Climate change and C₄ and C₃ grasses in a midlatitude dryland steppe

Climate change is projected to alter the structure of plant communities due to increasing temperatures and changes to precipitation patterns, particularly in midlatitude dryland ecosystems. Modifications to climatic suitability may lead to major community changes such as altered dominant plant functional types. Previous studies have indicated that climatic suitability is likely to increase for C₄ grasses and decrease for C₃ grasses in the Western United States. However, if no C₄ grass species currently exist to serve as a propagule source, expansion into areas of increased suitability will be limited. We conducted a field and modeling study in the Upper Green River Basin (UGRB) of Western Wyoming to determine if (1) C₄ grasses are present to provide a propagule source and (2) C₄ grasses are likely to increase in importance relative to C₃ grasses due to climatic changes. We searched 44 sites for C₄ grasses to establish presence, and modeled suitability at 35 sites using 17 Global Climate Models, two greenhouse gas Representative Concentration Pathways (RCPs; 4.5 and 8.5), and two time periods (mid- and late century; 2030-2060 and 2070-2099, respectively). We found C₄ grasses at 10 of the 44 sites, indicating that there is a present propagule source. Our model projected increases in suitability for both C₃ and C₄ grasses across sites for all RCPs and time periods. In the mid-century RCP 4.5 scenario, the C₃ functional type increased in projected biomass in 29 of 35 sites, and the C₄ type increased in 31 sites. In this scenario, C₃ grasses increased in projected biomass by a median 4 g m^-2 (5% change), and C₄ grass biomass increased by a median 8 g m^-2 (21% change). Our study suggests that climate change will increase climatic suitability for grasses across the UGRB, and that all requirements are in place for C₄ grasses to increase in abundance.