The droughts that have parched big regions of the country are killing forests. In the arid Southwest, the body count is especially high. Besides trying to keep wildfires from burning up these desiccated forests, there’s not much anyone can do. In fact, scientists are only now figuring out how drought affects trees. Park Williams studies trees at the Los Alamos National Laboratory in New Mexico, but not the way most scientists do. “We’re interested in trees that die,” he says — spefiically, death by heat and drought. Sure, lack of water kills trees, but which ones die first, how long does it take, how long can they go without water? “That’s a part we don’t understand very well as ecologists,” says Craig Allen, an ecologist with the U.S. Geological Survey. “We don’t know what it takes to kill trees.”