Giant sequoias in the Sierra Nevada range can grow to be 250 feet tall — or more. John Buie/Flickr

Thanks to National Public Radio (USA):

How Is A 1,600-Year-Old Tree Weathering California’s Drought?

CHRISTOPHER JOYCE

It’s been a brutal forest fire season in California. But there’s actually a greater threat to California’s trees — the state’s record-setting drought. The lack of water has killed at least 60 million trees in the past four years.

Scientists are struggling to understand which trees are most vulnerable to drought and how to keep the survivors alive. To that end, they’re sending human climbers and flying drones into the treetops, in a novel biological experiment.

From a distance, the forests of the Sierra Nevada look blotchy, with patches of dead trees standing right next to healthy green ones.

Nate Stephenson, an ecologist with the U.S. Geological Survey, says the drought and high heat combine to do things he hasn’t seen before. “We don’t really understand a lot of things,” he says, “like exactly how a drought kills a tree, or what’s going on underground. Where is the water flowing in areas we can’t see?”

Stephenson and his team of ecologists pull into a designated spot in the mountains, near Sequoia National Park, with truckloads of equipment they’ll have to carry in. Their mission: to find out what separates the surviving trees from the dead. Their ultimate destination is down a steep slope — through a blanket of pine needles, rotting tree limbs and a few yellow-jacket nests they are careful not to walk on.

The walk is worth it. Looming above is Odin, a green and thriving giant sequoia that’s more than 1,600 years old. 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 and many other sequoias are resisting this drought.

In hopes of finding clues to Odin’s survival, Cameron Williams, a researcher with the University of California, Berkeley, is heading up to the top.

“I consider myself a forest canopy biologist,” Williams says as he buckles on a climbing harness festooned with clips, carabiners, and an ascender — a kind of a clamp attached to the rope he will use to climb up.

He practices his emergency communication system: “Aaaaaaaaahhhhhhhhhhhhhhh,” he yells, then laughs. Williams can joke because he’s done this so many times. He and his climbing partner, Rikke Naesborg, also a research scientist at UC Berkeley, have spent hundreds of hours up in this tree. They take meticulous measurements, limb by limb, as though studying a patient etherized on a table. “Every single branch,” says Naesborg.

She takes notes dangling from the rope. “You get used to it,” she says. They check the tree’s growth rate, and how much moisture is in each branch and in the needles and cones.

“It’s very, very laborious,” Williams says.

Todd Dawson, the plant ecologist from UC Berkeley who is in charge of the expedition, says what’s happening to these forests is shocking and abnormal.

“There are a lot more dead trees in this forest than I’ve ever seen since we’ve been working here — since 2008,” Dawson says…

Read the whole article here.