Aspen regeneration in the Colorado Front Range: differences at local and landscape scales

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1 Landscape Ecology 14: , Kluwer Academic Publishers. Printed in the Netherlands. 231 Aspen regeneration in the Colorado Front Range: differences at local and landscape scales Kuni Suzuki 1, Harumi Suzuki 1, Dan Binkley 1, and Thomas J. Stohlgren 2 1 Graduate Degree Program in Ecology and Department of Forest Sciences, Colorado State University, Ft. Collins, CO 8523, USA; 2 Midcontinent Ecological Science Center, Biological Resources Division, U.S. Geological Survey and Natural Resource Ecology Laboratory Colorado State University, Ft. Collins, CO 8523, USA; (Corresponding author: dan@cnr.colostate.edu) (Received 13 February 1998; Revised 25 July 1998; Accepted 15 August 1998) Key words: elk, Cervus elaphus, herbivore effects, National Park management Abstract Elk (Cervus elaphus) populations in Rocky Mountain National Park are higher than at any time in the past century, and heavy browsing by elk may interfere with aspen (Populus tremuloides Michx.) regneration. We used aerial photographs to identify all aspen stands within Rocky Mountain National Park, and all aspen stands within the elk winter range range (defined as 24 to 28 m elevation) in three portions of the adjacent Roosevelt National Forest. From this population of aspen stands, we randomly selected 57 stands for evaluation of aspen regeneration. Stands that contained stems younger than 3 years and taller than 2.5 m tall were classified as regenerating successfully. Only 2% of the aspen stands in Estes Valley contained a cohort of regenerating aspen stems, whereas 45-to-75% of aspen stands across the larger landscape of the Front Range had regenerating cohorts of aspen. Within the elk winter range of the Roosevelt National Forest, 13 of 17 aspen stands were regenerating. In the elk winter range on the east side of the Park but outside of Estes Valley, 11 of 15 aspen stands were regenerating successfully. Only a few aspen stands exist in the elk winter range on the western side of the Park, and none of the five aspen stands sampled in Kawuneeche Valley had a regenerating cohort. The lack of regeneration in Kawuneeche Valley may result from locally heavy elk use in both winter and summer. In the summer elk range at higher elevations in the Park (28 to 32 m), 16 of 23 stands had regenerated. At landscape scales, all locations outside of the heavily impacted Estes Valley averaged about two cohorts/stand that regenerated after the mid-196s. All stands that lacked a regenerating cohort showed evidence of moderate-to-severe damage from elk browsing of stems. No regenerating stands showed evidence of severe browsing. We conclude that at landscape scales, regeneration within aspen stands is very common across the Front Range, except in local areas of the highest elk use where little regeneration has occurred in the past 3 years. Introduction Conifer forests dominate about 95% of the forests of the Rocky Mountains (USDA Forest Service 1982). The remaining portion of the forested area is covered by hardwood forests, primarily aspen (Populus tremuloides Michx.). Although aspen forests comprise a minor part of many Rocky Mountain landscapes, these forests have major ecological importance (DeByle and Winokur 1985). Aspen forests provide important habitat for birds and mammals (DeByle 1985a, b; Turchi et al. 1995), affect propagation of wildfires across landscapes (Brown and Simmerman 1986), and are hotspots of diversity for both native and exotic plant species (Stohlgren et al. 1997a, 1998). Given the minor extent and major importance of aspen forests, concern has developed about the state, dynamics, and future of aspen in the Rocky Mountains. The role of herbivory by elk may be particularly important. For example, Krebil et al. (1972) found that 9% of aspen mortality in the Teton National Forest in Wyoming resulted from damage by elk. Romme et al. (1995)

2 232 examined aspen regeneration in Yellowstone National Park, and concluded that the current extent of aspen forests resulted from favorable weather conditions in the 18s combined with low populations of elk. Currently, high elk populations in the northern regions of Yellowstone National Park impede aspen regeneration. Kay (1997) suggested that regional exclusion of natural fires and unusually high populations of elk have led to a regional decline of aspen; he cited unpublished USDA Forest Service data that showed a 6% decline in the area classified as aspen forest in Utah over the past century. Aspen stands cover only about 2% of Rocky Mountain National Park (RMNP) in Colorado, and the regeneration of these forests is important for the overall landscapes and species of the Park (Stohlgren et al. 1997b). Baker et al. (1997) recently examined the