Aspen: Ecological processes and management eras in northwestern Wyoming, 1807--1998

TitleAspen: Ecological processes and management eras in northwestern Wyoming, 1807--1998
Publication TypeThesis
Year of Publication2000
AuthorsHessl, A
Academic DepartmentGeology and Geography
UniversityUniversity of Arizona
KeywordsEcology, Geography, Paleoecology

Quaking aspen stands in many areas of the intermountain west are currently dominated by older (>100 year) age classes and may be in decline. The goals of my research are to: (1) place current observations of aspen decline into context by using historical and ecological data to investigate the interaction of fire, ungulate browsing, climate and human institutions in the regeneration of aspen stands over the last two centuries; (2) evaluate the challenges and limitations associated with using ecological history for management of aspen and other systems; and (3) compare current and historical aspen regeneration across three elk winter range areas in the intermountain West.

Based on results from stand age structures, aspen regeneration in the Jackson Valley has occurred episodically since 1830, with three major periods of regeneration: 1860-1885; 1915-1940; and 1955-1990. These multi-decadal episodes of aspen regeneration are related to similar variability in precipitation, where above average periods of annual precipitation are associated with aspen regeneration. However, significant levels of aspen regeneration have only coincided with low or moderate elk population estimates and fewer aspen have regenerated than expected when elk populations are high ( X 2 = 59.92, p < 0.0001). Current aspen reproduction, though minimal, is strongly affected by elk browse with percent browse significantly higher in elk winter range than outside of elk winter range (p = 0.051). Though extensive or frequent fires may have maintained aspen communities during the pre-settlement era, current management controlled fires have not affected aspen sucker density.

The influence of multiple interacting processes and drivers in the Jackson Valley suggests that reconstructing past ecosystems as benchmarks for ecological management should be considered carefully. Given future environmental variability, reconstructions of past systems should focus on ecological relationships rather than on single states or processes.

Comparison of aspen in the three elk winter range areas indicates that heavy browsing by elk populations has had a strong influence on episodes of aspen regeneration for the last 150 years in all three elk winter ranges. However, elk are not having strong impacts on aspen outside of elk winter range.