The Tree Whisperer

A Berkeley scientist is studying how climate change is affecting California’s giant sequoias, long considered dependable forest stalwarts.

Photo courtesy of Anthony Ambrose

For Anthony Ambrose, a University of California, Berkeley, tree biologist, fieldwork means getting up-close and personal with some of the world’s largest living things. In the wee hours of the morning, his team of intrepid researchers hike out to groves of giant sequoias, where they strap on climbing gear and scale the trunks to reach the upper branches before the sun comes up. Sometimes they even bed down in tree hammocks for the night. But Ambrose is far from just a tree-hugger. He’s a skilled diagnostician, documenting the trees’ health and future prospects as the earth’s climate continues to warm.

In the California mountain forests where they grow, giant sequoias have long been regarded as dependable stalwarts, surviving fires, earthquakes, and periods of widely fluctuating temperature. But although they can live more than 3,000 years, the evergreen behemoths aren’t invulnerable. Their survival is intricately tied to water that accumulates as snow high in the Sierra Nevada mountain range. As temperatures have climbed, this critical water source has shrunk, making it harder for the trees to get the hundreds of gallons they need each day.

In 2014, the third year of California’s most recent drought, Ambrose’s colleague Nate Stephenson of the U.S. Geological Survey noticed huge swaths of brown foliage as he walked through the groves at Sequoia National Park. Many dried-out branches were also falling off. “That was very unusual, and it hadn’t been observed before,” Ambrose says. “We really wanted to understand what was going on with the trees and how severe [the condition] was.” Through his ventures into the sequoia canopies, Ambrose hopes to quantify the effects of drought on the trees. The nighttime excursions provide him with a baseline water status on the trees—“a kind of resting heart rate equivalent,” as he puts it.

“It’s important to be looking at how climate change is going to affect this species,” says NRDC senior scientist Peter Miller. “The trees are dependent on the environmental conditions in which they’ve evolved, and those conditions are changing rapidly and dramatically.” The health of the environment is also dependent on the trees, of course, with these towering groves soaking up huge amounts of planet-warming carbon dioxide from the atmosphere.

The canopy of Sequoia National Park

Courtesy Anthony Ambrose

California’s landscape would be forever altered if these seemingly ageless forests were to disappear. “These 2,000- to 3,000-year-old monarch trees—each one is unique, reflecting their unique history,” Ambrose says. “If we lose them in their native groves, it’s a loss for biodiversity.” Sequoias provide habitats for a range of wildlife, including at least six species of bats, white-headed woodpeckers, and various salamanders. Douglas squirrels—which naturalist John Muir described in his book The Mountains of California (1894) as “without exception, the wildest animal I ever saw—a fiery, sputtering little bolt of life”—feast on scales from the trees’ cones.

At the start of a typical research day, Ambrose uses his crossbow to shoot a blunt-tipped arrow into the top branches of each tree under study. With fishing line tied to the arrow, he and his team pull a climbing rope up onto a branch more than 200 feet in the air. Once this “access line” is established, the researchers can ascend hundreds of feet to take cuttings from the tops of the trees, where dark-green foliage sprouts in dense clusters. “When you get up into their crowns,” Ambrose says, “you get a different perspective on how big and gnarly and complex they really are.”

Back on the ground, the scientists put the cuttings inside a pressure chamber, then monitor them carefully as they pump the chamber up with nitrogen gas. The pressure reading when water starts seeping out of the stem equals the pressure the water was under when the sample was cut. This measurement indicates the level of tension within the tree’s water column, which rises as trees become more drought-stressed. If the tension gets too high, “tiny little air bubbles form [in the vessels], and it blocks water movement,” Ambrose says. “It can basically kill the tree.”

Ambrose cutting leaves from the top of a giant sequoia in Sequoia National Park

Lincoln Else/KQED

Back at the lab, the researchers examine the cuttings for other signs of drought-related stress. It’s common, Ambrose says, to see sequoias closing tiny pores, known as stomata, on their needles. Though this is an effective strategy to conserve water in the short term, it also impairs the tree’s ability to perform photosynthesis and store carbon over time. And in some tree cuttings, Ambrose has observed those deadly air bubbles developing within the vessels. “When things get tough, the giant sequoias are able to withstand it for a period of time. They kind of hunker down,” Ambrose says. “But at some point, they may reach some threshold where they may not be able to regrow their foliage. The seedlings may not be able to survive.” What’s more, if drought conditions continue to parch or kill the giant trees, they could become vulnerable to severe wildfires that damage whole groves beyond recovery.

Assuming climate change continues as predicted, what’s the most reasonable projection of what could happen to the trees over the next century or so? “My personal opinion is it’s not looking good,” Ambrose says. “The trees were more stressed than we’d ever measured them before. As temperatures continue to increase, the drying power of the atmosphere is increasing; the snowpack’s melting faster.”

But Ambrose isn’t giving up on the earth’s most massive tree. He’s collaborating with the National Park Service (NPS) and the Carnegie Airborne Observatory to develop detailed maps that will reveal patterns of drought vulnerability within the sequoia groves. In areas hardest hit, other species like white fir could be thinned with controlled burning techniques to maximize the amount of water the giant sequoias can access. Ambrose says the NPS may also consider a targeted irrigation program, should things grow dire. With measures like these, he says, “you can hopefully increase the ability of the trees to [withstand] future droughts—give them that extra little benefit.” He cautions, though, that he’s not sure if these approaches will work in the long term. “If we get eight degrees of warming”—a rise climate experts predict could happen by the century’s end if we continue business as usual—“all bets are off.”

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