Climate change alters the vegetation composition and functioning of ecosystems. Measuring the magnitude, direction, and rate of changes in vegetation composition induced by climate remains a serious and unmet challenge. Such information is required for a predictive capability of how individual ecosystems will respond to future climates. In this study we identified the relationships between 20 climate variables and 39 large landscape ecosystems across the southwestern USA, covering an area of about 1,228,167 km2. We sought to understand the magnitude of relationships between variation in vegetation composition and bioclimatic variables as well as the amount of ecosystem area expected to be affected by future climate changes. Data used in this study consisted of mapped ecosystem distributions, field plot observations of plant species composition across each ecosystem, and bioclimatic variables at each field plot location. Bioclimatic variables best explaining the plant species composition of each ecosystem were identified. The strength of relationships between beta turnover within each ecosystem and bioclimatic gradients was calculated, the spatial concordance of ecosystem and bioclimatic spatialconfigurations was shown, and the area of suitable climate remaining within the boundaries of contemporary ecosystems under future climate projections measured.
Across the study area four climate variables account for most of the climate related variation in vegetation composition. The plant species composition of twelve ecosystems comprising about 238,285 km2—almost 20% of the study area—is highly sensitive to climate change. By the 2070s, two ecosystems are projected to lose 4,000 and 7,000 km2 of suitable climate area within their current boundaries. The climate areas of riparian ecosystems are expected to be reduced by half by the same decade. Results provide specific climate-vegetation parameters for anticipating how, where and when ecosystem vegetation is expected to transform with climate change. Projected loss of suitable climate for the vegetation composition of today’s ecosystems sets priorities for ecosystem conservation and restoration across the southwestern USA.