Context <p>Plant communities will experience compositional shifts as a result of climate change. Shifting functional type prominence, through changes in suitability of cool-season C₃ and warm-season C₄ grasses, could alter plant community phenology, which could prove impactful in ecosystems where plant communities provide valuable forage to local migratory species.</p> Objectives <p>We investigated: (1) the differences in phenology and early-season biomass contribution of key functional types and (2) the extent to which mule deer migration pathways could experience altered forage availability through plant community changes.</p> Methods <p>We tracked growth and biomass of C₃ and C₄ grass individuals in western Wyoming through the growing season to quantify the phenology and productivity of both functional types. We also compared results of a plant community model to regional mule deer migration pathways.</p> Results <p>C₄ grasses reached peak biomass 48–60&#xa0;days later than other functional types. Small C₃ grasses and forbs had similar phenological progression, followed by large C₃ grasses and then C₄ grasses. Large C₃ grass individuals contributed most to productivity and growth rate peaked 22&#xa0;days after small C₃ grasses. With climate change, all functional types are projected to increase across 60–70% of deer migration pathways by 3–5&#xa0;g/m2.</p> Conclusions <p>Our study indicates that mule deer migration most closely coincides with the phenology of small C₃ grasses and forbs, and there is up to a 2-month difference in the regional green-up between functional types. Climate change will have variable impacts on species and the timing of resource availability. These results provide valuable regionally specific information on early-season growth.</p>