Climate change said threat to US forests

Forests in the US Southwest could be crippled by climate change as rising drought brings stress and tree mortality, researchers say. Scientists from Los Alamos National Laboratory, the U.S. Geological Survey and the University of Arizona combined tree-ring growth records with historical information, climate records, and computer-model projections of future climate trends to forecast the future health of the region's forests. Southwestern forests grow best when winters with high precipitation levels are combined with a summer and fall that aren't too hot and dry, they said, predicting if the Southwest is warmer and drier in the near future widespread tree death is likely, leading to substantial changes in the distribution of forests and of species. To analyze which climate variables affect forests, the researchers combined some 13,000 tree core samples with known temperature and moisture data and with events known from tree-ring, archaeological and other paleorecords, and ASU release reported. One important climate variable is known as the summer-fall atmospheric evaporative demand, a measure of the overall dryness of the environment. "Atmospheric evaporative demand is primarily driven by temperature," A. Park Williams of Los Alamos National Laboratory said. "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." In the current drought, which began in 2000, average summer-fall evaporative demand has been the highest on record, researchers said.