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 (<I 700 m) by shrub Patterns in species diversity and geographical and grassland formations. The present study is an variation in forest composition have been treated in investigation of this belt of forest vegetation as earlier papers (Peet, 1978a, 1978b). Changes in size represented in Rocky Mountain National Park and and age structure of tree populations will be the the adjacent foothills. subject of a future paper. Rocky Mountain National Park, Colorado, en- bedrock. The fine-textured soils which contribute compasses an area of over l 060 km 2 between to the formation of mountain grasslands are largely 40 ° 10' and 40032 , north latitude and 105031 ' and confined to the wider bottomlands. 105°41 ' west longitude• In addition, the study area A detailed soil survey has not been conducted for includes the foothills to the east, largely in the the study area and an intensive study of the rocky, Roosevelt National Forest and west of 105 ° 15'west heterogeneous, immature soils of the region was longitude (Fig. 1). The study was confined to the not undertaken as part of the present study because east side of the range as defined by the continental of time constraints. Instead, a general portrait of divide south of 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