seed supply. mon, form of disturbance in Picea, Abies forests. Day (1963) suggested that bursts of Picea and Abies Blowd owns give the large populations of suppressed regeneration frequently correspond to favorable A bies seedlings and saplings opportunity for release years for seed production. If a site is adjacent to with the result that trees of Abies soon outnumber unburned Picea, Abies forest, the establishment Picea (6 to 1 after 24 yrs of recovery for plot 202, rates are likely to be high (Horton, 1956, 1959). For Table 9). However, given additional time the tess example, in the present study a gradient was numerous but faster growing Picea can be expected observed in Picea and Abies density on a 75-year- to overtop some of these Abies with the net result old burn with stands adjacent to relic old-growth being a more equitable division of importance. having a high tree density and stands only 200 m On xeric sites Picea and Abies share dominance away remaining very open. with an occasional Pinus contorta or P. flexilis During the second stage in stand development being present in peripheral stands. Here again rapidly growing Picea dominate, having over- successional patterns are variable. On the mesic topped and competitively suppressed many of the edge of the series, stand development is similar to contemporaneously established Abies. Stand 185 that described for mesie Pieea, Abies forest. At low (Table 9) illustrates a bell-shaped diameter distribu- elevations, pronounced bell-shaped diameter dis- tion typical of this stage and usually best developed tributions often occur while at middle to high between 200 and 250 yrs after fire. Cover and basal elevations and on the driest sites, such bell-shaped area are high but Abies has remained suppressed in distributions are absent. Specifically, on environ- the understory. With increasing age, basal area mentally extreme, high-elevation sites establish- slowly increases toward a site-specific maximum ment rates are typically very low and few individuals near 55 m2/ha while tree density decreases. After are established in any one yr. Chronic reproductive around 350 yr, owing to continued mortality, the failure has been reported previously for similar sites canopy starts to thin and regeneration resumes, (Bollinger, 1973; Fonda & Bliss, 1969; Habeck & thus marking the start of the third stage. With Mutch, 1973; Noble & Alexander, 1977; Stahelin, thinning and eventual loss of the original dominant 1943). individuals, both Abies and Picea approach all-size Stand 305 (Table 8) depicts a forest known to distributions. This pattern is consistent with the age have burned in excess of 85 yrs before sampling. structure reported by Whipple & Dix (1979) for Here density of Picea and Abies seedlings is low, the Picea, Abies forests of the west slope. canopy is open; basal area is only 7 m2/ha. Yet, this In the absence of further disturbance, the greater stand has had a higher recovery rate than other reproductive success of Abies in the shaded under- portions of the same burn where basal area re- story favors its steady increase in dominance. The mains below 1 m2/ha after 85 yr. Of particular fourth stage, a second generation Abies forest, interest on these slow establishment sites is the occurs only after in excess of 500 yrs have passed. formation of a reversed-J diameter distribution Stand 138 (Table 9) is such a stand where Abies has early in stand development with the competitive greatly increased at the expense of Pieea. Basal inhibition of regeneration typical of even-aged areas of the two species are roughly equal in this stands rarely occurring. In such stands basal area 46 and density only slowly increase with no evidence of with approach to steady-state conditions is less the overshoot observed with even-aged develop- consistent. On the dryest types including xeric ment. Stand 198 (Table 8) illustrates an older stand Picea, Abies forest and xeric Pinus contorta - on a slow establishment site with Picea still domi- Abies, Picea forest diversity appears to exhibit little nant. Given sufficient protection from fire it is recovery with approach to steady-state, while more likely that Abies will steadily increase in domi- mesic types including wet Picea, Abies forest, mesic nance, just as on the more mesic sites. Picea, A bies forest, and Pinus contorta forest show The effects of fire at or near timberline can be marked increases. dramatic and long-lasting owing largely to the low establishment rates of tree seedlings. Several old Pinus flexilis forests burns in excess of 100 yr in' age were observed Early successional, post-fire stands of Pinus during the study on which very few trees had flexilis - Picea, A bies forest are typically even-aged, become reestablished. Bollinger (1973) studied and composed of either pure Pinus flexilis or a establishment at the alpine-forest ecotone farther combination of Pinus and Picea. With increasing south along the Front Range and concluded that age Pinus assumes dominance, with Picea when competition from herbaceous species had effectively present remaining suppressed in the lower portion halted tree establishment on many old burns. of the canopy. With maturation and death of the Fonda & Bliss (1969) reported a similar phenome- initial Pinusflexilis population some 200 to 300 yr non in the Olympic Mountains of Washington. It later, Picea is released to assume dominance. appears likely that timberline in the Front Range is During this period regeneration of both Picea and often the result of a dynamic equilibrium between Abies resumes. On the dryest sites few Picea and fire and climate, rather than being a purely climatic Abies are established before canopy breakup. In phenomenon. Fire periodically removes the high either case Picea and Abies eventually dominate elevation forest vegetation which returns, but only with Pinus flexilis being retained mostly in small very slowly. Because of the high natural frequency patches on particularly rocky substrate. of fire, it is probable that timberline rarely reaches Diameter distributions for three developmental its true climatic limit. stages of Pinus flexilis forest are illustrated by Following the initial stage of stand development, stands 228,229, and 210 (Table 11). In these stands species richness drops dramatically in all three of sufficient Pinusflexilis became established early in the community types summarized in Table 10. This stand development to significantly inhibit subse- is consistent with the changes observed in Pinus quent seedling establishment, thus inducing typical contorta forests both in absolute drop, and in being bell-shaped frequency distributions. As in the other attributable almost entirely to changes in the num- forest types described, the bell-shaped diameter ber of herbaceous species. The degree of recovery 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 6 4 2 2 2 2 2 2 1 Abies lasiocarpa 49 47 20 19 5 12 6 4 1 4 1 2 1 Picea engelmannii 31 6 4 8 5 1 3 1 3 1 0 3 1 2 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 tl 7 8 8 9 7 9 2 3 8 1 3 0 l 0 0 0 t Picea engelmannii 6 7 7 5 5 3 3 4 5 5 2 2 0 0 0 1 Abies lasiocarpa 18 8 9 I 3 3 0 1 2 0 1 1

eventually disappearing due to ingrowth and can- number of seedlings per hectare has increased, in opy tree mortality. The oldest of the three stands in part due to the presence of Picea and Abies in the Table 9 (210; N240 yr) has no Pinus regeneration, understory. There is little evidence that Picea or and reversed-J distributions are present for Picea Abies will replace Pinus Jlexilis, but they may and Abies. Basal area in such forests appears to eventually share dominance on more mesic sites. increase steadily toward approx. 45 m2/ha (though On the more xeric sites most Picea and Abies stems stands of this type were recorded with over 60 die before reaching 12 cm dbh. In the final stage m2/ha). Other stand data suggest a subsequent basal area increases to what appears to be a steady- drop to a steady-state level of perhaps 35 to 40 m 2 state level between 35 and 40 m2/ha. with death of the initial cohort of Pinusflexilis, Unlike Pinus contorta forests, Pinus ftexitis Structural dynamics of the montane Pinusflexi- forests rarely show an extreme overshoot in basal lis forest vary with substrate. On relatively deep-soil area or cover. Rather, initial establishment is slow sites patterns similar to those of the Pinus contorta due to severe environmental conditions, thus reduc- forest can be observed. Very few seedlings occur in ing the likelihood that a large initial cohort will stand 118 (Table 12) which has a regular, bell- severely reduce seedling and sapling growth. Seed- shaped diameter distribution. On xeric, shallow- lings become established slowly, reaching peak soil sites the pattern is different; post-fire regenera- density only after the canopy has started to close. tion is gradual and similar to that of the environ- Thereafter basal area continues to increase with a mentally extreme xeric Picea, Abies forests. noticeable but minor decrease in seedling density An age sequence of four Pinusflexilis stahds is resulting. With maturation and natural thinning of shown in Table 12. Stand 113 is an early (--70 yr), canopy trees, Pinus flexilis regeneration attains post-fire forest containing a few relic trees. Relic steady-state levels. individuals are not uncommon on these sites, largely because the sparce vegetation on rocky soils Pinus ponderosa woodlands does not carry ground fire particuNrly well. With The types of size structure encountered in foothill continued stand development (e.g., stand 112) basal woodland communities are markedly different area and mean diameter increase as does density of from those of higher elevation forests. First, nega- mature trees. The diameter sequence of stand 112, tive exponential or reverse-Jdiameter distributions has a bulge reflecting the nearly synchronous initial typical of supposed steady-state populations are establishment and subsequent overcrowding. These rarely encountered. Second, the bell-shaped distri- trends are continued in stand 122, and culminate bution type, usually resulting from an early burst of with stand 97 where the diameter sequence is regeneration following disturbance with subsequent approaching the standard reverse-J shape. Here the competitive suppression of regeneration, is also 48

