Herbivory by mule deer and cattle suppress aspen basal shoot height; threatening aspen stand resilience across three National Forests

Background/Question/Methods

High ungulate densities in forest ecosystems alter tree species composition which leads to drastic changes in plant and animal communities. In many regions with aspen forests, wildlife management policies, and extirpation of natural predators have promoted historically high ungulate densities and aspen is considered preferred browse species of elk, deer, and important forage for cattle. While herbivory is recognized as a factor in the degradation of aspen stands, the differential and interactive effects of herbivory on aspen regeneration are unstudied. Further, evaluating whether tree species composition and topographical features such as elevation, slope, and aspect at a stand predict successful aspen regeneration would improve management of aspen forests. The objective of this study is to characterize the impact of herbivory across broad geographical ranges and track aspen regeneration and recruitment as a function of ungulate density, stand type, and topographical features. We address the following questions: Does the density of ungulates have a direct effect on the regeneration of aspen? Does aspen regeneration differ based on stand tree species composition? Does herbivory by elk, deer, and cow differ in their impact on aspen regeneration? Do topographical features predict the success of aspen regeneration?

Results/Conclusions

We selected 186 sites across the Dixie, Manti-La Sal, and Fishlake National Forests in Utah, and characterized the following; overstory composition, ungulate density, and aspen regeneration. Using GIS, we extracted topographical features at each site from digital elevation models. We evaluated aspen basal shoot height and density using AIC model selection as a function of stand type, ungulate density, slope, elevation and aspect. Preliminary results show that high deer densities correlate to slightly increased aspen densities and strongly drive decreased aspen height, while cattle have slight impacts on decreases of both density and height. Surprisingly, elk -which are often blamed for aspen stand degradation in Utah- were not part of our top models and their density was not correlated with aspen density or height. High elevations (3000m-3200m), southern aspects and steep slopes (>30%) were correlated with high aspen basal shoot density. In conclusion, we identify possible refuge areas at high elevation, steeps slopes and southern aspects, and find that both deer and cattle adversely affect the success of aspen regeneration. Our study could inform current management strategies by accounting for topographical features and the differential effects of ungulate species.