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."
One thing he's watching are the tiny holes in the trees' needles, called stomata. The trees absorb carbon dioxide through them, to make food. In drought, though, the stomata close up to conserve moisture. That means they can't make food. "Now that's no problem if it's just for a day or week," McDowell points out, "but you know the question is how long can they tolerate that before they run out of their stores? It's like if we didn't eat."