Table 13. Typical diameter distributions for Pinusponderosa 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 10000 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 1 212 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

uncommon. Instead, patterns largely restricted to Arizona similar episodic establishment has been woodland vegetation are dominant. Often a mix- shown to correlate with years climatically favorable ture of tree sizes is encountered, but with several for both establishment and seed production bulges in the diameter distribution, suggesting (Cooper, 1960; Schubert, 1974). Hoffman & irregular episodes of successful regeneration (Table Alexander (1976) suggested episodic establishment 13, stands 155,105, 177). On some sites uniform size for two widespread, low elevation Pinusponderosa distributions are encountered such as illustrated by woodland types of the Bighorn Mountains of stand 269 (Table 13). Stand 171 represents a more northern Wyoming. typical intermediate case. Disturbance factors provide an alternative expla- Examination of age structure for two represen- nation for periodic establishment. Woodland com- tative woodland stands confirmed the correspon- munities with open, grassy understories doubtless dence between periodic establishment and groups burned regularly before settlement. Many trees still of similar diameter. The only major result not show multiple firescars as evidence of repeated evident from the diameter distributions was that burning. The irregular frequency of these fires un- establishment periods were much shorter than doubtedly influenced seedling establishment. Dau- indicated by the broad humps in the diameter benmire (1943) reported distinct age groups of distribution. Such humps can, in fact, consist of saplings corresponding to series of consecutive sum- several bursts of regeneration. For this reason only mers without fire during which seedlings had time stands with long periods between establishment to grow into fire resistant saplings. However, obser- episodes (>50 yr) show a clear pulsed diameter vations of both unburned and recently burned structure. Stands with frequent establishment epi- stands in the Front Range revealed no bursts of sodes usually exhibit more even diameter distri- regeneration on either type. In addition, there was butions (e.g., stand 271). no evidence of a change in regeneration dating from Pulsed regeneration appears most characteristic the drastic increase and subsequent decrease in fire of the least favorable of the low-elevation sites, the frequency which occurred in the early part of the Pinusponderosa shrublands and the mesic foothill present century. Such release has been documented woodlands which together form the transition to in neighboring regions (e.g., Cooper, 1960, 1961; grassland. On these chronically drought-stressed Marr, 1961; Weaver, 1959) and can be found in the sites with limited potential for tree growth and higher elevation Pinus ponderosa forests of the establishment, it is likely that only in an unusual study area. While fire may increase the interval year will both weather and seed production be between regeneration events, it can not be con- appropriate for abundant tree regeneration. In sidered 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 l 0 1 1 0 0 l 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

regeneration in Front Range woodland communi- of periodic ground fires. In aU of the other wood- ties. land community-types the overriding factor con- On some sites all tree size-classes have roughly trolling size structure appears to be establishment equivalent representation (Table 13, stand 269). as influenced by site and climatic variables. This pattern is difficult to explain based on short- term observations, but most likely very old, stable Pinus ponderosa, Pseudotsugafi)rests stands are involved, with little mortality between Forests of the Pinus ponderosa, Pseudotsuga achievement of sapling size (>5 cm diameter) and series are transitional between Pinus ponderosa attainment of a diameter in excess of 50 cm. A small woodlands and Pinus contorta forests. Not surpris- but consistent decrease in radial growth with diam- ingly, characteristics of both groups can be seen in eter could reconcile the inevitable death of some their development. intermediate aged individuals with the retention of Two edaphic phases of most Pinus ponderosa, the flat diameter distribution. Alternatively, a series Pseudotsuga forests can be recognized. Fine-tex- of frequent and regularly spaced regeneration tured soils support relatively open forests, similar pulses could produce these distributions. It is not to the lower elevation woodlands. Here the well- uncommon for competition from grasses to limit established sod of grasses effectively precludes most Pinus ponderosa regeneration to a limited number tree establishment. In contrast, rocky, coarse-tex- of micro sites found next to rocks and in other gaps tured soils generally support dense standsr usually in the grass sod. If availability of micro sites rather with bell-shaped diameter distributions indicative than competition or climate were the critical factor of prior disturbance. limiting Pinus establishment, recruitment rates Pinus ponderosa and Pseudotsuga share domi- could be expected 'to be closely coupled to mortal- nance on open-phase sites, tree densities ranging ity, thus generating the observed regular or flat between 150 and 600 per ha. Diameter distributions diameter distribution. are not unlike those of lower elevation woodland The mesic montane woodland is the only com- types with bulges resulting from episodes of high munity-type in the Pinus ponderosa woodland establishment, though diameters appear more series in which the impact of inter-tree competition evenly distributed (e.g., Table 14; stands I39, 223). is evident. Here bell-shaped diameter distributions Competition-induced natural thinning is relatively resulting from competitive inhibition of regenera- unimportant on these sites and mortality is low tion can sometimes be seen. For example, stand 3 except during the establishment period. (Table 13) has a Pinus ponderosa, Pseudotsuga The dense-phase sites usually support even-aged canopy with a dense cohort of smaller Pseudotsuga stands of Pseudotsuga and Pinus ponderosa with which probably became established after cessation densities ranging up to 2 000 trees/ha in the 50 younger, 50-100 yr old stands. Pseudotsuga is of tree populations are not uniform throughout the usually dominant in the foothill ravine forests but study area, or even within a species or community on drier sites the two species often codominate. type. Rather, they vary in response to environment Both the bell-shaped diameter distributions (e.g., and chance historical events. To understand these Table 14; stands 48, 161) and the frequent occur- forests it is necessary to understand not only rence of charcoal are suggestive of a post-fire origin general patterns of development but also the varia- for most of these stands. Not infrequently stands tion in the patterns. will contain a few large Pinus ponderosa or Pseu- Although dominated almost exclusively by two dotsuga as relics from a previous stand. species, the Picea engelmannii, Abies lasiocarpa None of the dense-phase stands encountered forests exhibit considerable structural variation were old enough to support the eventual steady- relative to both site factors and successional devel- state structure. Natural thinning had not reduced opment. Failure to recognize this variability has led tree densities to the point where regeneration was to considerable confusion in the literature. In possible. While the eventual composition of the particular, the compositional and structural stabil- steady-state is uncertain, the dominance of the ity of the Picea, Abies forests has been variously smaller size-classes by Pseudotsuga suggests it to be interpreted. Working in the Medicine Bow Moun- increasing