regeneration of aspen stands in the Estes Valley portion of RMNP. They reported that only about 1% of the aspen stands contained a cohort that successfully established in the past 3 years under the National Park Service s policy of natural regulation of elk populations. Aside from rocky sideslopes, successful regeneration occurred only in areas protected by fences that excluded elk. They concluded that few aspen suckers now alive would grow into trees (Baker et al. 1997). Olmstead (1979) had sampled 13 aspen stands in the same area and forecasted the probable disappearance of most of the stands in the near future. A recent panel of experts concluded that During the last 3 years, there has been little or no recruitment of new tree-sized aspen stems on the (elk) winter range (Berry et al. 1997). These conclusions and predictions were based on samples taken from less than 7% of RMNP, so extrapolation to the whole Park or to adjacent areas may not be warranted. In this study, we applied the methods used by Baker et al. (1997) to examine the state of aspen regeneration (since 1965) in the other portions of RMNP, and also in similar locations in the adjacent Roosevelt National Forest (RNF). Quantitative information on elk use is not available for most of these areas, but substantial hunting outside the Park has limited elk populations to lower levels than inside the Park. We hypothesized that aspen stands would commonly include a regenerating cohort in areas of RMNP and the adjacent RNF where the impacts of elk would be lower than in the high-impact area of Estes Valley of RMNP. Site description and methods Rocky Mountain National Park is in the Front Range of east-central Colorado. Elevations in the Park range from 23 m to 43 m, with annual mean temperatures ranging from 8 Cto 3 C, and precipitation averaging 5 to 7 mm/yr (half or more falls as snow, even at the lower elevations). Aspen stands generally occur in small (<1 ha) stands or patches (Elizabeth and Foster 199) at elevations <32 m. At low elevations, these aspen are often surrounded by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) or lodgepole pine (Pinus contorta Engelm.), especially in north-facing valleys. At higher elevations, aspen occur on all aspects and may form larger stands. Aspen stands tend to occur along meadows below hills with conifers (Mutel and Emerick 1992), but they also occur on rocky slopes. Aspen trees also occur in mixtures with conifers, and vigorous aspen stands sometimes develop following stand-replacing disturbances such as fires. We omitted these types of forests from our study, and focused only on aspen stands that did not have major conifer encroachment, and that had dominant individual trees that were >5 yr old. We divided the RMNP and RNF landscapes into four zones relative to the intensity and season of elk use: (1) elk winter range in Estes Valley, between 24 and 28 m elevation (using data from Baker et al and additional data from this project); (2) elk winter range inside RMNP, but outside Estes valley; (3) elk winter range in the RNF; and (4) elk summer range inside RMNP, above 28 m elevation. In each zone, all aspen stands were mapped and then a subsample of stands was chosen at random for measurement. In the elk winter range of Estes Valley, Baker et al. (1997) identified a total of 72 stands of aspen (stand defined as >1 trees), and they selected 17 stands for measurement. We mapped 4 additional stands at the northeast corner of Estes Valley in Black Canyon, and sampled 2 of these stands. For the other three zones, we mapped all aspen stands visible on aerial photographs (1:4 ), and randomly selected stands within each area for sampling. In the elk winter range inside the Park but outside Estes Valley, we mapped 34 stands (in McGraw Ranch, Black Canyon, Meeker Park, Wild Basin, and Kawuneeche Valley) and sampled 16 stands. Inside the Park in the elk summer range, we mapped 387 stands (279 on the eastern side of the Park, 18 on the western side) and sampled 23 stands. In the RNF, we mapped a total of 328 aspen

3 233 McGraw Ranch Stand R1-2 # of trees/plot Age DBH (mm) DBH (mm) Kawuneeche Valley Stand K # of trees/plot Age DBH (mm) dbh (mm) Figure 1. Distribution of tree diameters and relationship between diameter and tree age for a representative stand with a regeneration cohort (McGraw Ranch) and a stand lacking regeneration (Kawuneeche Valley). stands among the three study areas (Lonetree Mountain, Buckhorn Canyon, and Crosier Mountain) identified as elk winter range by the Colorado Division of Wildlife (CDOW 1996), and sampled 16 stands. The sampling intensity in RNF was lower than in RMNP because of the low variance in aspen regeneration among stands. We followed the measurement procedures used by Baker et al. (1997) to allow comparisons between the studies. In each sampled stand, a 1 