Modeled climate change impacts on vegetation of the Mojave-Sonoran Desert interface

Background/Question/Methods: Modeled predictions for levels of climate change indicate that arid lands of the southwestern U.S. may experience greater departures from current climate conditions than will other temperate portions of North America. High temperatures and extended droughts that characterize many deserts may already approach physiological tolerances for many species and so the fate of biodiversity in the network of desert National Parks and refuges whose mission includes protecting that biodiversity here is an open question. Here we examined climate change sensitivity for an array of woody vegetation species at the Mojave-Sonoran Desert interface in Joshua Tree National Park, California. We modeled climate change impacts for six species with strong Mojave Desert affiliations, including Joshua Trees, Yucca brevifolia, three species with strong Sonoran Desert affiliations including Ocotillo, Fouqueria splendens, and four generalists whose distributions span both desert ecoregions (such as creosote, Larrea tridentata). Using abiotic variables we employed the Mahalanobis D² statistic to model their niche spaces and then assessed climate-change sensitivity by altering climate variables along a gradient of increasing temperature. While shifting climate variables, we held terrain and soils variables that otherwise define these species’ preferred habitat constant, providing a more realistic prediction of available niche space.

Results/Conclusions: The modeled distribution of suitable habitat shifted predictably up slope for each species, although the extent of that shift and the area of remaining habitat varied substantially among species. Plants with broad distributions such as creosote generally fared better than those with more narrow niches. With a simulated 3 degree Celsius increase in summer maximum temperatures creosote lost just 26% of its current areal extent whereas more narrowly distributed species such as Blackbrush, Coleogyne ramossisima, lost 86% of its current extent.  An overall result was a future blurring of the currently relatively discrete boundaries of the Mojave and Sonoran Deserts. Previous research had modeled a widespread retreat of Joshua trees away from their current southern distribution due to predicted levels of climate change. While our findings indicated a high sensitivity to increasing temperature in Joshua Trees, at least at moderate temperature shifts (a 2 degree Celsius increase in summer maximum temperature) 31% of their current distributions still remained within climate refugia. Successful recruitment in Joshua Trees is currently limited to that region identified as a future refugium, indicating impacts of climate change may already be apparent on this landscape.