Persistent Drought Kills Millions Of Trees In California Forests

KELLY MCEVERS, HOST:

Wildfires spreading across this state are bigger and stronger than normal thanks to five years of drought. The temperatures are higher. The area is drier, and many trees are dead. Biologists say this drought has killed millions of trees. NPR's Christopher Joyce is here in California with a team of tree-climbing scientists who are assessing the damage. I talked to him earlier today.

So you're in Sequoia and Kings Canyon National Park. That's in the Sierra Nevada mountains. And you are scientists who are climbing giant sequoia trees. What's that been like?

CHRISTOPHER JOYCE, BYLINE: Oh, fantastic. I mean watching these people go up 250 feet into the crown to study these trees is amazing. What they're doing is measuring everything - measuring the size of the branches. They're trying to determine how much foliage there is, how they're responding to the drought. They're doing, I guess you could call it, a biopsy of the tree and not just giant sequoias but actually all the different kinds of trees here that are suffering from the drought.

MCEVERS: I mean we know this drought's been going on for several years. What else are they finding out about it by climbing up in these trees?

JOYCE: They've been at it a couple of years. It's part of a program called Leaf to Landscape. It's the U.S. Geological Survey, and they're working with scientists from the University of California, Berkeley. They've found, for example, that trees at low elevation seemed to be a lot more vulnerable to drought. I suppose that's not a great surprise. You get a little more precipitation and cooler temperatures up higher.

But drought is killing all trees, mature as well as the young trees. The fires generally, if they're not too big, will only kill the young, small trees, and the big, mature trees survive. But drought is not as generous as fire in that sense.

And of course they're also finding that as these trees in the Sierra Nevada which have been hit hardest by the drought - they get sick. I mean they get weak. They don't necessarily die, but that's when the beatles move in.

MCEVERS: Oh.

JOYCE: It's like predators on the Serengeti that attack the weak.

MCEVERS: I mean, these are - these forests are really old - right? - like, centuries old. They've been through this before, haven't they?

JOYCE: They have, and certainly there have been droughts before but not a drought like this one because in addition to being dry or a lack of water, which is what a drought is, it's also incredibly hot, much hotter than it has been in the past because of climate change. And so the addition of heat does all sorts of things to trees.

It sucks the moisture out of the soil. It sucks the moisture out of the tree itself and gets sucked up into the atmosphere. It causes oxygen to bubble out of the water that's inside the trees and actually creates a sort of an embolism inside the tree, and they choke. And finally it causes the trees to close the little holes in their leaves, the stomata in their needles in their leaves, and that's how they breathe. But once they close those off, they can't take in CO2, carbon dioxide, and they starve to death.

MCEVERS: I mean so what can be done? I mean it's not like you can go out and, like, water all these trees.

JOYCE: Well, no. I mean it's a huge job. And for the trees that are already stressed and obviously the ones that are dead, I mean it's too late. But what scientists want to know is which trees are vulnerable? Which - how do they respond? The sequoias seem to be responding by letting some of their vegetation die back, and that's a good thing. But they want to know what elevation, for example, or what kind of soil confers any benefit in a drought.

And they actually can do things like do mechanical clearing or prescribe fires to get rid of some of the smaller trees. That reduces the competition for water so that the big trees can survive. And those are the sorts of things they want to do. And it's also a learning moment because, you know, scientists haven't seen a drought like this in decades and decades, so they're trying to figure out its course and where the beatles move in.

MCEVERS: That's NPR's Christopher Joyce reporting from Sequoia and Kings Canyon National Park here in California. Thanks a lot.

JOYCE: It's a pleasure.

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