in importance. Given sufficient time tains of southern Wyoming, Oosting & Reed (1952) without fire, these stands will most likely develop reported density ratios for Abies over Picea of into more open, all-aged forests of reduced basal seven to one in the seedling stratum and four to one area and with minor persistence of Pinus pon- in the transgressives, despite a dominance of Picea derosa. If the present fire suppression activities in the overstory. While similar shifts in dominance continue on lower elevation sites, such steady-state from overstory Picea to understory Abies have forests should become increasingly prevalent. frequently been reported (e.g., Alexander, 1974; Hansen, 1940; Hobson & Foster, 1910; Langen- Mesic montane forests heim, 1962; Loope & Gruell, 1973; Marr, 1961; Of all Front Range forest types, conditions Miller, 1970; Oosting & Reed, 1952; Schmid & appear most favorable for tree growth in the mesic Hinds, 1974), their significance is not obvious. montane forests with the result that after distur- Marr (1961), Amundsen (1967) and Oosting & bance even-aged stands develop rapidly. As in the Reed (1952) have all suggested that most Picea, Pinus contorta forests, an overshoot in basal area Abies forests are in a steady-state condition. and cover occurs with a subsequent decline due to Amundsen, however, allowed that periodic fires inhibition of tree regeneration. The major distin- occur, while Marr maintained that the forests have guishing characteristic of these forests is the high almost always been wet enough to inhibit fire. diversity of tree species. After fire, when com- Numerous workers in similar forests have suggested petitive pressures are low, several tree species that Picea will gradually be replaced by Abies usually become established. Abies lasiocarpa, Pi- (Bloomberg, 1950; Daubenmire & Daubenmire, nus contorta, Picea engelmannii, Picea pungens, 1968; Hansen, 1940; Horton, 1956; Loope & Gruell, Pinus ponderosa, Populus tremuloides, Pseudo- 1973; Moss, 1953). In contrast, Alexander (1974), tsuga and Populus angustifolia can all be important Miller (1970), and Schmid &.Hinds (1974) expect in first generation forests. This pattern of high Picea to increase in dominance because of greater initial diversity is strikingly similar to that observed longevity. Fox (1977) has argued that the climax for post-fire forests on favorable sites in the Lake Picea, /tbies forests of the Medicine Bow Moun- States by Goffand colleagues (Goff& Zedler, 1968; tains have a cyclic form of stability with Picea being Auclair & Goff, 1971). The eventual steady-state replaced by Abies which is in turn replaced by forest is usually dominated by some combination of Picea. A similar cyclic alternation of species was Abies lasiocarpa, Pseudotsuga and to a lesser proposed by Schmid & Hinds (1974), but they extent Picea engelmannii and Picea pungens. argued that insect-induced mortality rather than Variation in forest development biological interactions of the two species was the driving force. Forest structural development and the dynamics While none of the previously described studies 51 suggested forest structure to be site dependent, of climax Pinus contorta forests in the Fraser examination across elevation and so~l moisture Experimental Forest southwest of the study area. gradients reveals marked variation. Picea appears Recognition of several Pinus contorta types with codominant at steady-state on wet sites, whereas differing successional and structural characters can on mesic and dry-mesic sites the species is gradually clarify this apparent ambiguity. While most Pinus replaced by Abies. The lower elevation Picea, Abies contorta forests are successional to either Pseu- forests develop even-aged stands after fire with in dotsuga or Picea and Abies, there is a limited range excess of 500 yr required for the demise of the of site conditions over which Pinus contorta ap- original cohort. On high elevation sites, even-aged pears able to form climax stands. Evidence for forest structure is largely absent, owing mostly to steady-state Pinus contorta in the study area takes the low rate of reestablishment after fire. the form of reversed-J diameter distributions in Marr (1961) and Douglas (1954) have suggested older stands (>250 yr) which suggest continuing that Pinusflexilis forests are not self-maintaining regeneration. In addition tree age data are con- but are successional to Picea and Abies. Working sistent with the interpretation of continuing regen- within the southern portion of the study area, eration. Such stable Pinus contorta stands have Amundsen (1967) described Pinus flexilis stands also been reported from the central Rocky Moun- which he interpreted to be successional to Picea and tains of Montana and Wyoming (Despain, 1973; Abies. In the present study both successional and Pfister & Daubenmire, 1975; Reed, 1976). compositionally stable Pinus flexilis types were identified, with the successional stands found on A gradient model of forest development somewhat more mesic sites. Within the stable Pinus flexilis stands, structural development varied from, It is useful to recognize three primary types of on the mesie sites, typical even-aged development forest development within Front Range forests, as described for the Pinus contorta forests to, on the remembering that these refer only to arbitrary most xeric sites, the low establishment, gradual points in a continuous field of potential variation. approach to steady-state pattern observed for high- These types can be referred to as characteristic of 1) elevation xeric Picea, Abies forests. favorable sites, found at middle elevations near the Most Front Range Pinus contorta forests are of center or on the mesic side of the moisture gradient, post-fire origin and with sufficient time are replaced 2) severe sites, found at highest elevations or on by Pseudotsuga or Picea and Abies (Clements, very dry sites at middle elevations, and 3) episodic 1910; Langenheim, 1962; Marr, 1961; Mason, 1915; sites, found at lower elevations along the transition Moir, 1969). Amundsen (1967), Moir (1969) and from forest to grassland. Several charaeteristics of Whipple & Dix (1979), taking exception with earlier the first two types are illustrated in Fig. 9. workers, suggested the existence of self-maintain- The forest development sequence for favorable ing Pinus eontorta forests at intermediate eleva- sites resembles in part those proposed for northern tions. Moir's conclusion was based on the lack of Rocky Mountain forests by Daubenmire & Dau- reproduction by other species in a series of 60 to 100 benmire (1968) and Day (1972). These workers yr old stands. However, as Pinus contorta was not divided the developmental process into four stages. reproducing eitfier, and Pinus contorta stands in The first stage is that of initial invasion, during the 60 to 100 yr class are well known for being too which most species which occur in the region can dense for significant regeneration (Alexander, 1974; become established, including the potential climax Clements, 1910; Mason, 1915), conclusive evidence species. The second stage consists of a dense forest was lacking. Amundsen (1967) provided less data with stagnated growth, dominated by initially rapid- for his three alleged stable Pinus contorta stands, growing, seral species such as Pinus contorta. At but it is evident that virtually no Pinus contorta this stage the climax species, which became estab- seedlings were present. While numerous saplings lished during the initial invasion phase, are present were present, these were most likely suppressed in the understory, though they are largely sup- individuals, virtually the same age as the canopy pressed and overtopped by the seral species. During trees. Whipple & Dix (1979), however, presented the third phase trees of the seral species die and are convincing tree age data supporting the occurrence replaced by the suppressed individuals of the cli- 52

