COS 82-4 - Big sagebrush plant community response to wildfire and invasive annual grasses in a changing climate

Abstract: Climate change and altered disturbance regimes have accelerated the spread of invasive species globally. In the western U.S., cheatgrass (Bromus tectorum L.), an invasive annual grass, has invaded large areas dominated by big sagebrush (Artemisia tridentata Nutt.) and increased wildfire frequency with negative implications for native plant communities. Although a substantial portion of the big sagebrush region is currently relatively uninvaded, additional warming may enhance wildfire probability and habitat suitability for cheatgrass thereby facilitating invasion. Our goal was to determine how wildfire and cheatgrass invasion will affect big sagebrush plant communities under a future climate and to identify geographic areas that will become vulnerable to and those that will remain resistant to cheatgrass invasion. We simulated the response of big sagebrush plant communities to climate change and wildfire under multiple climate scenarios using STEPWAT2, an individual-based plant simulation model. Wildfire was simulated based on predicted wildfire probability as a function of mean climate conditions (mean annual temperature, annual precipitation, proportion of precipitation in summer) and fine fuels (aboveground biomass of annual grasses and forbs, aboveground biomass of perennial grasses and forbs). Under warmer future conditions, we simulated continued suitability for cheatgrass in the majority of currently invaded areas and increased suitability in minimally invaded areas located in the northeastern portion of our study area. Simulated wildfire frequency increased under future conditions, particularly for the northern portion of the big sagebrush region, due to larger projected increases in temperature. We simulated declines in the abundance of big sagebrush, C₃ perennial grasses, and perennial forbs throughout large areas in response to the joint effects of climate change and increased wildfire frequency. In contrast, we simulated slight to moderate increases in habitat suitability for C₄ perennial grasses due to warming. Among all of the native plant functional types, big sagebrush was the most negatively affected by wildfire (up to 48% declines in biomass relative to simulations with no wildfire), while native herbaceous plant functional types generally responded slightly positively to or were unaffected by wildfire. These results suggest that the joint impact of climate change and increased wildfire frequency may enhance habitat suitability for cheatgrass in currently uninvaded sites in the northeastern portion of the big sagebrush region, while suitability in currently invaded areas will be sustained. Collectively, warming, increases in wildfire, and enhanced or sustained cheatgrass suitability may have negative implications for most native plant functional types, but especially for big sagebrush.