Forests of the southwestern U.S. may be on the verge of dramatic changes in the coming decades, as a warming climate may squeeze many species of their narrow ecological niche. New research shows that Southwest drought conditions in recent years are as intense as they were during the historic megadroughts of the 1200s and 1500s. Southwestern forests grow best when total winter precipitation is high combined with a summer and fall that aren’t too hot and dry, but many climate models suggest the region will be warmer and drier. New Mexico just experienced its driest 24-month stretch on record. If those conditions persist, it would likely result in widespread tree deaths and significant changes in the distribution of species on a regional landscape level, according to a new report published in the journal Nature Climate Change last week. To measure the impacts of climate change, the scientists developed a stress index, factoring winter precipitation, late summer and fall temperatures, and late summer and fall precipitation into one number. “The new ‘Forest Drought-Stress Index’ that Williams devised from seasonal precipitation and temperature-related variables matches the records of changing forest conditions in the Southwest remarkably well,” said co-author Thomas W. Swetnam, director of the University of Arizona Laboratory of Tree-Ring Research.

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