m 1 m plot was subjectively located to represent a typical portion of the stand. Stand edges were avoided. Many stands had more than one apparent age cohort of stems, and a separate plot was placed in representative portions for each cohort. Stands that appeared to have only a single cohort were sampled with two plots, unless they were too small. In each plot, we tallied (by 2-mm diameter classes) the number of live aspen trees >2.5 m tall (above the reach of elk browsing, a value used by Baker et al. 1997). Ten or more increment cores were taken in each plot to determine stem ages, covering the entire range of diameter classes. Cores were taken from 1 cm above the base, or as high as.5 m above the ground when heart-rot prevented lower coring. Cores were stored in paper straws, with only one end sealed to let them dry. In the laboratory, the cores were glued to wooden trays and dried in a microwave oven. After sanding, a binocular microscope (4 ) was used to count rings and to estimate the ages of the trees. Annual rings in aspen can be difficult to discern, so the accuracy of our counts was verified by Peter Brown (USDA Forest Service); we are confident that tree ages are accurate within five years. We used the relationship between diameter and age within each plot to estimate the age of the uncored trees. The correlation of tree age to diameter was generally strong for trees less than about 1 mm dbh; larger trees showed greater variation in age within size classes. Regeneration timetables were developed to identify the regeneration frequency for each stand. The time period after 1965 (the period of natural regula-

4 234 tion of elk populations in the Park) was segmented into five 5-year intervals. If a stand contained a regenerating cohort that established within a 5-yr interval, the interval was counted as one. If regeneration did not occur, the count was zero. The number of 5- year intervals with successful regeneration represents the regeneration frequency for the stand. The regeneration frequencies were not normally distributed, and the variances among zones were unequal. Therefore we tested for differences in regeneration frequency among the study areas using the Kruskal Wallis test followed by nonparametric multiple comparisons by STP (Sokal and Rohlf 1981). We used a p value of.5 for the probability of a Type I error in our comparisons of the regeneration frequencies among the 4 zones. Results and discussion Typical patterns of tree size classes and age distributions are illustrated in Figure 1. The McGraw Ranch example represents the common pattern for stands that contained a regeneration cohort, and the Kawuneeche Valley example represents stands with no successful regeneration. Aspen regeneration was low in Estes Valley, with 2% of the aspen stands developing a regeneration cohort in 3 years (Figures 2, 3). Regeneration was significantly higher in other areas of elk winter range in RMNP (45% of stands), in the RNF (75% of stands), and in the elk summer range (high elevation) of RMNP (7% of stands). No pattern was apparent among the 5-year intervals (Table 1); regeneration appeared strong in all periods (low rates from probably reflect the time needed to attain the defined 2.5 m height). Many stands developed a regeneration cohort during more than one of the 5-year periods; the average number of regeneration events/stand in Estes Valley was.2 (standard error of the mean =.9), significantly lower than the 1.8 (s.e. =.5) average for other elk winter range in RMNP, 2.1 (s.e. =.4) for RNF, and 2.7 (s.e. = 1.8) for elk summer range in RMNP. These results support our hypothesis that aspen regeneration was greater outside Estes Valley. Patterns of aspen regeneration showed high variability within zones and locations. Although aspen regeneration in most of Estes Valley was low (as reported by Baker et al. 1997), we found two regenerating stands of aspen on the southwestern flank of Lumpy Ridge in the Black Canyon portion of Figure 2. Sampled stand locations and landscape pattern of aspen stands with regneration cohorts established since the 196s (cones), and non-regenerating stands (dots). Estes Valley. These anomalous sites showed very low impacts of elk (little damage on stems). High local variability in aspen regeneration was also evident on the west side of the Park. Stands sampled in the elk winter range of Kawuneeche Valley % Stands with regeneration N = 34 N = 76 n = 16 n = 19 N = 328 n = 16 Estes Valley Other RMNP RNF winter range winter range N = 387 n = 23 RMNP summer range Figure 3. Percentage of stands that contained at least one regeneration cohort that established since the mid-196s (N = number of mapable stands in the area, n=number of sampled stands). Regeneration in Estes Valley was significantly lower than in the other 3 areas, which did not differ among themselves.

