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Changes in aspen communities over 30 years in Gunnison County, Colorado.

INTRODUCTION

Aspen (Populus tremuloides Michx.) is a widespread species common to the montane and subalpine forests of the southern Rocky Mountains where it has traditionally been considered a seral species (Mueggler, 1985). Shoots (sprouts) arise from the often widespread root system allowing quick regeneration of an aspen stand after disturbance (Schier and Campbell, 1976; Mueggler and Bartos, 1977; Parker and Parker, 1983; Jones and DeByle, 1985a; Shepperd, 1990). At higher elevations, aspen seres typically yield to spruce and fir, and a transition from aspen forest to spruce-fir forest may occur in just one aspen generation (Mueggler, 1987). This successional relationship between aspen and conifer populations does not appear to be true for all stands; aspen stands appear to form persistent communities at some locations (Peet, 1988).

In Gunnison County, Colorado, aspen occurs at elevations between 2600 m and 3400 m (Langenheim, 1962) where it may grow in mixed stands with subalpine conifers, commonly Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa), or as relatively pure aspen forests. In the Crested Butte area of Gunnison County, some aspen forests appear to be seral while others have been interpreted to be climax communities (Langenheim, 1962; Cox, 1968; Johnston, 1997), particularly those that abut fescue grassland rather than coniferous forests (Morgan, 1969). Our understanding of aspen community dynamics and succession in Gunnison County comes from static studies inferring community change through stand characteristics such as the species presence and their size-class distributions (Langenheim, 1962; Morgan, 1969). While these studies have contributed to our understanding of the dynamics of aspen forests and the successional role of aspen in Gunnison County, interpreting differences in community structure among neighboring stands as representing seres, or chronosequences, can lead to erroneous inferences of successional trends (e.g., Fastie, 1995).

Morgan (1969) studied 25 aspen stands in the Crested Butte area in 1964. His analyses included the characterization of the understory vegetation as well as tree species basal area and size-class distributions. The chemical and physical attributes of representative soils were also analyzed. The objective of our study was to assess the successional status and persistence of aspen communities in Gunnison County by duplicating the vegetational measures and analyses of Morgan (1969) 30 yr later. A second purpose was to establish permanent plots and additional baseline data within the stand to facilitate long-term monitoring of aspen succession in this area.

MATERIALS AND METHODS

The study area was located in Gunnison County, Colorado (38 [degrees] 50 [minutes] N, 107 [degrees] 00 [minutes] W, elev. 2800-3100 m above sea level), where Engelmann spruce-subalpine fir forests, subalpine meadows and aspen forests form a mosaic over sedimentary and granitic parent materials. The sandy loam soils are cryoborolls (Morgan, 196.5, 1969; Fox, 1977; Johnston, 1997). While charcoal down to soil depths of 75 cm indicates past fires, there is no evidence of fire in this century within the aspen stands we sampled (Morgan 1965, 1969) nor does the United States Forest Service have records of fires or other major disturbances within these stands. Grazing has been common in the area since the 1870s but historic livestock use of specific aspen stands is unknown. In the past 30 yr grazing has been limited or nonexistent in most stands; the Kebler Pass stands (stands 21-25; [ILLUSTRATION FOR FIGURE 1 OMITTED]) are the exceptions where cattle and sheep grazing was evident in both 1964 (Morgan, 1965) and 1994. The montane climate, regional geology, soil chemical and physical characteristics, and grazing and fire histories of the Crested Butte area were more thoroughly described by Langenheim (1962) and Morgan (1969).

Morgan (1969) sampled 25 aspen stands, and we were able to confidently locate 19 of these [ILLUSTRATION FOR FIGURE 1 OMITTED]. Using the sampling technique of Morgan (1969), two 70-m transects were established well within each stand to minimize edge effects. Four 10 m x 10 m plots were established at 20 m intervals along each transect, consistently on the uphill side of the transect. Diameter at breast height (dbh) of all trees was recorded and used to calculate the basal area for the eight plots within each stand. Paired t-tests were used to test the null hypotheses that aspen basal area per stand and the total number of trees per stand were the same in 1964 and 1994. Aspen within each stand were grouped into size classes (Morgan, 1969), and a chi-square analysis was used to test the null hypothesis that the aspen size-class distributions were the same in 1964 and 1994. Identical analyses were used to assess differences in conifer basal area, total number of trees, and size-class distributions between 1964 and 1994.

Nested within each large plot were 4 m x 4 m and 2 m x 2 m quadrats used to measure the cover of shrub and herbaceous species, respectively. Cover of all understory species was measured using Braun-Blanquet cover classes (Morgan, 1969). Because we lacked stand-specific understory data for 1964 and only 19 of the original 25 stands were resampled in 1994, qualitative comparisons were used to compare species constancy (the percentage of stands in which a species was in one or more plots) and mean cover class between 1964 and 1994.

To facilitate locating the transects in the future, 30-cm segments of reinforcement bar were driven at each end of both transects. In addition, GPS readings were recorded, stand locations indicated on 7.5-min USGS topographic maps and slides were taken both inside and outside of the stands.

