Water & Drought

Sierra Nevada giant sequoias respond to water stress with clever adaptations

The giant sequoia canopy.
The giant sequoia canopy.

The leaves atop giant sequoias in the Sierra Nevada are better at storing water than those closer to the ground, an adaptation that may explain how their treetops are able to survive 300 feet in the air, researchers at American River College and Humboldt State University have found.

“It can take over a week for water to get from the ground to the top of the tree,” says Alana Chin, who led the study and is an instructor at American River College. “When you’re that tall of a tree, you’re under tremendous water stress.”

Chin and her colleagues used ropes to climb the trees and collect samples. They found that leaves higher up the tree have less surface area and more transfusion tissue compared to leaves near the trunk. These traits give leaves more succulence, or ability to store water, which serves as a buffer against a water deficit that would otherwise damage the shoot system.

The findings were published in the American Journal of Botany in May.

In the laboratory, the researchers also simulated acute drought conditions by submerging shoots in wax. Results from the experiment showed samples collected from the upper crown compared to the lower crown had less shoot damage when water was restored. The crown consists of branches and leaves that comprise roughly the top half of giant sequoias.

Giant sequoias in the Sierra Nevada have demonstrated other adaptations, such as foliage die-back, in response to prolonged water stress such as the recent drought conditions.

Giant sequoias have survived California’s severe drought conditions better than most other tree species. Although an estimated 66 million trees have died in the Sierra Nevada since 2010, giant sequoias were only a handful among that total. To stay hydrated in the summer, these trees rely on a healthy snowpack which may be threatened by climate change.

Nate Stephenson, a forest ecologist with the U.S. Geological Survey, regularly visits trees in the Sierra Nevada as part of a long-term forest monitoring program. Giant sequoias typically carry 2 billion leaves on each tree, but in recent years, he and his colleagues have witnessed unprecedented numbers of leaves shed, which is believed to be driven by the need to conserve water. Interestingly, trees shedding large amounts of foliage have leaves with similar water content as leaves from trees with more typical foliage.

“Even though they lost half their leaves, their remaining leaves are happy,” Stephenson said.

Leaves contain pores that allow the entry of carbon dioxide for use in photosynthesis. But pores also represent a potential source of water loss. Thus, shedding leaves can reduce water leaving the tree. There was no pattern apparent in leaf shedding, which was widely distributed across the crown of giant sequoias, according to Todd Dawson, a UC Berkeley professor who worked with Stephenson to survey the trees.

Dawson and his team are hopeful as they gear up for an upcoming survey of the giant sequoias this August. With a better rainfall last winter, they hope to see signs of improved tree health. The survey is expected to reveal how quickly giant sequoias respond to climate change.

“Are they regrowing leaves that they lost in the 2014 and 2015 summers?” Dawson wants to know. “Or will it take some years to make a comeback?”