Giants in the Face of Drought

Published on November 29th, 2016

As millions of California’s trees die, what will become of the largest tree there is, the giant sequoia?

Giants in the Face of Drought

Thayer Walker
November 27, 2016
The Atlantic


By the time John Muir and his trusty mule Brownie splashed across the Marble Fork of the Kaweah River in the fall of 1875, the Scottish-born naturalist had already seen his fair share of California grandiosity: Yosemite Valley; the high Sierra; Mariposa Grove. Muir had a thirst for exploration and a talent for storytelling. He founded the Sierra Club and dubbed its synonymous mountains the “Range of Light.” When Muir sauntered upon a montane plateau in what is now known as Sequoia National Park on that autumn day, he found a very large stand of very large trees. Drawing his poetry from the obvious he named it, quite simply, the Giant Forest.

The dominant feature of the Giant Forest is the giant sequoia (Sequoiadendron giganteum), the biggest tree on Earth. Thousands of them grow in this 2,300-acre grove, including five of the ten largest specimens in the world. They reach heights of nearly 300 feet; their trunks can span more than 30 feet; and they’re nearly impossible to miss if you’re tromping beneath their canopy. “In every direction Sequoia ruled the woods…” Muir waxed in Our National Parks, “a magnificent growth of giants grouped in pure temple groves.” And yet, at 4:00 a.m. on a warm August morning, our hearty group of scientists and climbers is having a tough time finding the damn things.

“I feel like we’ve gone too far,” says forest ecologist Wendy Baxter, 36, stopping the group. The ivory glow of a full moon offers enough illumination to hike without fear of face-planting, but it makes for a poor navigational beacon.

It’s the fourth day of two weeks of fieldwork led by Baxter and fellow forest ecologist Anthony Ambrose. Scientists at UC Berkeley’s Dawson Research Lab, the two are part of Leaf to Landscape, a program in collaboration with the United States Geological Survey, the National Park Service, and the Carnegie Airborne Observatory, that is focused on studying and managing the health of the giant sequoias.

California, of course, is in the middle of a historically punishing drought at a time when there’s never been more demand for water. According to the United States Forest Service, 62 million trees have died in California this year alone. Since 2011, a total of 102 million trees have perished, with tens of millions more on death's doorstep. California’s forests generate fundamental ecosystem services by creating healthy watersheds, providing wildlife habitat, and sequestering atmospheric carbon, and they’re dying at unprecedented rates. Even the great giant sequoias are showing concerning signs of stress. It’s Ambrose and Baxter’s goal to collect and analyze tree samples to understand how the sequoias are faring under these rapidly changing conditions, and what might be done to protect them. But first we have to find them.

“Have we come to any intersections at all?” asks Ambrose, 48, whose attention has been focused on answering my questions instead of spotting landmarks.

“I remember this tree, for sure,” someone chirps, a sentiment that seems more appropriate as an epitaph on a lost hiker’s headstone than as a vote of directional confidence. After a brief parley, we correct our course up a gentle rise, down into a shallow basin, and past a pair of landmarks, unmistakable even at this dark hour.

The path splits twin sylvan towers standing inches apart and hundreds of feet tall. It’s still too dark to marvel at their height, but the base of each tree inspires awe enough, gnarled and bulbous and swelling with woody knuckles the size of a Toyota Prius. A few hundred yards farther, the trail continues through the hollowed-out center of another sequoia. Fire, the great creator and destroyer, Kali of the Giant Forest, raged here long ago, burning out the tree’s core. The wound is enormous, 40 feet tall or more and nearly the size of the tree’s entire 12-foot diameter. Yet the grand monarch survived the blaze, which also would have cooked off the thick layers of duff that choke seedling growth, offering tiny sequoias a chance to one day touch the sky and survive their own infernos.

The group splits apart at the meadow, each climber heading to the tree they’ll be sampling. The scientists have targeted 50 sequoias for study—“the biggest, gnarliest trees in the forest,” Ambrose says—and this morning he’ll climb a 241-footer. In most other forests a tree like this would be a star attraction with an honorific name and perhaps even a viewing area. Here, it’s simply known as “tree 271.”

Ambrose has striking blue eyes and wears a woodsman’s beard with a chinstrap of white whiskers. He slides on his climbing harness and tugs the rope anchored to the crown some 24 stories above. He’s been studying trees for more than two decades, first with a focus on coast redwoods (Sequoia sempervirens) as an undergraduate and master’s student at Humboldt State University, and then on giant sequoias for his doctoral and post-doc work at Berkeley. “From an aesthetic perspective to a biological one, these trees are some of the most spectacular organisms on the planet,” he says with the enthusiasm of a boxing promoter. “They are the pinnacle of what a plant can become. They force you to think about life and your own place in it.”

He clips on a pair of jumar ascenders—mechanical devices that attach to the rope and allow him to pull himself up. “You can’t really understand the true character of a tree from the ground,” he says. Ambrose turns off his headlamp, cranes his head toward the canopy, and begins the long, dark climb into a world of mystery.