RESULTS

The majority of aspen trees in the 19 stands were under 10 cm dbh and few were over 33 cm dbh (Table 1). The five stands in the Kebler Pass area (stands 21-25) and stand 20 in Washington Gulch were notable exceptions having the majority of live aspen in the four largest size classes. In 1994 there were fewer total live aspen in the 19 stands than in 1964 (Table 2). While tree density significantly decreased 23% from 3151 trees per ha in 1964 to 2425 trees per ha in 1994 (t = 3.67, df = 18, P = 0.002), total aspen basal area remained constant (t = 1.06, df = 18, P = 0.304) with 744 [m.sup.2]/ha in 1964 and 789 [m.sup.2]/ha in 1994. There was a significant shift in size-class distribution from smaller to larger trees ([X.sup.2] = 428, df = 5, P [less than] 0.001; Table 2). The total number of dead aspen trees remain unchanged (t = 0.209, df = 18, P = 0.897) though there was a significant shift in mortality to smaller trees ([X.sup.2] = 207, df = 2, P [less than] 0.001). Over 30 yr, there was a 215% increase in dead trees below 2.5 cm dbh, accompanied by a 34% decrease in dead trees above 2.5 cm dbh (Table 2).

The number of stands with conifers increased in 30 yr, from six of 19 stands having conifers in 1964 to 10 of 19 stands in 1994. However, conifers have not been quick in invading these aspen forests. In fact, there was no significant difference in the total number [TABULAR DATA FOR TABLE 1 OMITTED] [TABULAR DATA FOR TABLE 2 OMITTED] of conifer trees found in the stands (t = 0.317, df = 18, P = 0.755) and the conifer basal area (t = 0.704, df = 18, P = 0.491) between 1964 and 1994. Like the aspen, conifers are shifting towards larger trees within the aspen stands ([X.sup.2] = 27.0, df = 2, P [less than] 0.001; Table 3). Of the 127 conifer trees observed in 1994, 96 were on N-facing slopes (stands 12, 13 and 18) and 16 were in the Kebler Pass area (stands 21, 23 and 24).

The understory of the aspen stands was dense herbaceous growth and few shrubs. The species composition in 1994 was similar to that in 1964. Three of the four most dominant species in 1964, as measured by mean cover class, were still among the four most dominant species in 1994 (Table 4). Likewise, four of the six species that had constancies over 80% in 1964 also had constancies over 80% 30 yr later (Table 4). However, several species had changed substantially in their mean cover class and constancy over 30 yr. The two most dramatic changes were the increase in Elymus glaucus and the decrease in Taraxacum officinale.

DISCUSSION

Over the last 30 yr, the aspen stands studied have remained remarkably constant in tree composition. Though the aspen stands are maturing and size distribution is shifting to larger trees, root sprouting is occurring and replacement of older trees in most stands appears likely. In both 1964 (Morgan, 1969) and 1994, 26% of the trees were smaller than 2.5 cm dbh. Conifers have yet to substantially increase in any of the sampled stands. Langenheim (1962), Cox (1968) and Morgan (1969) also found conifer reproduction to be limited within local aspen forests. In many cases (notably stands 12 and 13 on a ridge below Gothic Mountain and stands 2 and 20 in Washington Gulch), abutting forests of mature, seed-producing conifers have not appreciably invaded the aspen stands. Conifer establishment within aspen stands has been primarily on the cooler, moister N-facing slopes which is consistent with the observation that in the Crested Butte area, aspen on northern slopes are more commonly seral to coniferous forests (Johnston, 1908). While no significant change in canopy structure over 30 yr was observed in the Kebler Pass stands, a notable lack of aspen root sprouts and the presence of understory conifers indicates that with the continued absence of fire, these stands may experience more rapid change in tree species composition than other stands studied.

The understory composition remained relatively unchanged over 30 yr. Dominant species in both 1964 and 1994 are similar to those listed by Langenheim (1962) who reported Thalictrum fendleru and Ligistichum porteri as understory dominants in stable aspen stands. Hoffman and Alexander (1980) and Komarkova et al. (1988) identified Populus tremuloides/Thalictrum fendleri and Populus tremuloides/Pteridium aquilinum community types in the southern Rocky Mountains as climax communities, and our stands would be classified as such. The aspen regeneration, limited conifer invasion, and unchanging understory suggest the aspen stands in the study area are relatively persistent (Mueggler, 1985, 1987).
TABLE 3. - Conifer dbh class (cm) distributions, 1964 and 1994
(total trees in 19 stands)

Year          0-2.5      2.6-12.7      [greater than] 12.7

1964           119          25                 10
1994            63          56                 8
% change       -47        +124                -20


[TABULAR DATA FOR TABLE 4 OMITTED]

