Herbivory strains resilience in drought-prone aspen landscapes of the western United States
Aspen forests in the northern hemisphere provide richer biodiversity compared to surrounding vegetation types. In both North America and Europe, however, aspen stands are threatened by a variety of human impacts: clear felling, land development, water diversion, fire suppression and both wild and domestic ungulate herbivory. We conducted a landscape assessment of quaking aspen (Populus tremuloides) for the purpose of identifying key components of resilience. Specifically, we tested novel measures linking plant–animal interactions, compared crucial functional differences in aspen types and made restorative recommendations based on the outcome of these assessments.
Book Cliffs region, eastern Utah and western Colorado, USA.
Seventy-seven 1-ha plots were sampled for forest structure, composition, regeneration and recruitment, landscape elements, browse level and herbivore use. Use was determined by counting the number of pellet groups from ungulate species at each sample location. We tested the efficacy of a visual stand condition rating system when compared to objective metrics. A series of non-parametric analyses were used to compare functional aspen types and stand condition groups by key variables. Non-metric multidimensional scaling (NMS) allowed us to explore all our data to find the most critical measures of aspen stand conditions for the purpose of better informing future aspen monitoring.
Plots differed significantly in seral or stable aspen functional types, stand condition rating and browse species use. Ordination analysis revealed that regeneration level and herbivore use were the strongest objective indicators of aspen stand conditions, while stand condition rating proved a valuable subjective index of forest status. While ungulate herbivory of aspen is problematic internationally, our results show acute impacts where moderate slopes, relatively low water availability and intense browsing predominate.
Appropriate measures for aspen communities, informed by crucial functional divisions, allowed us to gain a clear understanding of conditions across this large landscape. Overall, aspen in our study landscape is highly vulnerable to collapse due to narrow physiographic and climate limitations and browse levels. Without herbivory reduction, future conservation in such areas will be strained and widespread system failure may occur.