Favorable Site A

z :'\ ."< ...... "',, :: %," \ ...... \ /.." -"'....".~ / I - ,,'x ,, ...... 2x-j~~>--~ / ,, \x "'" // \ . . 2Y_ ...... "---._2 ...... ,'~,"" 7, TIME

Unfavorable Site B ~.~-~ ~.,-.~ ...... :...... _.

~;~"-'-\ , TIM E

...... Biomoss ...... Species Richness ...... Production -- EstoNishment Rote

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 creases along with annual basal area increment, opens sufficiently to allow seedling establishment. owing to reduced competitive pressures. Whether The final phase is the true climax or steady-state the cycle repeats depends largely on the synchrony forest wherein the last of the relics have died and a of mortality and the resulting dispersion of sizes of true all-aged forest has developed. Other students the newly established trees. If mortality is con- of Rocky Mountain and adjacent forests have centrated during a short period, perhaps a decade reported similar patterns (e.g. Bloomberg, 1950; or two, a new though damped cycle can be ex- Horton, 1956; Moss, 1953; Raup, 1946). These pected. Episodes of severe pine beetle (Dendrocto- stages correspond closely to those recognized in nus ponderosae) infestation have been reported to Table 10 for Picea - Abies stands as well as the result in two or three different cohorts of Pinus stages recognized in Tables 4 and 6, except that in contorta on the same site. The increasing suscep- the latter two the second stage has been divided into tibility of Pinus to beetle attack with increasing age two for a total of five stages. can yield a short, 30-40 yr cycle of establishment While the Dauhenmire-Day scheme works well (Amman, 1977). In contrast, if mortality is gradual, for environmentally favorable sites in the present biomass, productivity, diversity and establishment study area, several additional observations can be are likely to asymptotically approach steady-state made. Toward the end of the first stage, growth of levels. This would be accompanied by development understory species becomes strongly inhibited with of a steady-state (reversed-J) diameter distribution. diversity dropping to a minimum early in stage 2. Forests dominated by Pinus contorta, Pinus pon- On many sites diversity increases again late in stage derosa, Pinus flexilis, Pseudotsuga, Populus tre- 3 or early in stage 4 when the canopy is more open muloides, Picea engelmannii and Abies lasiocarpa and competitive pressures are reduced. Basal area all exhibit this type of stand development under increases well into the third stage, despite natural appropriate site conditions. thinning and decreasing productivity (wood vol- A frequent observation is that the rate of post- ume increment). With the approach of late stage fire seedling establishment is lower at or near 3, the remaining trees become increasingly suscep- treeline than at lower elevations (e.g., Bollinger, tible to fungal infection, insect attack and wind- 1973; Bunin, 1975; Habeck & Mutch, 1973; Noble throw. This marks a period of rapidly declining & Alexander, 1977; Stahelin, 1943). Stahelin (1943) basal area (and biomass) as the large trees die. At in particular noted that where grasses and forbs the same time seedling establishment rapidly in- dominate early in the sequence, establishment rates 53 for tree speies are greatly reduced. Environmentally can be found for population development under severe sites are often characterized by such low various site conditions. Because of the quantity of establishment rates (Fig. 9). Tree seedlings become data required for studies of this kind, most investi- established so infrequently that the canopy can only gators have failed to relate their results to site or very gradually develop to the point where seedling successional variation (Peet, 1981). Of the few establishment is inhibited by competition from studies which have examined biomass or produc- canopy trees. In certain cases establishment is tion relative to site factors, none have included a actually enhanced by tree growth owing to the successional component. If future studies of com- sheltered conditions in the understory. Gradual munities and ecosystems are conducted so as to reestablishment on these sites is paralleled by a document variation over a range of site and succes- gradual increase toward steady-state levels of basal sional conditions, the overall interpretability of the area, annual basal area increment, and diversity. work will be greatly enhanced. The dramatic oscillations observed on the favor- able sites and induced by near synchronous estab- lishment are missing. The size structure approxi- mates the reversed-J shape throughout the devel- References opmental process. Diversity typically peaks early in the sequence and then drops gradually to steady- Aichinger, E., 195l. Vegetationsentwicklungstypen ats Grund- state levels. lage unserer land- und forstwirtschaftlichen Arbeit. Angew. Episodic-site forests are similar to those of severe Pflanzensoziol. (Wien) 1: 17-20. sites in their low seedling establishment rates. They Alexander, R. R., 1974. Silviculture of subalpine forests in the differ in that seedling establishment is episodic and southern Rocky Mountains: the status of our knowledge. the forests are rarely devastated by fire or other U.S.D.A. For. Serv., Rocky Mr. For. Range Exp. Star., Res. Pap. RM-121, 88 pp, disturbance factors. Rather, fire is a frequent, but Amman, G. D., 1977. The role of the mountain pine beetle in low intensity phenomenon which keeps the vegeta- lodgepole pine ecosystems: impact on succession, In: tion open and park~like. Thus, temporal variation Mattson, W. J, (ed.), The role of arthropods in forest in forest structure is almost entirely attributable to ecosystems. S pringer-Verlag, N.Y. 3-18. establishment events; only during unusually favor- Amundsen, C. C., 1967. The subalpine forest of Wild Basin, Front Range, Colorado. Ph.D. Thesis, Univ. Col., Boulder. able years do significant numbers of seedlings enter 129 pp. the tree population. The resultant size structure is Auclair, A. N. &Goff, F, G., 1971. Diversity relations of upland commonly one with bulges resulting from periods forests in the western Great Lakes area. Amer. Natur. 105: of successful regeneration. 499--528. The three forms of stand development identified Bailey, R. L. & Dell, T. R., 1973. Quantifying diameter distribu- tions with the Weibull function. For. Sci. 19: 97-104, provide reference points in a continuum of struc- Barry, R. G,, 1972. Climatic environment of the east slope of the tural variation which can be used as a framework Colorado Front Range. Inst. Arct. Alp. Res., Occ. Pap. 3, for interpreting numerous aspects of forest ecology. Univ. Col., Boulder. 206 pp. Diversity, basal area, biomass, productivity and Barry, R. G., 1973. A climatological transect on the east slope stability all can be seen as responding to the of the Front Range, Colorado. Arct. Alp. Res. 5: 89-110. Bates, C. G., 1924, Forest types in the central Rocky Mountains dynamics of the dominant tree populations, which as affected by climate and soil, U.S.D.A. Bull. 1233. 152 pp. in turn are responding to local environmental Billings, W. D., 1969. Vegetational pattern near alpine timber- conditions. The implications for future studies line as affected by fire-snowdrift interactions. Vegetatio 19: should be clear. Investigations of most community 192-207. and ecosystem properties including diversity, Biswell, H. H., 1973. Fire ecology in ponderosa pine - grassland. Tall Timbers Fire Ecol. Conf., Proc. 12:69 96. biomass, and productivity need to be conducted Bliss, C. I. & Reinker, K. A., 1964. A tognormal approach to within a multidimensional framework including diameter distribution in even-aged stands, For. Sci. 10: both site condition and stand development. Simi- 350-360. larly, studies of population change during succes- Bloomberg, W. G., 1950. Fire and spruce. Forest. 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Appendix In addition to prevalent species, modal species are indicated. Modal species are those species with their highest constancy in Community summary tables the community type or series under consideration. Each species is modal in one series and one type. Ties were broken on the basis Each of the following eight tables summarizes the composi- of average cover. Modal species are designated by a line under tion of one community series as described in detail in the text. the constancy value. When a species is prevalent in one type in The series as a whole is summarized first followed by summaries the series, and modal but not prevalent elsewhere, frequency is of the component community-types. Summaries are for under- omitted from that community list in which the species is not story vegetation defined as leaf area below one m in height, prevalent. Those species which are modal but never prevalent Species are separated into three growth forms: trees, shrubs and are not listed because of space limitations. An expanded set of herbs. summary tables listing all modal species and other supplemental Composition is summarized in terms of prevalent species. The information is available from the author upon request. number of prevalent species is equal to the average number of In addition to the species lists, a number of summary statistics species per stand (0.1 ha) of that type. Prevalent species are those are presented for each community. These include the number of species with highest constancies (percentage of stands where stands included in the group, Curtis" (1959) index of homo- present). The prevalent species of the series as a whole are listed toneity (average constancy of prevalents), average species num- first. These are followed by species which are prevalent in one or ber, average understory cover (below one m in height), and Hill's more of the community-types, but not in the series as a whole. (1973) first (1/Simpson's Index) and second (Exp[H']) diversity Constancy values are given for each prevalent species. Fre- statistics (see Peer, 1974) calculated for each stand using quency values (25 0.5 X 2 m subplots) are also given based on importance values equal to the average of relative frequency and those 0. l ha sampling units in which a species was present. relative cover. Community table I. Pinus ponderosa woodland seriesi

Pinus ponderosa Mesic Foothill Xeric Foothill Mesic Montane Xeric Montane Series A Shrubland (AI) Woodland (A2) Woodland (A3) Woodland (A4) Woodland (A5)

Number of stands 35 9 6 4 6 10 Homoteneity .529 ,580 .645 .690 .652 .640 Diversity: EXP(H') 17.2 16.6 17.9 16.6 20.0 15,8 Diversity: 1/h 11.6 10.6 13.3 11,3 12.9 10.7 Species per stand 42.6 44,8 44.7 40.3 45.0 38.8 Understory cover % 44.9 64.0 28.2 33.8 38.0 46.4 Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. TREES (Below I m) Series Prevalents Pinus ponderosa 7,1 48.6 4.0 44.4 11,2 83.3 6.0 50.0 4.0 50.0 8.0 30.0 Pseudotsuga menziesii 6.5 31.4 5.6 83.3 10.7 50.0 Additional Prevalents Juniperus scopulorum 4.0 50.0 Betula occidentalis 0.0 25.0

SHRUBS Series Prevalents Ribes cereum 12.5 85.7 4.0 55.6 12.0 100.0 15.0 100.0 14.7 100.0 15.1 90.0 Purshia tridentata 22.1 80.0 20,8 55.6 20.0 100.0 5.3 75.0 17.6 83.3 32.4 90.0 Rubus deliciosus 3,6 54.3 3.2 83.3 7.0 100.0 2.4 50.0 Opuntia polyacantha 9.5 51.4 14,0 88.9 1.0 66,7 14.0 50.0 7.0 40.0 Rhus trilobata 18.0 34.3 30.8 77.8 0.0 66.7 Yucca glauca 4.0 34.3 4•9 100.0 1.3 50.0 Rosa sp. 8.7 31.4 4.0 50.0 18,7 50.0 5.0 40,0 Additional Prevalents Cercocarpus montanus 25.7 32.6 77.8 36.0 50.0 Echinocereus viridiflorus 11.4 2.0 44,4 Jamesia americana 6.0 66.7 Mahonia repens 25.3 50.0 Prunus virginiana 1.3 50.0 8.0 50.0 0.0 40.0 Juniperus communis 5.0 66.7 Arctostaphylos uva-ursi 6.7 50.0 Physocarpus monogynus 6.7 50.0 Symphoricarpos oreophilus 5.3 50.0 HERBS Series Prevalents Artemisia ludoviciana 49.5 94.3 40.5 88.9 46.7 100.0 38.0 I00.0 55.3 10Q,O 60,4 90.0 Artemisia frigida 35.7 94.3 36.8 100.0 28.7 100.0 45.0 100.0 21.6 83.3 43.1 90.0 Carex rossii 36.0 80.0 16.8 55.6 47.3 100.0 22.0 100.0 41.6 83.3 43.0 80.0 o~ Community table 1 (cont.) Freq. Const. Freq. Const. Freq. Const. Freq. Const, Freq. Const. Freq. Const.