In the eastern and northwestern United States, aspen is considered to be successional to other hardwood or coniferous forests (Sakai et al., 1985; Leak, 1991; Palik and Pregitzer, 1991; Peterson and Squiers, 1995). Previous workers (Langenheim, 1962; Mueggler, 1985, 1987, 1988; Komarkova et al., 1988; Cryer and Murray, 1992) in the intermountain West, however, have concluded that aspen-dominated communities are often stable and can be considered climax in some areas. This may be due in part to the fact that aspen lives longer and grows more slowly in the intermountain West than elsewhere (Jones, 1967). Mueggler (1987) also observed that while the occurrence of fires has decreased in western states, there is little evidence of a decrease in aspen-dominated woodlands, which suggests that aspen may not need repeated disturbance to persist. In contrast, other authors have assumed that the establishment and maintenance of aspen in the intermountain West are dependent on periodic disturbance (Bartos et al., 1994). Mueggler (1985) suggests that a community which can self-propagate without disturbance for several centuries should be considered climax, but he also concludes that the distinction between seral and climax aspen communities is not necessarily clear. In fact, the entire concept of climax communities has been questioned including the importance of differing spatial and temporal scales, disturbance regimes, and equilibrium and nonequilibrium states in interpreting community dynamics (e.g., Pickett et al., 1987; Pickett and McDonnell, 1989; Allen and Hoekstra, 1992; Glenn-Lewin and van der Maarel, 1992; McCook, 1994). Despite this ongoing debate, questions regarding the ecological factors that contribute to the long-term persistence of some aspen stands remain of interest.

Whether an aspen community persists or not may depend upon the site, particularly soil characteristics, vegetational history, and past disturbances. Parker and Parker (1983) reported finding aspen in newly burned sites associated with a deep, rich humus layer, whereas lodgepole pines in adjacent burns were associated with soils lacking this layer, indicating that the soils present at the time of a disturbance have a significant effect upon the ability of aspen to regenerate and ultimately become a stable community. Cryer and Murray (1992) proposed a model of aspen regeneration in western Colorado. On favorable sites, aspen and the associated understory species are able to maintain sufficient nutrient-rich litterfall which enhances the development of a mollic epipedon. These mollisols are not only ideal for aspen regeneration, they characteristically have a higher pH, a greater percentage of organic matter, and a greater cation exchange capacity and thus are less suitable for conifer growth (Hoff, 1957; Jones and DeByle, 1985b). On sites with a less-developed mollic epipedon, older aspen forests produce less litter and the mollic epipedon is not maintained. With time, an albic horizon forms and the resulting alfisol allows conifers to establish (Cryer and Murray, 1992). In 1964, Morgan (1965, 1969) sampled soils in nine aspen stands and in nearby coniferous forest and subalpine meadows of the Crested Butte area. In general, the mollisols (Fox, 1977) of aspen stands had more organic matter, higher pH, greater cation exchange capacity and more nutrients than nearby spruce-fir or meadow. Whether these aspen soils will retain their mollisol characteristics or eventually lose the mollic epipedon and become suitable for conifer growth per the soil-cycle models described by Cryer and Murray (1992) is uncertain.

Many of the aspen stands in the Crested Butte area have been grazed by livestock in the past and a few continue to provide cattle and sheep forage, particularly in the Kebler Pass area (stands 21-25). Pteridium aquilinum was found only in the five Kebler Pass stands where it had a mean cover class of 1.20 in 1964 (Morgan, 1965) and 1.58 in 1994; the abundance of P. aquilinum, which is unpalatable to livestock and reproduces abuntlantly with rhizomes (Mueggler, 1988), may reflect a long history of grazing in the Kebler Pass area. Likewise, the general lack of aspen sprouting in these stands (Table 1) may be attributable to heavy cattle grazing (Mueggler and Bartos, 1977; Bailey et al., 1990). However, the overall impact of grazing in the Crested Butte area appears to have decreased over the 30-yr period. Taraxacum officinale and Lathyrus leucanthus, which in abundance indicate heavy grazing (Mueggler, 1988; Komarkova et al., 1988), had higher cover and constancy in 1964 than in 1994 (Table 4). Similarly, the observed increase in grass species, such as the substantial increase in Elymus glaucus cover and constancy, indicates better range condition (Mueggler, 1988).

Establishment of aspen stands so common in the Crested Butte area is thought to have occurred in the past when climatic conditions facilitated aspen reproduction by seed (Langenheim, 1962). Repeated fires accompanied by suitable climate and low levels of herbivory (e.g., Romme et al., 1995; Baker et al., 1997) may have suppressed conifers and favored root sprouting of aspen and stand growth. Many of these stands appear to be persisting, without conifer invasion, despite the lack of fire for at least a century (Langenheim, 1962; Morgan, 1969). In contrast, other stands, particularly those on N-facing slopes and in the Kebler Pass area, show evidence of conifer invasion. Although Picea engelmannii and Abies lasiocarpa may gradually replace these aspen stands in the continued absence of fire, this replacement may take centuries.

Acknowledgments. - We thank Susan Lohr for her assistance in accessing stands in the vicinity of the Rocky Mountain Biological Laboratory. This research was funded in part by a Thornton Biology Research Grant.

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Author:Crawford, Jeremy L.; McNulty, Seth P.; Sowell, John B.; Morgan, Michael D.
Publication:The American Midland Naturalist
Date:Oct 1, 1998
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