PARENT SESSION
Oral Session 8: Forest Ecology I: Dynamics and Succession.
Presiding: A Fiala and D Kashian
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room B 117.

Variable trajectories of aspen succession and decline in the northern Colorado Front Range.

Kashian, Daniel^*,1, Romme, William¹, Regan, Claudia², ¹ Colorado State University, Fort Collins, Colorado² USDA Forest Service Region Two, Lakewood, Colorado

ABSTRACT- Many investigators have suggested that factors such as climate fluctuations, conifer invasion, fire suppression, and/or increased elk browse have created an alarming trend of aspen (Populus tremuloides) decline in the western United States and Canada. Few studies, however, have documented the variability of trends in aspen succession across large landscapes, particularly where aspen is not the dominant cover type. To predict the long-term sustainability of aspen forests on the eastern slope of the Rocky Mountains, we assessed aspen successional trends in 92 stands across the northern Colorado Front Range. Almost 60% of the stands in our study area were less than 0.5 ha in area, and nearly 70% included ramets 100-140 years old. Approximately 19% of aspen stand age distributions confirmed a significant lack of recent recruitment and appeared to declining, most often at lower elevations where significant elk browse is heavy and chronic (p < 0.05) and where fire suppression is most effective. However, 38% of the stands had multiple cohorts indicative of self-replacing stands and exhibited little evidence of future decline. Furthermore, over 40% of all aspen stands were seral to conifers, most often when aspen occurred at high elevations among lodgepole pine forests. Although these stands had one cohort and were clearly declining, reconstructions of relative basal area and density of aspen and lodgepole pine suggest cyclical dominance of these species, where aspen stands are succeeded by conifers over long fire intervals, then vigorously re-establish following stand-replacing disturbances. Because fire suppression is less effective and elk browsing pressure is reduced in high-elevation forests, aspen stands may persist indefinitely in these ecosystems. Both natural and human disturbances are critical drivers of aspen succession in the West, and their relative influences in shaping western landscapes demand a broad-scale approach in assessing aspen decline. Although some aspen stands are declining in the northern Front Range, the presence of multiple successional trends makes it unlikely that aspen is in danger of disappearing from the region.

Key words: Colorado Front Range, lodgepole pine, aspen decline, succession