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.