Potential changes in the distributions of western North America tree and shrub taxa under future climate scenarios

Increases in atmospheric greenhouse gases are driv-ing significant changes in global climate. To project potential vegetation response to future climate change, this study uses response surfaces to de-scribe the relationship between bioclimatic vari-ables and the distribution of tree and shrub taxa in western North America. The response surfaces il-lustrate the probability of the occurrence of a taxon at particular points in climate space. Climate space was defined using three bioclimatic variables: mean temperature of the coldest month, growing degree days, and a moisture index. Species distributions were simulated under present climate using ob-served data (1951–80, 30-year mean) and under future climate (2090–99, 10-year mean) using sce-narios generated by three general circulation mod-els—HADCM2, CGCM1, and CSIRO. The scenarios assume a 1 % per year compound increase in green-house gases and changes in sulfate (SO4) aerosols based on the Intergovernmental Panel on Climate Change (IPCC) IS92a scenario. The results indicate that under future climate conditions, potential range changes could be large for many tree and shrub taxa. Shifts in the potential ranges of species are simulated to occur not only northward but in all directions, including southward of the existing ranges of certain species. The simulated potential distributions of some species become increasingly fragmented under the future climate scenarios, while the simulated potential distributions of other species expand. The magnitudes of the simulated range changes imply significant impacts to ecosys-tems and shifts in patterns of species diversity in western North America. Key words: future climate change; response sur-face models; western North America; trees