Grazing and climate change interact to influence big sagebrush plant community functional type composition

Kyle A. Palmquist, Department of Biological Sciences, Marshall University, Huntington, WV, Rachel R. Renne, Montana Natural Heritage Program, Missoula, MT, Rachel R. Renne, School of Forestry & Environmental Studies, Yale University, New Haven, CT, Daniel R. Schlaepfer and John B. Bradford, Southwest Biological Science Center, U.S. Geological Survey, Flagstaff, AZ, Daniel R. Schlaepfer, Center for Adaptable Western Landscapes, Northern Arizona University, Flagstaff, AZ, William K. Lauenroth, School of the Environment, Yale University, New Haven, CT

Background/Question/Methods
Livestock grazing is one of the most widespread land uses globally and can modify plant biomass and community composition. In the future, grazing regimes may interact with shifting climate to influence plant communities in unexpected ways, especially in drylands that are already water limited where recovery after disturbance is slower than in more mesic environments. We explored the individual and combined impact of climate change and grazing intensity in 200 big sagebrush (Artemisia tridentata) plant communities across the western U.S. and ask: (1) how will grazing intensity interact with climate to influence plant functional type biomass and composition? and (2) which environmental settings will be most vulnerable to grazing in the future? We simulated plant community responses under 52 CMIP5 climate scenarios (13 GCMs, 2 RCPs, 2 future time periods) and four grazing intensities (none, light, moderate, heavy) using an individual-based plant simulation model that incorporates a process-based representation of soil water availability and competition between individuals for soil water resources. We characterized changes in biomass and composition in response to climate and grazing intensity for the dominant plant functional types in these widespread dryland plant communities.
Results/Conclusions
Plant functional types responded differently to climate change and grazing intensity: big sagebrush was relatively resistant to climate change and grazing, while grazing modified perennial C3 grass, perennial C4 grass, and perennial forb biomass under future climate. We simulated small mean differences in future big sagebrush biomass under different grazing intensities (differences of 39 g/m2, 6%). However, a shift from light to heavy grazing under RCP8.5, end-century conditions resulted in slightly larger reductions in big sagebrush biomass than under other climate scenarios. In contrast to big sagebrush, shifts from light to heavy grazing exacerbated declines in perennial C3 grasses projected due to climate change, leading to mean reductions of 35% and 38% across all sites, depending on the RCP-time period combination. Perennial forb and perennial C4 grass biomass exhibited similar responses to perennial C3 grasses when grazing intensity shifted from light to heavy under future conditions (33% to 36% declines). Collectively, these results suggest a shift to greater shrub dominance, especially under heavy grazing and by end-century. Our simulations suggest grazing intensity will interact with climate change in drylands dominated by big sagebrush, with important implications for ecosystem management.