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California’s water-starved sequoias show signs of stress

October 14, 2015 at 6:25 PM EDT
California's giant sequoias are special. These massive trees stretch hundreds of feet and live for thousands of years. But they are also being stressed: requiring an enormous amount of water, they are showing signs of suffering through the state's historic drought. Special correspondent Scott Shafer and producer Gabriela Quirós of KQED report.

JUDY WOODRUFF: California’s historic drought has created a long list of problems for the Golden State, including killing millions of trees in the Sierra Nevada Mountain Range.

Now even the iconic giant sequoias, which can live thousands of years, are starting to show signs that they’re not getting enough water.

Our colleagues at public TV station KQED in San Francisco joined a team of biologists from the University of California, Berkeley, as they climbed 50 of these giant trees this summer in Sequoia National Park.

The story was produced by Gabriela Quiros and narrated by Scott Shafer.

SCOTT SHAFER: Anthony Ambrose is climbing this giant sequoia to find out how it’s faring after four years of drought.

ANTHONY AMBROSE, University of California, Berkeley: We are at about 240 feet at the top of a giant sequoia tree.

SCOTT SHAFER: These leaves will tell him how stressed out the tree is due to the lack of water.

ANTHONY AMBROSE: You need to measure them kind of at the most relaxed time of the day, before the sun rises, before they start to lose water to the atmosphere. They require an enormous amount of water, way more than any other tree that has ever been documented.

SCOTT SHAFER: His colleague Wendy Baxter is near the top of another giant sequoia.

WENDY BAXTER, University of California, Berkeley: Giant sequoias are just such special trees. They have been able to persist and live in this exact place maybe for thousands of years. Some of them live to be 3,000 years old.

SCOTT SHAFER: Over the course of their long lives, sequoias can grow as tall as a 30-story building.

WENDY BAXTER: Well, there’s a beautiful view up here.

SCOTT SHAFER: Even at these great heights, leaves contain water.

WENDY BAXTER: There is higher concentrations of water in the soil than in the air. So, that gradient is actually pulling the water up through the tree.

SCOTT SHAFER: Inside each of the trees’ cells, water gets pulled up to the top as if it were being sucked up through a straw. Researchers can determine how much tension the water is under as it travels upward and into each leaf.

WENDY BAXTER: When we clip it, the water retracts back into the stem, kind of like a rubber band.

SCOTT SHAFER: Researcher Ken Schwab places the leaves in a pressure chamber.

WENDY BAXTER: When we put our stem into the pressure chamber, the amount of pressure that it takes to force the water back out is an indication of how much tension it was under.

MAN: And I’m beginning to see darkening and water.

SCOTT SHAFER: The higher the pressure required to push the water out, the more stressed the tree is.

MAN: The trees are definitely as stressed as we have ever measured giant sequoia. We have been measuring giant sequoia water status periodically over the last few decades, under non-drought conditions, and most of the trees seem to be kind of at that level or exceeding it.

SCOTT SHAFER: Trees move water from soil to the atmosphere, which helps create rain and snow. When little water is available, the pull of the dry atmosphere breaks the water column. Air in the water forms bubbles, which prevent water from moving up. If bubbles stop water flow in cell after cell, the tree dies.

Biologist Nathan Stephenson says sequoias’ main water source is snow.

NATHAN STEPHENSON, U.S. Geological Survey: So the last two winters here have been by far the warmest on record, and what that’s meant is there’s been almost no snow on the ground.

SCOTT SHAFER: The weak snowpack has led to new signs that the giant trees are under stress.

NATHAN STEPHENSON: I looked up, and I saw a big, mature, giant sequoia, and its foliage was turning brown. At least half of its foliage had gone brown. No one has ever reported that before.

SCOTT SHAFER: While only a handful of the park’s sequoias have recently perished, the Forest Service says the drought is taking a toll on more than six million trees of other species in the Sierra Nevada Mountain Range.

NATHAN STEPHENSON: We’re seeing firs, pines, incense cedars and oaks are all dying at a rate we have never seen before. Even during the 1977 drought in California, we didn’t see this many trees dying.

SCOTT SHAFER: In Sequoia National Park, Koren Nydick says the giant trees are causing concerns.

KOREN NYDICK, Sequoia & Kings Canyon National Park: A lot of our sequoias are still appearing healthy, still doing well during the drought. But there are some that are showing symptoms, and we want to learn more about that and be able to track that stress.

SCOTT SHAFER: To help researchers study the effects of the drought on the tens of thousands of giant sequoias in the Sierra Nevada, scientists aboard the Carnegie Airborne Observatory use instruments to measure trees’ water content. Blue trees are getting the most water, followed by yellow, orange, and red trees, which are getting the least.

The park has several options for helping the most vulnerable giant sequoias. Burning part of the forest would reduce the number of overall trees, and since sequoias are resistant to fire, more would survive.

KOREN NYDICK: There’s less competition for the larger trees that remain behind. So the larger trees have more access to water and nutrients, and helps them get through the drought.

SCOTT SHAFER: If the trees’ health continued to deteriorate, Nathan Stephenson of the Geological Survey says humans may need to intervene more directly and provide trees with drip irrigation.

NATHAN STEPHENSON: If humans continue to warm the climate by adding greenhouse gases to it, we might have to consider some unnatural actions.

SCOTT SHAFER: For the PBS NewsHour, I’m Scott Shafer.