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Herbivory by domestic and wild ungulates as drivers of aspen recruitment and understory composition throughout arid montane landscapes
Environmental and Natural Resource Sciences
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Herbivory by wild and domestic ungulates influences tree recruitment and alters plant community composition in forests throughout the world. Herbivore-driven declines in tree recruitment have been widely observed for quaking aspen (Populus tremuloides), a foundation tree species that provides numerous ecosystem services and habitat for a variety of species. Livestock fencing is widely implemented as a management practice to promote aspen regeneration, but its effectiveness in achieving this goal is rarely tested quantitatively. Using a livestock reduction experiment, we evaluated the effects of ungulate herbivory on aspen in the Great Basin and southern Cascades, an expansive and environmentally heterogeneous region where aspen faces the interacting threats of drought, elevated temperatures, conifer encroachment, and herbivory by native, domestic, and feral mammals. We found that livestock fencing only reduced the intensity of ungulate browsing on aspen when wild ungulate abundance was low and did not result in increased stem densities of aspen recruits. Wild ungulate abundance was a strong driver of browsing intensity on juvenile aspen within aspen stands surrounded by fencing, but did not predict browsing intensity in unfenced aspen stands. In addition, browsing intensity on aspen was increased by the presence of shrubs and perennial grasses in the stand understory. The density of aspen recruits decreased with browsing intensity and was mediated by variables relating to fire history and intra- and interspecific competition, including adult stem density and the number of years since the stand last burned. Finally, we assessed the effects of livestock fencing on plant communities in aspen stand understories and found that fencing forb cover but increased shrub species richness and the cover of exotic annual grasses, a group dominated by Bromus tectorum. The interaction of livestock fencing with wild ungulate abundance and the association of fences with greater cover of exotic annual grasses suggests that although fences may be useful in reducing the influence of domestic herbivores, they may not be an appropriate management tool in areas with abundant populations of wild ungulates and/or high cover of exotic annual grasses. Rather, land managers may wish to erect fences that excludes both wild and domestic ungulates or focus on reducing wild ungulate populations through actions such as hunting, culling, and reintroducing apex predators. In sum, our findings indicate that aspen recruitment is limited by browsing by both wild and domestic ungulates and is mediated by competition with neighboring trees and fire history.