Herbivory impacts of elk, deer and cattle on aspen forest recruitment along gradients of stand composition, topography and climate

Abstract

The abundance and composition of large ungulate communities are changing dramatically across terrestrial ecosystems due to human activities. This has resulted in novel herbivory regimes that create strong top-down effects on biological communities, and can reduce ecosystem resilience. Biotic and abiotic conditions that influence the population structure and behavior of ungulate herbivores are highly variable across space and time, making it difficult to predict ungulate impacts across complex landscapes. Where multiple ungulate species co-exist, it can be challenging to differentiate species impacts on plant communities. We examined the effects of cattle, American elk, and mule deer on aspen regeneration and recruitment along gradients of forest composition and physiographic conditions in 3 National Forests of Utah, USA. Aspen regeneration and recruitment increased with winter precipitation, elevation and higher abundance of overstory aspen. Binned fecal count densities of elk, mule deer, and cattle showed strong inverse correlations with aspen height (elk: r2=0.83; deer: r2=0.92; cattle: r2=0.35) and aspen recruitment (elk: r2=0.51; deer: r2=0.59). Aspen stem density was negatively correlated with elk (r2=0.35) and cattle fecal densities (r2=0.71). We found that 60% of the sites we surveyed had low ungulate activity and good aspen regeneration, 32% were intermediate use sites that warrant additional monitoring, and 8% of sites showed high ungulate use that exhibited aspen regeneration failure and lack of recruitment where targeted management is recommended. We conclude that ungulate impacts in aspen forests should be considered on a site by site basis and should primarily be evaluated based on the total abundance of the ungulate community with precipitation, elevation and stand composition as secondary factors.