Climate change could cripple southwestern US forests: Trees face rising drought stress and mortality as climate warms

Science Daily: Combine the tree-ring growth record with historical information, climate records, and computer-model projections of future climate trends, and you get a grim picture for the future of trees in the southwestern United States. That's the word from a team of scientists from Los Alamos National Laboratory, the U.S. Geological Survey, the University of Arizona, and other partner organizations.

If the Southwest is warmer and drier in the near future, widespread tree death is likely and would cause substantial changes in the distribution of forests and of species, the researchers report this week in the journal Nature Climate Change. Southwestern forests grow best when total winter precipitation is high combined with a summer and fall that aren't too hot and dry.

The team developed a Forest Drought-Stress Severity Index that combines the amount of 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 UA Laboratory of Tree-Ring Research.

"Among all climate variables affecting trees and forests that have ever been studied, this new drought index has the strongest correlation with combined tree growth, tree death from drought and insects, and area burned by forest fires that I have ever seen."

A. Park Williams of Los Alamos National Laboratory in New Mexico is the lead author of the paper, "Temperature as a potent driver of regional forest drought stress and tree mortality." Six of the paper's 15 authors are at the UA. A complete list of authors is at the bottom of this release. ...Williams said, "Atmospheric evaporative demand is primarily driven by temperature. When air is warmer, it can hold more water vapor, thus increasing the pace at which soil and plants dry out. The air literally sucks the moisture out of the soil and plants."...

Weathered growth rings in a horizontal cross section cut through an tree felled around AD 1111 used for the western building complex at Aztec Ruins National Monument, San Juan County, New Mexico, USA. There is the cross section cut of the tree located in the outer wall of the building. Shot by , Wikimedia Commons, under the Creative Commons Attribution 3.0 Unported license