LINDA WERTHEIMER, HOST:
This is WEEKEND EDITION from NPR News. I'm Linda Wertheimer.
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 trees, there's not much anyone can do.
In fact, as NPR's Christopher Joyce reports, scientists are only now figuring out how drought affects trees.
CHRISTOPHER JOYCE, BYLINE: Park Williams studies trees at the Los Alamos National Laboratory in New Mexico, but not the way most scientists do.
PARK WILLIAMS: We're interested in trees that die.
JOYCE: Specifically, 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?
CRAIG ALLEN: That's a part we don't understand very well as ecologists. We don't know what it takes to kill trees.
JOYCE: Craig Allen, an ecologist with the U.S. Geological Survey. Together, these two ecologists have tracked droughts through the centuries. They know there've been bigger droughts than the one we're in now, but not many. And Williams says this one is different.
WILLIAMS: In past droughts, temperatures always rebounded and precipitation rebounded and went to a wet time. What we have now is a gradual trend towards warmer temperatures.
JOYCE: Climate change is making the region ever hotter. And when you have consistently hotter weather, the atmosphere gets thirsty; it sucks water out of the ground and out of plants. You get more droughts and deeper droughts, and less water for everything.
Computer models of climate change predict that this region in the Southwest will warm by several degrees before the century is out. Allen says the new normal will look like the worst of the past.
ALLEN: So it suggests basically that the main tree populations on our mountains will die. They can't - they will not be able to survive there. It doesn't mean there won't be any trees anywhere on these landscapes. But it means the current dominant trees will die.
JOYCE: Which ones survive and how many is the subject of research at the Los Alamos lab.
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NATE MCDOWELL: Please try to watch where you step.
JOYCE: Nate McDowell is giving me a tour of his tree torture lab. It's actually outside in the desert. He's growing a group of Pinon and Juniper trees, about 15 feet high. Plastic gutters keep rain away from the tree roots to simulate drought. The trees themselves are growing inside clear plastic chambers - tubes with no tops. Silvery hoses carry heated air into the chambers.
MCDOWELL: OK, so we climb in through this little hole. So this is a heated chamber and you can feel it if you put your hand...
JOYCE: Oh, yeah.
MCDOWELL: It's a little bit warmer air.
JOYCE: About seven degrees hotter than the outside; about the increase predicted by computer models of climate change over the next 80 years or so.
McDowell is simulating drought and a warmer climate. He measures how the trees respond. There are instruments stuck into and all over the trees, even wrapped around the stems.
MCDOWELL: Every few minutes, they measure the diameter of that tree.
JOYCE: Every few minutes?
JOYCE: It kind of looks like a patient in intensive care...
MCDOWELL: Yeah. Yeah.
JOYCE: ...all wired up and tubes coming in and out of it, to see what it takes to kill it.
MCDOWELL: Yeah, exactly. Everyone knows it gets hot and dry, beetles show up, the trees are dead, but we don't really understand it.
JOYCE: One thing McDowell's watching are the tiny holes in the trees' needles, they're 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.
MCDOWELL: Now, that's no problem if it's just for a day or a week, But, you know, the question is how long can they tolerate that before they run out of their stores? It would be like if we didn't eat...
JOYCE: We'd start burning up our own stores of fat, and that's what drought-stressed trees appear to do. They start consuming themselves, though it's not fat, it's carbon. And when trees die, whatever carbon they've got left goes into the atmosphere. Carbon is the very thing that's already warming the planet.
MCDOWELL: If there's more surface area of the Earth dying, more forests are dying, there could be more carbon coming out into the atmosphere. It accelerates the warming of the Earth.
JOYCE: When the climate warms, trees in dry places like this may not survive. But the what the work here will help predict where the losses will be, where there might be refuges, too. So we'll know more about what climate change will bring.
Christopher Joyce, NPR News.
WERTHEIMER: NPR photographer David Gilkey captured the fire and drought-ravaged landscape of the Southwest. You can see his pictures at npr.org.
(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright National Public Radio.