The giant sequoia has dominated its landscape for millions of years and captivated global imagination since the mid-19th century when rumors of trees the size of fairy-tale beanstalks came roaring out of the Sierras. One of four redwood species, the giant sequoia is not the world’s tallest tree; that crown belongs to its northern cousin, the coast redwood. But in terms of sheer volume of biomass, no living organism ever to walk, swim, fly, or stand on this planet comes close. They are of such stature that people struggle to describe them and so compare them to other very big things: blue whales, 747s, dinosaurs, the Statue of Liberty, elephant herds, space shuttles. Giant sequoias make mice of them all.

More than 100 million years ago, when the planet was warmer and wetter, the sequoia’s earliest relatives thrived across much of the Northern Hemisphere. Redwood fossils have been found everywhere from Northern Mexico and the Canadian Arctic to England. During the late Miocene, some 10 to 20 million years ago, the closest direct ancestor of the giant sequoia lived in what is now southern Idaho and western Nevada. As the Sierra Nevada Mountain Range continued its uplift and the climate became drier, the giants’ range shrank. Today, the last remaining sequoias are limited to 75 groves scattered along a narrow belt of the western Sierra Nevada, some 15 miles wide by 250 miles long.

Giant sequoias are among the longest-living organisms on Earth. Though no one knows the trees’ absolute expiry date, the oldest ever recorded is 3,200 years old. Muir claimed to have found a stump with 4,000 tree rings, one per year. During their early years the trees are subject to predation and the volatile whims of nature. Once they reach adolescence after a few centuries, however, sequoias become well-nigh indestructible. Their bark is soft and fibrous and contains very little pitch, qualities that make the trees extremely resistant to fire. The tannins that give their wood a rich cinnamon hue also repel insects and fungi.

When a mature sequoia does die, mortality is usually a function of its marvelous size. Root rot can deprive a tree of a solid anchor and fire can undermine its base, but rarely will either actually kill a 30-story monarch. Gravity is the ultimate culprit, for a giant sequoia with an uncertain foundation faces a violent and certain end. The persistent tug of gravity can pull an unbalanced tree to the forest floor with such a thunderous crash that the reverberation can be heard miles away. The sequoia’s fate is an Icarian allegory, met not by flying too close to the sun, but by stretching too far from its roots.

Thanks largely to their ability to withstand disease and drought, it’s extremely rare for a giant sequoia to die standing upright. “You don’t get to be 2,000 years old without surviving a few dry spells,” Ambrose tells me. Which is exactly why United States Geological Survey forest ecologist Nate Stephenson was so alarmed when, in September 2014, he went for a walk in the Giant Forest and saw something unexpected.

“I had been saying with confidence for decades that if you hit a big drought, the first signs of climatic changes would show up in seedlings,” recalls Stephenson, who has studied trees in Sequoia and Kings Canyon National Parks since 1979. “I was completely wrong.”

He surveyed an area that had burned a few years prior, where seedlings had taken root. Crawling around on his hands and knees, Stephenson was surprised to see that the seedlings were rigid and full of water, their leaves a vibrant blue-green. This was the third year of drought in California, and the summer of 2014 was particularly brutal. There should be some evidence of drought stress, he thought. Sitting on the ground, he leaned back, craned his head toward the heavens to ponder the mystery, and found his answer.

Above him stood a grand old monarch. The crown of the tree was almost entirely brown, a scale of dieback he’d never seen. He searched for other trees displaying similar stress and when he found one with branches close to the ground, he touched it. The foliage crumbled off. In more than 30 years of studying these trees Stephenson had only seen two die on their feet. Five years into the current drought, he’s now seen dozens of standing dead.   

Stephenson quickly assembled a team to survey the 2014 dieback before autumn storms could blow away the evidence. The National Park Service (NPS) enlisted Ambrose and Baxter to begin their fieldwork in 2015. While the NPS and scientists working in Montana’s Glacier National Park might already be resigned to a glacier-free future as the climate changes, no one is ready to consider the possibility of Sequoia without its namesake trees.   

“HEADACHE!!!” Ambrose Yells.

His warning, the tree-climbing vernacular for plummeting deadfall, fills the forest moments before a branch whooshes passed, inches from my head. It happens so quickly, the broken limb has already hit the ground before I have a chance to move.

“And that’s why we wear helmets when we work around trees,” he explains to the small group of us standing at the base of the sequoia.

The lessons come quickly on our first day of fieldwork. We set up on a steep hillside and Baxter demonstrates how to prepare the rigging for a climb. Tall and lean with a strong jawline and a soft voice, she’s as comfortable doing stable isotope analysis in the lab as she is setting a 600-foot static line in a tree. “I love the combination of physical exertion and intellectual stimulation,” she tells me. “It’s a struggle to get to the top of the tree. You’re sweating and huffing and puffing, but that’s when you start collecting your samples and the science begins.”