Geranium fremontii 19.3 80.0 7.2 55.6 12.8 83.3 24.0 75.0 26.7 100.0 23.1 90.0 Penstemon virens 11.4 80.0 13.3 100.0 10.0 100.0 8.0 83.3 14.0 100.0 Chrysopsis villosa 18.8 71.4 21.3 66.7 17.6 83.3 4.0 75.0 36.7 50.0 16.0 80.0 Potentilla fissa 12.5 68.6 8.0 83.3 23.0 100.0 22.4 83.3 7.0 80.0 Eriogonum umbellatum 16.0 68.6 16.6 66.7 20.0 66.7 16.0 50.0 17.3 90.0 Muhlenbergia montana 49.0 65.7 23.0 66.7 45.0 100,0 41,6 83,3 70.0 80.0 Stipa comata 27.2 60.0 23.5 100.0 12.0 33.3 30.0 33.3 37.1 70.0 Scutellaria brittonii 7.2 60.0 10.0 100.0 6.0 100.0 5.7 70.0 Helianthus pumilus 19.0 57.1 14.6 100.0 26.0 50.0 18.0 33.3 26.0 60.0 Bromus lanatipes 7.4 57.1 5.6 55.6 7.0 66.7 10.0 100.0 5.3 50.0 9~0 40.0 Sitanion longifolium 8.8 54.3 14.7 33.3 16.0 83.3 5.0 80.0 Bromus tectorum 42.0 51.4 65.5 88.9 16.0 33.3 14.0 50.0 34.4 50.0 Harbouria trachypleura 9.5 51.4 8.8 83,~ 12.0 50.0 9.3 60.0 Agropyron albicans 25.4 48.6 .43.0 44.4 16.0 50.0 8.8 83.3 34.4 50.0 Poa fendleriana 11.2 .45.7 8.0 50.0 6.0 50.0 12.8 83.3 12.8 50.0 Cystopteris fragilis 5.5 45.7 6.0 66.7 8.0 100.0 Cryptantha virgata 6.0 45.7 6.7 50.0 8.6 70.0 Arabis drummondii 5.3 45.7 9.3 50.0 5.8 90.0 Koeleria gracilis 17,6 42,9 4.0 33.3 16.7 100.0 29.6 50.0 Festuca saximontana 13.0 16.0 33.3 12.0 100.0 13.3 60.0 Antennaria parvifolia 11.2 42.9 5.3 50.0 16.7 100.0 10.4 50.0 Grindelia subalpina 11.2 42.9 9.7 77.8 21.3 50.0 6.0 50.0 Leucopoa kingii 32.9 40.0 32.0 83.3 6.0 50.0 64.0 66.7 Astragalus flexuosus 15.1 40.0 8.0 44.4 8.0 33.3 23.4 70.0 Sedum lanceolatum 22.3 40.0 6.7 50.0 40.0 83.3 18.4 50.0 Pulsatilla patens 16.6 40.0 9.6 83.3 48.0 50.0 Phacelia heterophylla 2.3 40.0 1.0 44.4 2.0 50.0 4.0 50.0 Senecio fendleri 14.8 37.1 8.0 83.3 23.0 66.7 Achillea lanulosa 7.4 37.1 8.0 100.0 8.8 50.0 Allium cernuum 7.0 37.1 14.7 50.0 4.0 50.0 6.0 60.0 Agropyron trachycaulum 24.6 34.3 32.0 33.3 30.0 50.0 24.8 50.0 Campanula rotundifolia 10.3 34.3 13.3 33.3 18.0 66.7 1.0 66.7 Additional Prevalents Bouteloua hirsuta 28.6 14.0 66.7 4.0 50.0 Cirsium undulatum 31.4 9.3 66.7 4.0 50.0 Sporobolus cryptandrus 20.0 6.7 66.7 Verbascum thapsus 31.4 4.0 66.7 0.0 50.0 Bouteloua gracilis 25.7 11.2 55.6 Gutierrezia sarothrae 14.3 12.0 55.6 Bouteloua curtipendula 14.3: 7.2 55.6 ~Unknown (dicot) 6,4 55.6 Community table 1 (cont.) Freq. Const. Freq. Const. Freq. Const, Freq. Const. Freq. Const. Freq. Const.

Unknown 2 (dicot) 14.3 3.2 55.6 Cerastium arvense 25~7 19.0 44.4 36.0 33.3 Ambrosia psilostachya 11.4 15.0 44.4 Tradescantia occidentalis 17.1 14.0 44.4 Erysimum asperum 31.4 11.0 44.4 4.0 75.0 Tragopogon dubius 28.6 7.0 44.4 Andropogon scoparius 8.6 45.3 33.3 Gramineae sp. 20.0 33.3 Andropogon gerardii 8.6 8.0 33.3 Chrysothamnus viscidiflorus 17.1 10.7 33.3 Galium aparine 11.4 10.7 33.3 Mertensia viridis 12.0 33.3 13.0 66.7 Solidago canadensis 25.7 5.3 .~b.o 20.0 100.0 Paronychia jamesii 11.4 4.0 50.0 6.9 70.0 Woodsia oregana 34.3 2.7 50.0 Selaginella mutica 16.0 33.3 Lathyrus leucanthus 28.0 33.3 Selaginella underwoodii 4.0 33.3 Eriogonum alatum 25.7 0.0 100.0 Parietaria pensylvanica 17.1 10.7 ~570 Mimulus glabratus 8.6 9.3 75.0 Brickellia grandiflora 31.4 8.0 75.0 Chertopodium teptophyllum 16.0 50,0 Androsace septentrionalis 6.0 50.0 Euphorbia robusta 31.4 4.0 50.0 6.7 60.0 Hackelia floribunda 11.4 2.0 50.0 Cryptantha virgata 6.7 50.0 0.0 50.0 8.6 70.0 Selaginella densa 26.0 66.7 Erigeron compositus 11.0 66.7 Thermopsis divaricarpa 4.0 66.7 Antennaria rosea 2.0 50.0 22.7 50.0 Poa pratensis 4.0 50.0 Solidago missouriensis 13.3 50.0. Potentilla hippiana 22.9 10.7 50.0 16.0 40.0 Solidago spathulata 8.0 50.0 Lithospermum multiflorum 14.3 6.6 50 ..._2_0 Carex foenea 16.0 33,3 Conyza canadensis 25.7 15.2 50.0

o~ 62

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

Foothill Pinus ponderosa - Xeric Xeric Pinus Foothill Ravine Pseudotsuga Pseudotsuga ponderosa Series B Forest (Bl) Forest (B2) Forest (B3) Forest (B4)

Number of stands 40 7 16 I1 6 Homoteneity ,530 ,601 ,552 ,579 .596 Diversity: EXP(H’) 11.3 9.9 12.1 9.3 14.8 Diversity: 1/A 8.0 6.9 8.4 6.6 11.0 Species per stand 30.6 28.0 33.0 28.3 31.8 Understory cover % 18.2 29.5 19.2 9.0 19.3

Freq. Const Freq. Const. Freq. Const. Freq. Const. Freq. Const. TREES (Below 1 m) Series Prevalenrs Pseudotsuga menziesii 12.5 80.0 15.4 100.0 12.5 87.5 8.0 45.5 12.6 100.0 Juniperus scopulorum 4.2 40.0 7.2 71.4 3.2 31.3 3.2 45.5 Additional Prevalents Betula occidentalis 0.0 14.3 Populus tremuloides 11.3 37.5 Pinus flexilis 10.0 33.3

SHR CBS Series Prevalents Juniperus communis 9.5 90.0 15.3 85.7 12.3 93.8 1.3 81.8 9.3 100.0 Ribes cereum 6.9 80.0 16.0 42.9 7.1 83.1 6.4 90.9 2.7 1oo.o Arctostaphylos uva-ursi 14.0 60.0 23.0 57.1 18.5 68.8 4.8 45.5 4.0 66.7 Purshia tridentata 4.0 57.5 0.4 56.3 7.5 81.8 5.3 50.0 Physocarpos monogynus 28.2 55.0 34.3 100.0 30.2 56.3 15.0 36.4 24.0 33.3 Jamesia americana 17.1 55.0 36.0 85.7 10.2 68.8 Acer glabrum 3.3 42.5 7.0 57.1 2.5 50.0 1.3 50.0 Additional Prevalents Symphoricarpus oreophilus 40.0 8.8 71.4 Rubus deliciosus 2.0 36.4 Artemisia tridentata 20.0 33.3

HERBS Series Prevalents Carex rossii 29.2 95.0 8.7 85.7 29.6 93.8 37.8 100.0 32.7 100.0 Potentilla fissa 14.6 80.0 13.7 100.0 20.0 81.3 5.3 54.5 13.3 100.0 Penstemon virens 10.1 72.5 7.2 71.4 10.4 62.5 6.7 81.8 18.4 83.3 Geranium fremontii 71.0 70.0 1.0 57.1 6.5 68.8 4.8 90.9 25.3 50.0 Leucopoa kingii 24.6 67.5 25.3 75.0 22.5 72.7 16.0 50.0 Artemisia ludoviciana 22.3 65.0 33.0 57.1 20.9 56.3 20.8 90.9 28.8 83.3 Senecio fendleri 1.3 60.0 6.0 62.5 8.9 81.8 7.2 83.3 Sedum lanceolatum 14.7 52.5 15.6 68.8 14.0 54.5 9.3 50.0 Pulsatilla patens 10.1 47.5 1.3 85.7 10.3 43.8 12.0 27.3 25.3 50.0 Poa fendleriana 10.0 45.0 10.0 50.0 5.6 45.5 18.7 50.0 Antennaria rosea 6.2 45.0 2.7 42.9 9.8 56.3 4.0 50.0 Frasera speciosa 2.7 45.0 0.8 71.4 3.0 50.0 6.7 50.0 Koeleria gracilis 5.2 42.5 4.5 72.7 4.0 50.0 Antennaria parvifolia 5.6 42.5 4.4 56.3 10.0 36.4 4.0 66.7 Artemisia frigida 1.7 40.0 10.3 63.6 9.0 66.7 Muhlenbergia montana 17.3 37.5 12.0 63.6 21.3 50.0 Heuchera bracteata 4.6 35.0 10.7 42.9 3.4 43.8 Cystopteris fragilis 5.7 35.0 17.0 57.1 1.5 50.0 Arabis drummondii 5.1 35.0 4.0 63.6 Poa pratensis 4.6 32.5 4.0 37.5 8.0 27.3 2.7 50.0 63

Community table 2 (cont.) Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const.

