KELLY MCEVERS, HOST:
It's been a brutal fire season in California, but the record-setting drought is actually a greater threat to the state's forests. Over the past four years, the drought has killed at least 60 million trees. Scientists are struggling to find ways to keep the survivors alive, and as NPR's Christopher Joyce discovered, they're trying some completely new ideas.
CHRISTOPHER JOYCE, BYLINE: The Sierra Nevada Mountains are known for rocky pinnacles and tall forests. These days, though, the forests are blotchy. Patches of dead trees stand right next the healthy, green ones. I went into the forest to talk to Nate Stephenson, an ecologist with the U.S. Geological Survey. He says the drought and high heat combine to do things he has not seen before.
NATE STEPHENSON: We don't really understand a lot of things, like exactly how a drought kills a tree or what's going on underground - you know, where is the water flowing in areas we can't see.
JOYCE: So a team of ecologists has come here, as well, with truckloads of equipment to find out what separates the survivors from the dead.
UNIDENTIFIED PERSON: (Unintelligible).
UNIDENTIFIED MAN: Yeah.
JOYCE: Their destination is down a steep slope, through piles of pine needles and rotting tree limbs.
STEPHENSON: Yellow jacket nests down here, so watch out.
JOYCE: Looming above is Odin, a green, thriving giant sequoia. The top is 250 feet up. Its base is as wide as a city street. Odin was a sapling when Rome was still an empire, and for some reason, it's resisting this drought. These scientists want to find out why.
CAMERON WILLIAMS: My name is Cameron Williams. I consider myself a forest canopy biologist.
JOYCE: Williams buckles on a climbing harness festooned with clips, carabiners and an ascender - a clamp attached to the rope he's now uncoiling that he will use to climb up into the canopy. I put a small microphone on his shirt, and he practices his emergency communication drill.
(SOUNDBITE OF SCREAMING)
WILLIAMS: Williams can joke because he's done this so many times. He and his climbing partner, Rikke Naesborg, have spent hundreds of hours up in this tree, measuring it like a patient etherized on a table.
RIKKE NAESBORG: Every single branch.
JOYCE: Taking notes while you're hanging from a rope?
NAESBORG: We are taking notes while we're hanging on rope, yes (laughter). Well, you get used to it.
WILLIAMS: It's very, very laborious.
JOYCE: They measure to check growth rate and how much moisture is in the branches and the needles and cones. Todd Dawson is running the show here. He's a plant ecologist from the University of California, Berkeley. Dawson says what's happening to these forests is not normal.
TODD DAWSON: There's a lot more dead trees in this forest than I've ever seen since we've been working here - since 2008.
JOYCE: Dawson is like an epidemiologist, studying disease in a large population - in this case, trees. But there are way too many trees here to climb each one, so while his climbers take Odin's measurements, Dawson is going to try something else. He's going to fly a drone around the giant sequoia, carefully avoiding branches on every side, and essentially do a botanical X-ray of it.
DAWSON: This is the first time for all of us, so we're taking our time and trying to be very cautious.
JOYCE: Tom Jennings is a drone jockey with a company called CloudD8TA.
TOM JENNINGS: We're dealing with the canopy, and that's a new hazard that I'm not used to.
JOYCE: The drones will fly to the top of the tree and then down around it, taking images.
JENNINGS: Go ahead, guys. We just figured we all should get started.
JOYCE: What the team hopes to do is compare what the climbers see with what the drones reveal. If drones can diagnose a tree as well as a climber can, they could cover a whole forest much faster.
(SOUNDBITE OF DRONE BUZZING)
JOYCE: The black drone rises from the ground. It's about three feet square and looks like something Darth Vader would have on his desk. It's carrying some very sophisticated cameras.
DAWSON: It records basically the reflected light off the canopy, and that reflected light gives us the health of the crown itself - water content and other chemicals in there, like chlorophyll content, which is directly related to photosynthesis, and nitrogen content.
JOYCE: All things that reflect how stressed the tree is.
(SOUNDBITE OF DRONE BUZZING)
JOYCE: The drone slowly descends from the treetop. Meanwhile, Cameron Williams pulls himself up a tree on a rope the size of his pinky finger.
(SOUNDBITE OF RUSTLING AND CLICKING)
WILLIAMS: Well, we just reached 160-ish feet above the ground. Looks like a long ways down there. And I can hear a drone overhead. Sounds like a giant bees' nest (laughter). Looking around at the landscape, you can really see a lot of dead trees. Wow. There's hundreds - potentially thousands - of dead trees that I can see in this one view.
JOYCE: These diagnoses are vital to understanding the drought. Already, Dawson has found that forests at low and mid-elevations - pine, fir, cedar - are getting slammed the hardest. They can choke to death. Heat and drought cause air bubbles to form in their trunks and block their flow of water. Or they die when beetles detect the weak ones and single them out like predators on the Serengeti. But the giant sequoias, like Odin, are different.
DAWSON: What is giant sequoia doing? Why is it able to be so resilient?
JOYCE: It could be that sequoias grow where there is more groundwater, or maybe it's the way they shed needles when they're stressed. Understanding how different species of trees respond will help scientists focus their rescue efforts. There are things that can be done. Forest ecologists like Nate Stephenson say, for example, you can thin out small trees and underbrush. That means fewer straws sucking water out of the ground.
STEPHENSON: That gives more water, more light, more nutrients for the biggest trees on the landscape, so they're better able to survive stresses in the future.
(SOUNDBITE OF RUSTLING)
JOYCE: The drought is a disaster, but also a huge natural experiment. And ecologists like Cameron Williams, still up in Odin's branches, say trees like this one could tell them how to help these forests survive.
WILLIAMS: Looks like this tree has been here for a long time and endured a lot of the elements that the environment has in store for it.
JOYCE: And that's exactly why these scientists think that Odin, going strong despite the drought, will help them save the rest of the forest. Christopher Joyce, NPR News.
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