Title: Evaluating Vegetation Dynamics Under Sheep Targeted Grazing in Mountain Big Sagebrush Rangelands

Program: Master of Environmental Management (MEM)

Committee Chair: Kelly Hopping

Committee: Kelly Hopping, Allison Simler-Williamson, April Hulet, Kerry Byrne

Abstract: Invasive annual grasses like cheatgrass (Bromus tectorum) are transforming fire regimes, reducing native biodiversity, and threatening the livelihoods of rural communities across sagebrush rangelands. Large-scale management efforts of invasive grasses have had limited success, highlighting the need for innovative management solutions tailored to the needs of the landscape that incorporate ecological, economic, and social considerations. This study evaluates the effects of sheep targeted grazing on plant community dynamics and cheatgrass abundance in higher elevation sagebrush ecosystems. We implemented spring, fall, and combined fall and spring grazing treatments over two years, assessing their impacts on species diversity (alpha diversity) and community composition (beta diversity). Additionally, we investigated how sheep may be driving changes in vegetation based on their selection preferences, as identified by sheep herders. Spring grazing led to the greatest reductions in species diversity and preferred forage cover, while also significantly reducing cheatgrass. In contrast, fall grazing alone reduced cheatgrass more effectively than any other treatment without significantly altering diversity metrics, suggesting it may be more compatible with the ecological characteristics of these systems. Combined treatments did not consistently produce interactive effects and often dampened the individual effects of spring or fall grazing alone. Assessing sheep impacts based on dietary preferences rather than plant growth form provided more detailed insights into grazing effects. Using this approach revealed that sheep significantly reduced cheatgrass cover despite it not being a preferred forage species, suggesting that non-consumptive mechanisms may be contributing to the observed reduction in cheatgrass. This underscores the potential utility of incorporating herders’ local knowledge to refine grazing management in higher elevation rangelands with diverse plant communities. Ultimately, these findings offer insights for improving targeted grazing strategies that balance invasive species control with native biodiversity conservation.