High and Dry?

Meet scientists studying how California’s drought is affecting the tallest trees in the world

March 13, 2017

By Greg Uyeno

Locked into a harness and suspended by ropes, Anthony Ambrose spends all day climbing a tree that reaches more than 300 feet into the sky. The tree he’s scaling is a sequoia, one of the tallest trees in the world.

Ambrose loves the sweeping views, but he’s mainly climbing these tall trees for science. He’s a biologist at the University of California, Berkeley, studying the state’s giant sequoias and coast redwoods. They are two of the tallest, largest, and longest-living tree species in the world. Ambrose is one of the many scientists trying to figure out how these giant trees have been handling California’s historic drought.

“It’s an amazing opportunity to learn about an incredible organism,” says Ambrose, “as well as a beautiful place to work.”

THIRSTY TREES

Found only in the mountains of central California, sequoias can grow taller than 300 feet and live for 3,000 years. Because they’re so big, sequoias need huge amounts of water, says Ambrose. A single tree can guzzle up to 800 gallons a day. That’s more than you drink in four years!

But California has suffered a devastating drought for the past several years. The area where sequoias live typically receives 40 to 45 inches of rain and snow each year. In recent years, many sequoia groves have received less than 22 inches. Although this winter’s record-breaking rain and snow has helped, the drought isn’t over yet.

More than 100 million trees, including redwoods, sequoias, and other species, have died in California since the drought began in 2010. Since it takes more than 500 years for sequoias and redwoods to reach their full height, growing new trees is a slow process.

UP-CLOSE STUDIES

High up in the treetops, conditions are different from lower down. There’s more wind and brighter sunlight, and the water gathered by the trees’ roots must travel that much farther to nourish the plants. As he’s climbing, Ambrose takes samples by snipping a few leaves to test the amount of water and carbohydrates (compounds made during photosynthesis to feed the tree) in them. He takes samples from different heights throughout the canopy to repeat the test.

The information Ambrose and his team gather tree-by-tree could help explain some unusual behavior scientists have noticed since the drought began. What concerns them the most is that redwoods are dropping many of their leaves. Since redwoods are evergreen, they do not normally lose their leaves with the changing seasons.

FOREST PROTECTORS

SOURCES: SAVE THE REDWOODS LEAGUE; TODD DAWSON, UNIVERSITY OF CALIFORNIA, BERKELEY

One of the main goals of this research is to better understand the biology of these giant trees and how they respond to the stress of drought. “This drought is probably the most severe drought we’ve had in the last couple of centuries and possibly a lot longer,” says Adrian Das, an ecologist with the U.S. Geological Survey who’s also studying California’s tall trees.

Das’s team is looking at the forest as a whole—as opposed to specific trees at the “leaf level” as Ambrose is doing. Part of his work involves counting how many trees are alive or dead in different areas. Those forest surveys can be compared with all of the measurements taken over the past 30 years.

Lack of water isn’t the entire story. The rate of tree death, or mortality, has also been tied to bark beetle infestations and changing climate. “We’re learning more and more about the [different] processes that drive mortality,” says Das.

Although the public is interested mostly in redwoods and sequoias, this research is part of a larger effort to understand California’s forests as a whole. The lessons learned may apply to other important California tree species, like the sugar and ponderosa pines, the white fir, and the incense cedar. “Sequoias are unique trees in a lot of ways, but they are trees,” says Das.

Knowing more about the basic biology and mortality could help save California’s trees. Pinpointing which areas have been hurt most by the drought—and how—will help the researchers create a “vulnerability map.” Forestry managers can use this map to decide how to support their forests in challenging times. “We can use what we’ve learned at smaller scales to inform what we’re seeing on the whole landscape,” says Das.

Although both the future of the drought and the fate of the tall trees remains uncertain, Ambrose is optimistic. “Redwoods and sequoias are doing fairly well compared to other trees on the landscape,” he says. “They’re tough.”

FINDING MEAN ABSOLUTE DEVIATION

Researchers are collecting all sorts of data on California’s giant trees. One way to analyze the data is to determine the variability in the data. One measure of the variability, or spread of a data set, is the mean absolute deviation (MAD). This is the average difference from the mean of all the numbers in the data set.

The tables below show the percent of dead trees in research plots in two California parks. Use the information to answer the questions that follow. Round all answers to the nearest tenth.