5 235 Table 1. Regeneration frequencies by zone and location. Zone Location Total Number Number of sampled stands showing number of stands regeneration of stands sampled Elk winter range Baker et al In Estes Valley, RMNP Black Canyon Zone total Elk winter McGraw Ranch Range outside Meeker Park Estes Valley in Wild Basin RMNP Kawuneeche Valley 2 5 Zone total Elk winter range Crosier Mtn RNF Buckhorn Creek Lonetree Mountain Zone total Elk summer West Creek range, RMNP Cow Creek Forest Canyon Fall River Black Canyon Tahosa Creek 23 2 Glacier Creek Upper Kawuneeche Ranger Creek N. Fork Big Thompson N. St.Vrain Grand Lake Zone total Grand total showed no cohorts established since Many of these existing aspen were scarred by elk, and most of suckers were heavily browsed. Live, heavily browsed aspen suckers were present, and elk feces were dense on the forest floor. The impact of elk in this valley appeared very similar to Estes Valley. Two nearby stands to the east of Grand Lake were within the elevational band of elk winter range, but outside the area mapped as actual elk use area in winter (Colorado Division of Wildlife 1996). Both stands contained regeneration cohorts that established within the past 25 years, and signs of elk impact on stems and suckers were minimal. Aspen regeneration was strong in many areas of the elk winter range, except for Estes Valley and Kawuneeche Valley. Elk density is extremely high in Estes Valley, where >9% of elk in RMNP spend the winter (Larkins 1997). The winter elk population is much lower in Kawuneeche Valley, but summer use is very heavy (Larkins 1997). Kay (1997) noted that aspen stands protected from ungulate herbivory typically form multi-age structures, which is consistent with the stands we examined

6 236 in the Front Range. In contrast, Shepperd (1981) examined 14 stands in the Central Rocky Mountains of Wyoming and Colorado, and found that 83% had single-age structure, 13% had two-age structure, and 4% had multi-age structure. This may imply that many stands have been strongly affected by herbivory, and that herbivory across the region is more severe than along the Front Range, or that environmental factors affecting aspen stand structure differ among regions. Aspen stands are declining locally in the heart of the winter range of RMNP where elk use is highest (Olmsted 1979; Baker et al. 1997). These localscale studies showed that without active intervention to manage elk populations or the distribution of elk impacts, many aspen stands are likely to disappear. The localized nature of aspen damage by elk provides some opportunities for elk management; the current population of elk may be less critical than the concentration of the population in small areas. Management strategies could be developed that aim to disperse elk use more broadly by temporarily fencing portions of high-elk-use areas, drawing elk to areas of improved winter range with prescribed fires, or other means. While local-scale studies (Olmsted 1979; Baker et al. 1997) are appropriate for addressing the decline of individual or groups of aspen clones, they cannot address the persistence of aspen at landscape scales. We found no evidence that aspen stands are declining at landscape scales throughout RMNP, or in the adjacent RNF. We conclude that aspen stands are commonly regenerating the Front Range, with no evidence of any region-wide failure of regeneration. Within this general picture, a number of localized areas are characterized by stands with no regeneration cohort, primarily as a result of excessive browsing by elk. Two features of aspen stands in Rocky Mountain landscapes warrant future study. An unquantified portion of aspen trees occur in mixtures with conifers, and the general successional trends in these stands tends toward conifer dominance and aspen suppression or elimination. After major disturbances, the trajectory of species composition in mixed stands probably depends on site factors and small-scale disturbances that create small gaps in the forest canopy and allow aspen to persist. The extent, structure, and dynamics of mixed aspen/conifer forests should be quantified at a landscape scale. We also stress that large-scale, stand-replacing disturbances may play a pivotal role in the continuation of aspen stands across landscapes (cf. Romme et al. 1995), including significant establishment of new trees from seeds (Romme et al. 1997). Large fires and insect outbreaks in the Front Range could lead to major recruitment of aspen stands across the landscape, modifying the effect of high elk populations on aspen regeneration. Additional work is needed to quantify the extent, structure, and dynamics of small aspen stands, mixed stands of aspen and conifers, and the importance of large disturbances (such as fire and insect outbreaks) in perpetuating aspen stands in the presence of high impacts of elk. Acknowledgements We thank William L. Baker (University of Wyoming, Laramie) for advice and for sharing unpublished information. We thank our colleagues for assistance with information on aspen, elk habitat, dendrochronology methods, and aerial photo interpretation: Wayne Shepperd, Frances Pusateri, Steve Steinert, Jerry Claassen, Peter Brown, Steve Johnson, Ron Thomas, Nancy Wilson, and Skip Smith. Two anonymous reviewers substantially improved this paper. The project was funded by McIntire-Stennis appropriations to Colorado State University, and by the USGS Biological Resources Division (Colorado Rockies Global Change Research Program and the program to Develop an Inventory and Monitoring Program in Rocky Mountain National Park). References Baker, W. L., Munroe, J. A., and Hessl, A. E The effect of elk on aspen in the winter range of Rocky Mountain National Park, Colorado, USA. Ecography 2: Berry, J., Decker, D., Gordon, J., Heitschmidt, R., Huff, D., Knight, D., Romme, W., and Swift, D Science-based assessment of vegetation management goals for elk winter range. Report of the Environment and Natural Resources Policy Institute, Colorado State University, Ft. Collins. Brown, J. K. and Simmerman, D. G Appraisal of fuels and flammability in western aspen: a prescribed fire guide. USDA Forest Service General Technical Report INT-25, Ogden. Colorado Division of Wildlife Elk Habitat Database. Colorado Division of Wildlife. Fort Collins. DeByle, N. V. 1985a. Wildlife. In Aspen: Ecology and Management in the Western United States. Pp USDA Forest Service General Technical Report RM-119, Fort Collins. DeByle, N. V. 1985b. Animal impacts. In Aspen: Ecology and Management in the Western United States. Pp USDA Forest Service General Technical Report RM-119, Ft. Collins. Kay, C. E Is Aspen Doomed? J. Forestry 95: Krebill, R. G Mortality of Aspen on the Gros Ventre Elk Winter Range. USDA Forest Service Research Paper INT-129, Ogden.

7 237 Larkins, K. F Patterns of elk movement and distribution in and adjacent to the eastern boundary of Rocky Mountain National Park. MA thesis, University of Northern Colorado. Mowrer, H. T. and Shepperd W. D Field Measurement of Age in Quaking Aspen in the Central Rocky Mountains. USDA Forest Service Research Note RM-476, Fort Collins. Mutel, C. F. and Emerick, J. C From Grassland to Glacier. Johnson Books, Boulder. Olmsted, C. E The effect of large herbivores on aspen in Rocky Mountain National Park. Ph.D. dissertation, University of Colorado, Boulder. Olmsted, C. E The ecology of aspen with reference to utilization by large herbivores in Rocky Mountain National Park. In: North American Elk. Pp Edited by Boyce, M. S. and Hayden-Wing, L. University of Wyoming Press, Laramie. Romme, W. H., Turner, M. G., Wallace, L. L., and Walker, J. S Aspen, elk, and fire in northern Yellowstone National Park. Ecology 76: Romme, W. H., Turner, M. G., Gardner, R. H., Hargrove, W. W., Tuskan, G. A., Despain, D. G., and Renkin, R. A A rare episode of sexual reproduction in aspen (Populus tremuloides Michx.) following the 1988 Yellowstone fires. Natural Areas J. 17: Shepperd, W. D Stand characteristics of Rocky Mountain aspen. Situation Management of Two Intermountain Species (Aspen and Coyotes) Symposium Proceedings. Vol. I. Aspen. Utah State University, Logan. Shepperd, W. D Vegetative reproduction and early clonal growth of aspen. The Ph.D. dissertation, Colorado State University, Fort Collins. Shepperd, W. D. and Fairweather, M. L Impact of large ungulates in restoration of aspen communities in a southwestern Ponderosa pine ecosystem. In Sustainable Ecological Systems: Implementing an Ecological Approach to Land Management. Pp Edited by Covington, W. W. and DeBano, L. F. General Technical Report RM-247, Fort Collins. Sokal, R. and Rohlf, F. J Biometry. Freeman, San Francisco. Stohlgren, T. J., Chong, G. W., Kalkhan, M. A., and Schell, L. D. 1997a. Rapid assessment of plant diversity patterns: A methodology for landscapes. Ecol. Monit. Assessment 48: Stohlgren, T. J., Coughenour, M., Chong, G., Binkley, D., Kalkhan, M., Schell, L. D., Buckley, D., and Berry, J. 1997b. Landscape analysis of plant diversity. Landsc. Ecol. 12: Stohlgren, T., Binkley, D., Chong, G. W., Kalkhan, M. A., Schell, L. D., Bull, K. A., Otsuki, Y., Newman, G., Bashkin, M., and Son, Y Exotic plant species invade hotspots of native plant diversity. Ecology, in press. Turchi, G. M., Kennedy, P. L., Urban, D., and Hein, D Bird species richness in relation to isolation of aspen habitats. Wilson Bull. 17: USDA Forest Service An assessment of the forest and range land situation in the United States. Forest Resource Report #22, Washington, D.C.