Harbouria trachypleura 3.7 32.5 2.9 43.8 2.7 50.0 Achillea lanulosa 9.0 30.0 10.4 3t.5 3.0 36.4 Sitanion longifolium 4.0 30.0 1.6 45.5 4.0 50.0 Additional Prevalents Mertensia viridis 4.0 57.1 Erigeron speciosus 29.3 42.9 Fragaria vesca 17.3 42.9 Galium boreale 10.7 42.9 7.2 31.3 Heuchera parvifolia 14.7 42.9 Clematis cotumbiana 2.7 42.9 Arabis fendleri 17.5 1.3 42.9 Polypodium vulgare 12.5 1.3 42.9 Selaginella underwoodii 10.0 18.0 28.6 Saxifraga bronchialis 5.1 43.8 Festuca saximontana 5.1 43.8 Solidago spathulata 5,7 43.8 Carex foenea 21.3 37.5 30.0 33.3 Erigeron compositus 6,0 37.5 4.0 36.4 Androsace septentrionalis 5,6 31,3 4.0 50.0 Solidago multiradiata 17.6 45.5 Selaginella densa 7.2 45.5 Helianthus pumilus 2.0 36.4 Chrysopsis villosa t7,3 50.0 Eriogonum umbellatum 17.3 50.0 Thermopsis divaricarpa 26.0 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 28 4 9 15 Homoteneity .518 .652 .595 .622 Diversity: EXP(H') 16.8 26.7 18.9 13.0 Diversity: 1/), 11.9 20.0 t3.1 9.0 Species per stand 40.9 60.3 46.8 32.2 Undevstory cover % 54,1 80.2 49.8 49.7 Freq. Const. Freq. Const. Freq. Const. Freq, Const, TREES (Below 1 m) Series Prevalents Populus tremuloides 21.5 67.9 22.0 t00.0 30.0 88.9 t 1.4 46.7 Picea engetmannii 18.2 64.3 21.3 33,3 17.6 1130.0 Abies lasiocarpa 35,5 60,7 38.7 t00.0 Pseudotsuga menziesii 9.1 50.0 11.3 66,7 Additional Prevalents Juniperus scoputorum 5.3 75.0 1.0 44.4 Betula occidentalis 10.7 6.0 50,0 Alnus tenuifolia 32. t 8.0 50._..L0 SHR UBS Series Prevalents Rosa sp. 29.3 96~4 30,0 I00.0 35.t I00.0 25.4 93.3 J uniperus communis 13,4 71.4 12.0 77.8 14,2 73.3 Physocarpus monogynus 20.2 67.9 12.0 50.0 34.9 77.8 11.6 66.7 Jamesia americana 22,7 57.1 26.0 66,7 23.0 53.3 Mahonia repens 19.0 57.1 ,~ 13.0 44.4 20.4 73.3 Acer glabrum I0.5 57.1 10.7 7!,0 I5.3 66.7 6,3 46.7 Ribes tacustre 10.3 50.0 20.0 50.0 5.0 44.4 t0.5 53.3 Vaecinium myrtillus 52.3 46.4 52.3 86.7 Lonicera involuerata 8,7 42.9 16.0 50,0 = Additional Prevalems Ribes inerme 24.0 75.0 Symphoriearpos oreophilus 25.3 75,0 18.3 77.8 Cornus stolonifera 10,7 14.0 50.0 Prunus virginiana 8.0 50.0 Ribes cereum 7.5 88.9 Rubus idaeus 42..__.~9 4.6 77.8 Linnaea borealis 35,7 53.2 66.7 HERBS Series Prevalents Arnica cordifolia 28,5 89,3 28.0 75.0 29.1 77.8 28.3 100.0 Fragaria vesta 16.2 71.4 6.0 50.0 21.8 100.0 12.9 60.0 Pyrola seeunda t8.8 7114 .... 2.0 50.0 15.0 44.4 22.3 93.3 Osmorhiza depauperata 16.6 71,4 34.7 75,0 18.3 77.8 t0,0 66.7 Clematis columbiana 12,8 ,67.9 9.5 88.9 Haplopappus parryi 15.8 57.1 l&3 100.0 Galium boreale 29.6 53.6 32.0 100,0 27.5 88.9 Carex rossii I 1.7 50.0 12,0 44.4 11.6 66.7 Pyrota virens 7.4 50.0 7.0 44.4 6.2 60.0 Epilobiam angustifolium 8.9 50.0 10,9 77.8 7.3 40.0 Calamagrostis canadensis 20.9 46.4 ..... 44.0 75,00 22.0 66.7 Erigeron speciosus 17.5 46.4 18.0 44,4 18,5 53.3 Smilacina stetlata 19.7 46.4 36.0 !00.0 12.0 77.8 Taraxacum officinale 19,0 46.4 27.0 16.0 77.8 65

Community table 3 (cont.) Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq, Const.

Achillea lanulosa 31.0 42.9 30.0 100.0 31.3 66.7 Galium triflorum 13.7 42.9 26.7 75.0 12.0 66.7 Oryzopsis asperifolia 14.7 42.9 21.3 33.3 11.5 53.3 Goodyera oblongifolia 15.3 42.9 16.4 73.3 Fragaria virginiana 29.8 39.3 24.0 75.0 30.4 55.6 Actaea rubra 4.0 39.3 4.0 75.0 4.0 44.4 Equisetum arvense 17.2 35.7 30,0 lO0.O 2.7 33.3 Heracleum lanatum I1.2 35.7 10.0 12.8 55.6 Chimaphila umbellata 20.8 35.7 20.8 66.7 Carex geyeri 10.4 35.7 20.0 50.0 10.7 33.3 10.4 Cystopteris fragilis 10.0 35.7 4.0 50.0 14.0 66.7 Thalictrum fendleri 4.4 35.7 5.0 iOO.O 1.6 55.6 Smilacina racemosa 9.2 35.7 12.0 50.0 4.7 40.0 Antennaria rosea 5.2 3517 2.0 50.0 8.0 33.3 Geranium richardsonii 32.9 32.1 49.3 75.0 20.8 55.6 Additional Prevalents Ligusticum porteri 28.6 16.0 100.0 Thermopsis divaricarpa 32.0 75.0 Dodecatheon pulchellum 17,9 30.7 75.0 Trifolium repens 14.3 21.3 75.0 Clematis columbiana 17.3 75.0 14.5 53,3 Hydrophyllum fendleri 25.0 13.3 75.0 7.0 44.4 Monarda fistulosa 14.3 6.7 75.0 Phleum pratense 54.0 50.0 Poa pratensis 28.0 50.0 27.0 44.4 Rudbeckia laciniata 14.3 52.0 50,0 Aralia nudicaulis 24.0 50.0 Viola canadensis 14.3 28.0 50.0 Carex canescens 10.7" 14.0 50.q ..... Carex microptera 10.7 20.0 50.0 Carex foenea 18.0 50.0 Mertensia ciliata 20.0 50.0 6.7 33.3 Pteridum aquilinum 21.4 8.0 50.0 Montia chamissoi 7.1 6.0 50.0 Aconitum columbianum 14.3 10.0 50.0 Unknown 4.0 50.0 Cirsium centaureae 10.0 50.O Pseudocymopterus montanus 6.0 50.0 Aquilegia caerulea 28.6 6.0 50.0 4.0 33.3 Rudbeckia hirta i0./ 6,0 50.0 Urtica dioica 7.1 8,0 50.0 Frasera speciosa 2.0 50.0 Geum macrophyllum 10.7 4.0 50,0 Potentilla fissa 4.0 50.0 5.0 44.4 Geranium fremontii 21.6 55.6 Campanula rotundifolia 7.2 55.6 Artemisia ludoviciana 0.0 44.4 Bromus richardsonii 28.6 14.7 33.3 Pedicularis racemosa 22.7 40.0 Calypso bulbosa 28.6 2.7 40.0 Hieracium albiflorum 3.3 ,40.0. Saxifraga bronchialis 8.0 33.3 Streptopus amplexifolius 4.0 33.3 Community tabte 4. Pinus contorta forest series.