In 2015, Baxter and Ambrose did much of the work themselves, identifying and rigging 50 trees, making six climbs a day, and collecting samples and measurements from each one. Their days began at 2:30 a.m. and ended at 10 p.m.—if they were lucky. “That was brutal,” Baxter recalls.

They have more help this time around. Over the course of two weeks, more than a dozen volunteers—students, professional arborists, climbing junkies—will rotate in and out. The schedule, while not nearly as frantic as the previous year, is aggressive. We wake up at 3 a.m. and begin our hike from the Crescent Meadow parking lot into the Giant Forest an hour later. After climbing trees and taking and analyzing samples all day, we head back to our campground for some R&R before collapsing into bed.

The immediate goal is to understand the severity of water stress the trees are facing, the water content in the leaves, and the amount of the stable carbon-13 (13C) isotope the tree uses during photosynthesis, which offers additional insight into how the trees are coping with drought. With that information, scientists and park officials can assess the trees’ health and begin to think about ways to protect giant sequoias through practices like controlled burns, which clear the ground for seedlings and eliminate less fire-resistant trees that compete for water.  

Ambrose’s first exposure to forest management came as a wildland firefighter following his senior year of high school in Chico, California. The experience, he recalls, involved “hours of boredom followed by long stretches of terror,” and gave him a first-hand look at how a policy of aggressive fire suppression can have an adverse effect on forest ecosystems.

For more than a century, the government’s approach toward forest fire has been one of suppression. But indiscriminately stamping out frequent, less intense, naturally occurring fires disrupts the natural process of consumption and rejuvenation that species like giant sequoias need to thrive. It also allows dangerous levels of fuels to pile up—until one explosive holocaust vaporizes everything. “You get these large landscape conversions, conifer forests turning into brush,” Ambrose says.

In 2013, the Rim Fire swept through the Sierras, consuming more than 257,000 acres. It was the third largest fire in California’s recorded history and burned for 15 months. It never reached Sequoia National Park, but it did sweep through parts of Yosemite some 100 miles north. As a precautionary measure, officials even set sprinklers around of some of Yosemite’s giant sequoias in case the fire got too close.

Giant sequoias, like all trees, play a central role in the hydrologic cycle. Storms drop rain and snow, which giant sequoias can chug to the tune of 800 gallons per day—more than any other tree. As the trees draw water out of the ground, the air surrounding the leaves draws water through the trees and, eventually, back into the atmosphere. That process, called transpiration, creates tension within the tree’s water columns. The drier the atmosphere and the less groundwater available, the higher the tension. Under extreme drought conditions, when that tension grows too high, those columns of water can snap like a rubber band. Gas bubbles form, creating an embolism that prevents the flow of water up the trunk. If this happens enough, a tree will shed its leaves and can, eventually, die.

To measure water tension and other biological processes, climbers sample each tree twice a day, once under cool pre-dawn conditions when the tree is least stressed, and once under the heat of the midday sun. The scientists clip foliage from the lower and upper canopies, which allows them to assess conditions at different parts of the tree.

After the safety talk and rigging demonstration, Ambrose grabs a laminated map from his pack and assigns the climbers to their trees. Pulling on a forest-green arborist harness, he clips a pouch onto each hip to carry his samples. Then he steps into the foot straps attached to the ascenders and begins the climb.

His arms, legs, and core work in an assembly line of movement. Hanging on the rope in a crouch, he slides his right arm up, follows with his left, pulls his knees to his chest, and stands up straight in the stirrups, at which point he repeats the routine—scores of times on his way to the top. Climbers call it “jugging,” a process as onomatopoeically laborious as it sounds.

About 100 feet up, Ambrose stops at the lower canopy, marked by the first significant limbs, which can grow up to six feet in diameter. He clips a handful of tiny branches, puts them into a plastic bag, shoves the bag into his hip pouch, and continues climbing. The tree’s leaves regulate gas exchange through tiny pores called stomata. The stomata take in carbon dioxide and release oxygen and water vapor. When a tree becomes too water-stressed it closes its stomata. This stops water loss through transpiration but also prevents the tree from absorbing atmospheric carbon dioxide and using it for photosynthesis. Sequoias have vast carbon stores to help them weather these lean times, but if the stomata stay closed for too long, the trees will eventually starve to death.

As Ambrose works in the tree, I take a short hike up to the top of a hill just above the study site, where the cost of California’s drought reveals itself in spectacular panorama. The Middle Fork of the Kaweah River plummets from the high Sierra into the agricultural empire of the San Joaquin Valley. Polished granite swells and the jagged sawtooth mountains of the Great Western Divide dominate the horizon; pine, fir, and cedar trees blanket the river basin. The colors are rich and electric, but they don’t all sit right. In a sea of green, huge islands of red metastasize across the landscape. These ochre forests are not sequoia. They are thousands and thousands and thousands of dead trees.

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