Big sagebrush (Artemisia tridentata) ecosystems of the western United States are facing threats of increasingly frequent wildfire and invasion of the annual grass cheatgrass (Bromus tectorum), which may be exacerbated by global climate change. Past studies have found cheatgrass to exhibit a positive feedback loop with wildfire, and climate change is expected to facilitate expansion of the cheatgrass potential range. However, it is unclear how future climate conditions will affect cheatgrass invasion and its effect on fire, or what ecological factors will affect susceptibility of sites to future invasion. The goal of our study is to simulate where cheatgrass invasion is most likely to occur, and to characterize the resulting impacts to fire frequency and the native plant community under various future climate scenarios. We modeled changes in functional type community composition and fire frequency under projected future climate conditions using an individual-based plant simulation model, STEPWAT2, for 91 sites across the big sagebrush ecosystem. We modeled future climate scenarios from global circulation models for established representative concentration pathway (RCP) emissions scenarios 4.5, and 8.5 for mid-century, and end-of-century. We examined changes in fire frequency and impacts to native plant functional types in response to future cheatgrass abundance.
Results/Conclusions
We simulated increased suitability and colonization of cheatgrass in 31 sites under future conditions. We found that sites where current climate is not yet suitable for cheatgrass within the Snake River Plain, the Wyoming Basins, and the Great Plains ecoregions were the most likely to become invaded by cheatgrass due to changing climate. Of these 31 sites, 18 became more suitable under RCP 8.5 conditions, while 13 became more suitable under RCP 4.5. The Colorado Plateau and the Great Basin sites maintained current high invasion levels. Fire return intervals across all sites with higher cheatgrass biomass (30g/m2) displayed shorter fire return intervals than sites characterized by low or no cheatgrass biomass (<3g/m2), decreasing from over 150 years to under 100 years under high invasion levels. Sites which supported higher cheatgrass biomass and more frequent fire had lower big sagebrush biomass than uninvaded sites (32g/m2 vs. 353g/m2, respectively). These results suggest that climate change may lead to the increased spread of cheatgrass and increased wildfire in big sagebrush ecosystems. These findings may help guide management decisions within these ecosystems in light of a changing climate.