Mesic Xeric Mesic Pinus Xeric Pinus Pinus contorta- Pinus contorta- contorta-Abies, contorta-Abies, Pinus contorta Pseudotsuga Pseudotsuga Picea Forest Picea Forest Series D Forest (D1) Forest (D2) Forest (D3) (D4) (D5)

Number of stands 84 23 12 8 13 28 Homoteneity .448 .526 .520 .571 .547 .521 Diversity: EXP(H') 7.8 7.6 7.2 11.4 t0.8 5.8 Diversity: 1/~. 5.7 5.6 5.0 8.0 8.0 4.4 Species per stand 19.5 18.7 23.8 26.9 25.3 13.6 Understory cover % 27.5 17.9 17.3 21.3 42.0 34.9 Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. TREES (Below 1 m) Series Prevalents Abies lasiocarpa 36.9 52.4 51.6 69.2 42.5 85.7 Picea engelmannii 15.0 46.4 14.7 39.1 26.7 69.2 10.3 75.0 Populus tremuloides 14.9 40.5 23.3 52.2 4.7 50.0 17.0 61.5

Additional Prevalents Pinus contorta 21.6 65.2 43.0 50.0 8.4 32.1 Pseudotsuga menziesii 11.1 75.0 Juniperus scopulorum 0.0 50.0 Pinus flexilis 12.0 75.0

SHR UBS Series Prevalents Juniperus communis 12.9 86.9 14.3 91.3 10.2 91.7 17.7 87.5 5.6 76.9 14.5 85.7 Vaccinium myrtillus 52.3 57.1 15.2 43.5 52.7 92.3 66.3 92.9 Arctostaphylos uva-ursi 17.5 51.2 16.8 82.6 7.4 58.3 36.5 100.0 Rosa sp. 20.1 46.4 19.2 43.5 4.6 58.3 2.0 25.0 34.2 84.6 20.0 32.1 Jamesia americana 11.6 44.0 7.7 60.9 22.9 91.7 7.0 50.0

Additional Prevalents Physocarpus monogynus 24.0 58.3 Acer glabrum 2.3 58.3 Ribes cereum 2.7 50.0 1.3 37.5 Mahonia repens 28.0 46.2 Lonicera involucrata 12.0 38.5 Rubus idaeus 9.0 30.8 Vaccinium scoparium 36.8 35.7

HER BS Series Prevalents Carex rossii 19.6 73.8 19.8 91.3 19.6 83.3 37.1 87.5 14.0 46.2 14.4 64.3 Pyrola virens 8.5 45.2 6.0 60.9 5.0 33.3 14.5 61.5 9.1 39.3 Community table 4 (cont.) Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const.

Epilobium angustifolium 10.7 44.0 2.4 43.5 18.8 76.9 12.0 53.6 Arnica cordifolia 21.3 42,9 14.9 30,4 32.0 100.0 16.9 50.0 Pyrola secunda 13.3 42.9 2.0 33.3 18.9 84.6 14.3 50.0 Haplopappus parryi 22.5 40.5 20.5 34.8 4.8 41.7 34.5 84.6 21.8 32.1 Potentilla fissa 11.8 40.5 11.3 52.2 8.0 66.7 13.3 75.0 6.4 38.5 Sedum lanceolatum 11.4 36.9 6.9 60.9 20.0 100.0 Penstemon virens 12.4 33.3 12.0 39.1 7.5 66.7 11.4 87.5 Thermopsis divaricarpa 29.5 32.1 39.6 43.5 45.0 50.0 28.0 30.8 17.3 21.4 Solidago spathulata 18.9 31.0 19.6 39.1 6.4 41.7 10.0 50.0 Senecio fendleri 5.1 29.8 4.4 39.1 9.5 100.0 mntennaria rosea 12.2 23.8 1.6 41.7 7.0 50.0 Additional Prevalents Saxifraga bronchialis 8.0 39.1 Fragaria vesca 12.8 41.7 Clematis columbiana 6.4 41.7 Selaginella densa 11.0 33.3 32.0 37.5 Solidago missouriensis 9.0 33.3 Antennaria parvifolia 3.0 33.3 5.6 62.5 Galium boreale 4.0 33.3 34.4 38.5 Artemisa ludoviciana 2.0 33.3 Leucopoa kingii 16.,0 50.0 Achillea lanulosa 11.0 50.0 10.7 46.2 Festuca saximontana 12.0 50.0 Frasera speciosa 5.0 50.0 Arabis drummondii 5.0 50.0 Astragalus parryi 8.3 26.7 37.5 Oxytropis lambertii 18.7 37.5 Artemisia frigida 17.3 37.5 Erigeron compositus 18.7 37.5 Harbouria trachypleura 18.7 37.5 Carex foenea 14,7 37.5 Erigeron speciosus 45.7 53.8 Osmorhiza depauperata 22.3 53.8 Campanula rotundifolia 7.2 38.5 Fragaria virginiana 73.0 30.8 Calamagrostis canadensis 27.0 30.8 Pyrola asarifolia 17.0 30.8 Chimaphita umbellata 4.0 30.8 Cystopteris fragilis 0.0 30.8 Penstemon whippleanus 3.6 32.1 ",M Community table 5. Picea. Abies forest series.

Wet Picea, Mesic Picea, Xeric Picea, Subalpine Picea, Wet Montane Bog Forest Abies Forest Abies Forest Abies Forest Abies Forest Series E Forest (El) (E2) (E3) (E4) (E5) (E6)

Number of stands 68 8 3 14 17 17 9 Homoteneity .435 .642 .684 .583 .535 .652 .620 Diversity: EXP(H') 11.7 16.5 15.4 16.0 8.1 7.3 14.4 Diversity: 1/h 8.2 12.1 11.1 11.5 5.8 4.8 9.7 Species per stand 28.2 34.1 37.0 35.1 21.5 21.4 34.8 Understory cover % 54.9 56.1 93.1 62.1 50.7 50.9 45.5

Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. TREES (Below 1 m) Series Prevalents Abies lasiocarpa 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 Pieea engelmannii 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 Sorbus seopulina 3.2 7.4 4.0 33.3 Additional Prevalents Alnus tenuifolia 16.0 25.0 Populus tremuloides 8.0 11.8 SHR UBS Series Prevalents Vaccinium myrtillus 49.1 69.1 18.7 37.5 46.7 100.0 32.6 50.0 62.3 94.1 56.8 88.2 Vaccinium scoparium 44.6 66.2 14.0 66.7 ' 20.5 57.I 34.9 64.7 67.0 94.1 50.9 77.8 Ribes montigenum 17.1 38.2 22.7 64.3 2.7 35.3 23.4 77.8 Ribes coloradense 6.1 33.8 4.0 57.1 3.0 44.4 Juniperus communis 8.0 27.9 4.0 33.3 11.2 58.8 4.0 44.4 Additional Prevalen ts Lonicera involucrata 15.5 100.0 5.7 41.2 Rosa sp. 13.7 87.~ Rubus idaeus 16.7 75.0 Ribes lacustre 1.5 31.2 62.5 20.0 35.3 Sambucus racemosa ~ 14.0 50.0 4.9 52.9 Linnaea borealis 12.0 37.5 Gaultheria humifusa 5.9 22.0 66.7 Salix planifolia 1.5 24.0 66.7 Kalmia polifolia 2.9 14.0 66.7 HERBS Series Prevalents Epilobium angustifolium 19.8 72. I 17.3 75.0 22.7 100.0 26.4 71.4 19.3 64.7 16.6 76.5 18.0 66.7 Arnica cordifolia 34.9 70.6 22.7 75.0 0.0 66.7 35.0 85.7 51.7 82.4 27.6 64.7 30.7 33.3 Polemonium delicatum 23.9 69.1 26.8 92.9 22.4 58.8 10.1 88.2 44.4 100.0 Community table 5 (cont.) Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const.

