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To Measure Drought's Reach, Researchers Scale The Mighty Sequoia

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<strong></strong>UC Berkeley tree biologist Wendy Baxter is about to begin her ascent of a giant sequoia.<strong></strong>

The giant sequoias in the Sierra Nevada are one of America's treasures, but for the first time in Sequoia National Park's history, the trees are showing visible signs of exhaustion due to the drought.

On a hike last summer, a scientist noticed that the leaves of the giant sequoias were browning and more sparse than usual. This finding got ecologists thinking: Did the drought cause this?

"We're just trying to get a better understanding of how giant sequoia trees respond to severe drought. We have very little understanding of ... how severe of a drought it takes to kill a giant sequoia tree," says Anthony Ambrose, a tree biologist at University of California, Berkeley.

This notion that the giant sequoias could die because of drought has brought together multiple agencies, including the National Park Service, Stanford University, the U.S. Forest Service, the U.S. Geological Survey and the Carnegie Airborne Observatory, for the first health-related study on the giant sequoia. Ambrose is part of a team working on — and climbing up — trees in Sequoia National Park.

Some of the sequoias in the park are over 3,000 years old and have faced many droughts in their lifespans. But perhaps this drought is too much for them.

"The good news is that there were lots of trees that still seem healthy, but there was this smaller amount that seemed to be stressed — and stressed in ways that we haven't seen documented before in the parks," says the study's lead scientist, Koren Nydick, with Sequoia National Park.

More than 40 trees are in the process of being analyzed for stress from four years of drought and warming temperatures. "That's the kind of stress that eventually could kill a tree," Nydick says.

Researchers want to compare data from healthy sequoias with those showing signs of decay.

Wendy Baxter is another team member in the park who is trying to understand how these sequoias respond to drought conditions.

"We're going to assess the water status of those samples that we collect. So that's sort of an instantaneous measurement of the water status of the tree at that point in time," Baxter says.

This is how it works: After a researcher loops a rope over a branch hundreds of feet high, tree biologists like Baxter attach themselves to the line and literally hoist themselves into the branches. Scientists set up rainfall sensors in the branches themselves, and at the end of August, they'll return and take clippings from different heights for testing in a pressurized chamber to measure water content in the leaves.

Researchers will also study the sprigs of foliage in the laboratory. The goal of this study is twofold. Scientists like Ambrose want to figure out how the drought is harming these historic trees, and he says they want to be able to fly over the forest, look at the color of the leaves and understand how stressed these areas are.

"Being able to relate the measurements that we get on the ground to the airborne data, assuming we get a nice relationship, then they'll just be able to fly periodically over the whole forest and get, like, a spatial map of tree water status or tree stress levels," he says.

But for now the researchers are rushing to collect data in case a strong El Niño brings a large amount of rain and snow this fall.

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