Topoclimates, refugia, and biotic responses to climate change

Plant distributions are strongly influenced by both climate and topography. In an analysis of geographic and topographic distributions for selected tree species in California, we found that tree populations are increasingly restricted to extreme topographic positions as they approach the edge of their geographic ranges, occupying cooler, pole-facing slopes (at the warm and dry edge) and warmer, equator-facing slopes (at the cool and moist edge). At a local scale, species distributions across topographic gradients also correlate with species geographic ranges (species that occupy cooler locations within the landscape have cooler, moister geographic distributions, and vice versa). Model outputs indicated that species found on pole-facing slopes and equator-facing slopes will experience population declines and population increases, respectively, in response to a warmer and drier future. As such, tree species occupying cooler landscape locations, which are viewed as refugia in some contexts, may be most threatened by anthropogenic climate change.