The Race to Save the World’s Biggest Trees

Extreme climbers and climate scientists work hundreds of feet in the air to study the effect of drought on sequoias.

For nearly two decades, Anthony Ambrose has been scaling trees to study them, but even this seasoned climber is awestruck by his view from the top of massive sequoias in the Giant Forest of western Sierra Nevada, Calif.

The biggest trees on Earth are in this forest, including the planet's single largest tree, General Sherman, which is 2,100 years old, weighs 2.7 million pounds, measures 100 feet wide at its trunk and towers 275 feet. General Sherman and the surrounding trees are considered to be among the ultimate survivors, yet they are showing worrisome signs of climate change-related stress.

Ambrose, a scientist from Berkeley, forest ecologist colleague Wendy Baxter and their team are on a mission to save these giants, and it sometimes means climbing to the very top of them. He summarized the experience in two words: "Absolutely amazing."

"You need to get up into the crowns of these tall trees in order to appreciate how large, complex and beautiful they are," Ambrose said. "You simply can't get the same perspective from the ground. Also, you get to see the other life forms high up in the canopy that you wouldn't otherwise be able to see on the ground, such as epiphytes (non-parasitic plants that grow on other plants), birds, spiders and ants. The views are incredible, both within the tree and surrounding canopy, as well as from the treetop."

They and other scientists, such as forest ecologist Nate Stephenson of the U.S. Geological Survey (USGS), were concerned about the trees following years of drought in California.

According to a story in the California Academy of Science's magazine, bioGraphic, Stephenson one day looked at the crown of a giant sequoia in the forest a few years ago and noticed that it was almost entirely brown, a scale of dieback that he had never witnessed before. When he found another tree showing similar distress lower to the ground, he touched one if its branches. The foliage crumbled off.

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Stephenson said that in his three decades of studying these trees, he had only witnessed "two die on their feet." Now dozens appear to be "standing dead" after five years of brutal drought.

A program called Leaf to Landscape is studying the problem, bringing together UC Berkeley's Dawson Research Lab, where Ambrose and Baxter work, as well as the USGS, the National Park Service and the Carnegie Airborne Observatory.

Ambrose and Baxter are charged with collecting foliage samples, at different heights of the trees and at various times of day and night. It's a risky undertaking both for the climber and those nearby.

"Large trees are more difficult to climb, and have more hazards than small trees, such as large dead limbs, which may fall out of the tree while climbing it," Ambrose explained.

When such a limb falls from above, he screams, "Headache!" to alert his colleagues below.

The foliage samples are carefully brought down from the tree and are used to measure water pressure, which Ambrose likened to studying a person's blood pressure.

Giant sequoias can take in about 800 gallons of water a day, according to BioGraphic. As the trees pull water from the ground, the air surrounding their leaves draws water through the trees and eventually back into the atmosphere. The process, transpiration, creates tension within a given tree's water columns.

This means that the drier the atmosphere is, and the less groundwater that's available, the higher the tension will be. The water columns can even snap like a rubber band, causing gas bubbles and an embolism to form, preventing the flow of water up the tree's trunk.

The process can help the tree to preserve valuable water, but if it happens enough, the tree will continue to shed leaves and will eventually die.

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"While there has been more rainfall this season so far, it has not been enough to really replenish the soil and ground water and California is still in a drought," Ambrose explained. "Our measurements indicate that the giant sequoias were doing a little bit better in June, but were experiencing a similar level of water stress in August 2016 as we measured in August 2015."

"Every little bit of rain and snow helps, though," he said, "and if we have a good wet winter they may recover more by next year."

The granddaddy of all trees, General Sherman, is among those being monitored. The General is "showing some signs of water stress, some brown foliage, but not too bad," Ambrose said.

Another aspect of the research involves flight surveys. Greg Asner, principal investigator at the Carnegie Airborne Observatory, flies a Dornier 228 airplane containing $12 million of custom-built equipment that permits measurement of the composition, chemistry and structure of this and other forests.

The data collected by Asner and the other scientists will be used by park officials to assess The Giant Forest trees' health and to inform management decisions. The latter may include controlled burns to remove less fire-resistant trees that compete for water with other, healthier ones.

Anyone can assist in the research effort, according to Scott Loarie, co-director of the California Academy of Science's iNaturalist platform. Loarie was previously a research fellow at the Carnegie Institute for Science and has expertise in tracking global biodiversity loss, including California's trees and plants.

"Climate change is one of the biggest threats facing giant sequoias," he told Seeker. "Citizen-scientists can use the free iNaturalist app to record observations of sequoia trees to help scientists understand how their distributions are changing. You can also help by recording other organisms that coexist with giant sequoias. This helps scientists understand how impacts on giant sequoias are cascading across the diverse ecosystems that they support."