Carex rossii 23.0 54.4 10.4 35.7 15.4 41.2 30.5 94.1 25.0 88.9 Pyrola secunda 23.0 54.4 31.0 100.0 28.6 82.4 14.7 52.9 Mertensia ciliata 26.5 48.5 23.3 75.0 47.7 92.9 4.0 47.1 19.0 44.4 Erigeron peregrinus 22.4 48.5 33.3 100.0 20.3 92.9 42.3 41.2 5.7 41.2 Penstemon whippleanus 11.2 44.1 11.2 88.2 17.8 100.0 Senecio triangularis 31.8 38.2 45.0 50.0 38.7 100.0 36.0 100.0 Luzula parviflora 14.2 38.2 5.6 62.5 29.7 50.0 14.0 47,1 Osmorhiza depauperata 20.3 36.8 5.3 75.0 30.7 64.3 21.8 52.9 Streptopus amplexifolius 29.0 35.3 49.1 87.5 29.3 100.0 25.7 50.0 12.0 41.2 Poa nervosa 20.0 32.4 15.3 64.7 32.0 77.8 Achillea lanulosa 13.3 32.4 16.9 52.9 16.8 55.6 Trisetum spicatum 4.2 32.4 6.0 47.1 5.0 44.4 Hieracium gracile 14.2 29.4 8.0 29.4 11.4 41.2 22.4 55.6 Mitella pentandra 21.5 27.9 37.6 62.5 2.0 66.7 23.5 57.1 Sedum lanceolatum 6.7 27.9 6.2 64.7 8.6 77.8 Cystopteris fragilis 5.6 26.5 14.4 62.5 Pedicularis racemosa 17.7 25.0 23.2 29.4 17.1 41.2 Angelica grayi 16.9 25.0 27,1 64.3 Castilleja rhexifolia 3.5 25.0 5.3 42.9 1.0 44.4 Selaginella densa 15.0 23.5 17.5 64.7 I 1.0 44.4 Additional Prevalents Galium triflorum 35.3 100.0 Carex disperma 10.3 34.3 87.5 Equisetum arvense 54.0 75.O 14.0 66.7 Heracleum lanatum 24.7 75.0 Ligusticum porteri 10.0 75.0 48.0 66.7 Cardamine cordifolia 16.2 21.6 62.5 47.2 35.7 Listera cordata 22.1 16.8 62.5 6.0 66.7 Moneses uniflora 23.5 4.8 62.5 2.9 41.2 Pyrola asarifolia 31.0 50.0 Cinna latifolia 5.9 24.0 50.0 Viola pallens 10.0 50.0 40.0 66.7 Carex aquatilis 16.2 89,3 100.0 20.8 35.7 Caltha leptosepala 14.7 61.3 100.0 37.1 50.0 Pedicularis groenlandica 8.8 38.7 100.0 Fragaria vesca 11.0 50.0 Calamagrostis canadensis 73.3 37.5 34.0 66.7 65.7 50.0 Habenaria hyperborea 34.7 37.5 Gymnocarpium dryopteris 4.4 16.0 37.5 . Aconitum columbianum 10.7 37.5 Sedum rhodanthum 13.2 22.7 100.0 13.6 35.7 Saxifraga odontoloma 22.1 0.0 100.0 34.5 78.6 Community table 5 (cont.) -.a Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const. Freq. Const.

Eleocharis pauciflora 2.9 30.0 66.7 Epilobium anagallidifolium 4.4 40.0 66.7 Carex illota 2.9 16.0 66.7 Juncus drummondii 17.6 8.0 66.7 9.7 50.0 Podagrostis humilis 2.9 0.0 66.7 Agrostis variablis 76.0 33.3 Calamagrostis scribneri 44.0 33.3 Juncus mertensianus 2.9 40.0 33.3 Viola adunca 4.4 80.0 33.3 Epilobium sp. 40.0 33.3 Unknown 12.0 33.3 Carex canescens 20.0 33.3 Carex occidentalis 4.0 33.3 Trollius laxus 19.1 28.0 64.3 Bistorta bistortoides 14.0 57.1 12.0 66.7 Ranunculus eschscholtzii 14.7 14.3 50.0 Adoxa moschatellina 19.1 18.3 50.0 Epilobium hornemannii 16.2 17.3 42.9 Veronica wormskjoldii 13.2 17.3 42.9 Pedicularis bracteosa 12.0 42.9 Luzula spicata 34.4 35.7 Delphinium barbeyi 7.4 33.3 15.0 28.6 Carex nova 5.9 9.0 28.6 Pyrola virens 5.0 47.1 Haplopappus parryi 4.0 35.3 Solidago spathulata 10.5 64.7 11.2 55.6 Arabis drummondii 4.5 47.1 Carex foenea 4.0 35.3 21.3 66.7 Potentilla diversifolia 14.3 77.8 Antennaria rosea 8.6 77.8 Androsace septentrionalis 7.4 77.8 Artemisia arctica 39.3 66.7 Festuca brachyphylla 17.3 66.7 Mertensia viridis 2.4 55.6 Draba streptocarpa 3.2 55.6 Sibbatdia procumbens 25.0 44.4 Geum rossii 14.0 44.4 Erigeron simplex 20.0 44.4 Carex elynoides 4.0 44.4 Draba aurea 14.7 8.0 44.4 Saxifraga debilis 13.2 6.0 44.4 Trifolium parryi 26.7 33.3 71

Communi O' table 6. Pinus flexilis forest series.

Montane Pinus flexilis- Subalpine Pinus flexilis Picea, Abies Pinus flexilis Series F Forest (F 1) Forest (F2) 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

Community table 6 (cont.) Freq, Const. Freq. Const. Freq. Const. Freq. Const

Polemonium delicatum 3.2 55.6 0.0 37.5 Festuca brachyphylla 7.0 44.4 Arabis drummondii 8.0 44.4 Trifolium dasyphyllum 38.7 33.3 26.7 37.5 Pedicularis parryi 36.0 33.3 Pyrola secunda 1.3 37.5 Thermopsis divaricarpa 24.0 25.0 73

Community table 7. Forest-Alpine transition series.

Mesic Xeric Krummholz Krummholz Series G (G1) (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. Freq. Const. Freq, Const. TREES (Below 1 m) Series Prevalents Picea engelmannii 16.0 100.0 12.7 100.0 22.7 100.0 Abies lasiocarpa 53.5 59.2 83.3 44.0 100.0 SHRUBS Series Prevalents Salix brachycarpa 40.0 66.7 34.0 66.7 52.0 66.7 Vaccinium scoparium 27.3 66'13 28.8 83.3 20.0 33.3 Ribes montigenum 12,8 55.6 13.0 66.7 12.0 33.3 Additional Prevalent Vaccinium myrtillus 48.0 16.7 HERBS Series Prevalents Arenaria obtusiloba 16.0 t00.0 9.3 100,0 29.3 100.0 Sedum lanceolatum 10.7 !OOrO 8.7 100.0 14.7 lOO.O Potentilla diversifolia 6.2 100.0 5.3 100,0 8.0 I00.0 Trifolium dasyphyllum 13.0 88.9~ 7.2 83.3 22.7 lOO,O Geum rossii 17.0 88.9 16.8 83.3 17.3 100.0 Polemonium delicatum 19.0 ~8K9' 22.7 100.0 8.0 66.7 Artemisia arctica 135 -g'ffT".'ff t5.3 100.0 8.0 Selaginella densa 7.5 88.9 8.0 83.3 6.7 100.0 Bistorta bistortoides 15.5 88.9 19.3 100.0 4,0 66.7 Antennaria rosea 6.5 88.9 8.8 83.3 2,7 100.0 Penstemon whippleanus 8.6 77.8 7.2 83.3 t2.0 66.7 Epilobium angustifolium 9.1 77.8 8.8 83.3 10.0 66.7 Trisetum spicatum 3.4 77.8 3.2 83.3 4.0 ,,, ,,, ,,,,,,,, 66.7 Artemisia borealis 28.0 66.7 32.0 50.0 24.0 100.0 Arenaria fendleri 10.0 66.'f 1.3 50.0 18.7 100.0 Castilleja occidentalis 3.3 ...... 66.7 0,0 50.0 6.7 ,, ,0 100.0 Hymenoxys grandiflora 0.7 66.7 1.3 50.0 0.0 100.0 "Carex foenea 21.6 55.6 21.3 50.0 22.0 66.7' Silene acaulis 10.4 55.6 14.7 I00.0 ,, ,,,,, Mertensia viridis 10.4 55.6~ 9.3 50.0 12.0 66,7 Bistorta vivipara 12.0 55£ 13.3 100.0 Mertensia ciliata 3.2 55.6 4.0 66,7 Saxifraga rhomboidea 7.2 55.6 10.7 50.0 2.0 66.7 Carex albonigra 9.0 44.4 t8.0 66.7 Anemone narcissiflora 19.0 44.4 25.3 50.0 Festuca brachyphylla 13.0 44.4~ 14.7 50.0 Pedicularis parryi 6.0 "-~ 1.3 50.0 20.0 33.3 Erigeron simplex 10.0 44.4 12.0 50.0 Draba streptocarpa 6.0 44.4 6.0 66.7 Poa alpina 4.0 44.4 2.0 66.7 74 Community table 7 (cont.)

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

Campanula rotundifolia 1.0 44.4 2.0 66.7 Carex chalciolepis 1.0 44.4 2.0 66~_..27 K obresia myosuroides 14.7 33.3 16.0 66 7 Poa fendleriana 1K7 33.3 26.0 33.3 Carex rossii 18,7 33,3 18.7 50.0 Additional Prevalents Poa nervosa 33._.~3 22.7 50.0 Polemonium viscosum 33,3 5.3 50,0 Solidago spathulata 0.0 50.0 Trifolium parryi 22.2 26.0 50.0 Gentiana algidd 22.2 16.0 33.3 Achillea lanulosa 10.0 33.3 Juncus parryi 22._2 10.0 33.3 Oreoxis alpina 33,3 9.3 100.0 Senecio taraxacoides 33.--'~ 4.0 100.0 Poa lettermanii 22.2 14.0 66.7 Calamagrostis purpurascens - 10.0 66.7 Hymenoxys acaulis 22.2 8.0 66.7 Androsace septentrionalis 33,J 10.0 647 Carex rupestris 22.)' 28.0 33.3 75

Community table 8. Populus forest.

Populus Forest Series H (H 1)

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