COS 122-2 - The effects of climate change on plant community composition in western Wyoming and implications for wildlife migration

Abstract: Climate change is projected to alter the structure of plant communities due to increasing temperatures and changes to precipitation patterns, particularly in water-limited dryland ecosystems. These climate-mediated modifications to habitat suitability can lead to major community changes such as altered dominant plant functional types. Previous studies have indicated that habitat suitability is likely to increase for C₄ grasses and decrease for C₃ grasses in the western United States. However, if no plants exist in an area to serve as a propagule source, expansion into areas of increased suitability could 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 have sufficient presence to provide a propagule source; 2) C₄ grasses are likely to increase in importance relative to C₃ grasses due to climatic changes; and 3) such a shift toward C₄ grasses could alter green biomass phenology and impact the spring migration of ungulates. We searched 44 sites in our study area for C₄ grasses to establish a source of propagules, and modeled suitability at 35 sites using 17 Global Climate Models, two Representative Concentration Pathways (RCPs; 4.5 and 8.5), and two time periods (mid- and late-century; 2030-2060 and 2070-2099, respectively). We compared the spring phenology of C₃ and C₄ grasses at 8 of the field sites and assessed proximity of C₄ locations to mule deer migration paths. We found C₄ grasses at 9 of the 44 sites, indicating a present propagule source. Our model projected increases in suitability for both C₃ and C₄ grasses across sites for both RCPs and time periods. In the late-century RCP 4.5 scenario, both functional groups increased in projected biomass in 32 of 35 sites. In this scenario, C₃ grasses increased in projected biomass by a median of 5 g/m² (7% change), and C₄ grass biomass increased by a median of 7 g/m² (20% change). Our field-based phenology comparison showed that C₄ grasses reached peak green biomass 6 weeks later than C₃ species. Mule deer migration paths overlapped with three sites containing C₄ grasses and were near three others. Our study suggests that climate change will cause increased habitat suitability for grasses across the UGRB. Due to the increase of C₄ species, the phenological differences of functional types, and the proximity of C₄ propagules to migration paths, climate change could result in altered forage availability during spring ungulate migration.