Forecast distribution shifts of montane forests in the Sky Islands under projected 21st century climate change
The sky islands of Southwestern North America consist of several dozen isolated mountain ranges which rise out of the Sonoran and Chihuahuan desert basins between the Sierra Madre Occidental of Mexico and the Southern Rocky Mountains. They are capped in montane forests whose species have expanded and contracted across the desert basins throughout the glacial/interglacial cycles of the Quaternary. Contemporary climate change is driving a further contraction of the montane forests. To what extent, then, will 21st century climate change drive contractions of montane forests?
Given the individualistic response of species to environmental change, we develop empirical climate-based distribution models for each coniferous tree species composing the montane forests of the Sky Islands. Each species model is built using observations from across their respective distributions in order to capture the breadth of their realized climatic niche. For each species an ensemble of five modeling techniques is built and applied to six general circulation models under two climate change scenarios. The resulting changes in species distributions are analyzed using landscape metrics characterizing patch size, shape, and landscape connectivity. In turn, these metrics are applied to the combination of all species to assess changes in the montane forests as a whole.
Every conifer species modelled is forecast to have an overall reduction in areal distribution. Several of the higher altitude conifers (Abies lasiocarpa, A. concolor, Picea engelmannii, P. pungens ,and Pinus flexilis) are regionally extirpated from the sky islands under one or both climate change scenarios. In all cases the number of patches and their landscape connectivity decline. Collectively, the number of montane forest patches decline by ~25% and total area declines by ~50%. Landscape connectivity and the perimeter/area ratio of patches, however, change little in comparison, declining by ~1% and ~5%, respectively.
The size and location of the Sky Islands makes them both biologically unique and exceptionally vulnerable to climate change. Our results suggest montane forest conifers will decrease in area and species richness. The relatively small change in patch shape and landscape cohesion is reflective of the form and isolation of each mountain. The forecast disappearance of some conifer species from the Sky Islands attests to the variation in regional impacts of climate change induced shifts in continental distributions.