America’s tallest, most spectacular trees are getting...heat stroke?

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They’re home to birds, salamanders, and even other trees. Rising temperatures are putting all of them at risk.

A grove of coast redwood trees is a forest of forests. Individual trees can live for thousands of years, and as they grow, their canopies develop mats of soils and ferns, creating something like a second forest floor hundreds of feet above the ground.

These elevated ecosystems are home to creatures like the wandering salamander, which can spend its entire life cycle on top of a single tree, and the marbled murrelet, a seabird whose nesting habits were a mystery until one day in 1974, when a tree-trimmer stumbled upon a nest 150 feet above the ground. Sometimes a stray seed — a sitka spruce or oak or even another redwood — finds its way up a massive redwood and grows into another tree entirely, nestled among the branches of a giant.

For millennia, redwoods have been the anchors of this incredibly stable ecosystem; they can even survive wildfires that devastate other tree species. They’ve also been the source of plenty of human inspiration and drive millions of dollars to California’s tourist economy. But like other living things, they aren’t immune to warming temperatures, and it turns out these trees have a particular weak spot.

A new study from researchers at University of California Davis, using measurements taken twice a month for two years, found that coast redwood leaves essentially dial down their photosynthesis during periods of extreme heat. This means trees are slowing their “breathing” and “eating” because they can’t cool down fast enough when temperatures rise. It’s a problem that’s only going to get worse as climate change continues to drive global temperatures up, and it raises urgent questions about the future of these iconic trees and the layers of life that live on and around their shadow-dappled lengt

To understand what’s happening to the redwoods — and what that may mean for the species that rely on them — first we need to take a quick foray into tree anatomy. Trees use the pores in their leaves, stomata, to do two things: take carbon dioxide in, and let water out. Open stomata mean the tree is “breathing” in carbon dioxide and photosynthesizing, but probably also letting water out. Closed stomata mean the tree is conserving water but also slowing down its photosynthesis, since it isn’t taking in any carbon dioxide.

“Because leaves are a physical, light-absorbing object, they heat up just like asphalt would in the sun,” said Lily Klinek, a PhD student at UC Davis who was the lead author on the study.

“They’re heating up faster than the air around them,” she said. “And one way that they can cope with this is by opening their stomata and allowing water to transpire out of the leaf, which is kind of like the way that humans sweat. When water evaporates off our skin, it cools us down, and it’s the same for leaves. But they can only really do that if they have enough water to lose.”

Klinek and her colleagues studied coast redwoods in Mendocino County, near the southern part of their range, where the researchers used specialized instruments to measure whether or not individual leaves were photosynthesizing. During heat waves, they found, coast redwoods hit a point where they can’t cool down fast enough, and they just keep their stomata closed. While that saves water, it also means the tree isn’t taking in more carbon — which means it’s not “eating” or building up energy stores it can use to keep growing.

This may sound like a technicality of tree physiology. But it could have a big impact on these forests, creating a vicious cycle where trees don’t have enough carbon stored to replenish their leaves in the future, which means they won’t be able to photosynthesize as efficiently, which will starve them even further, repeating until trees eventually die. And while this sort of behavior has been observed in tropical trees, it’s the first time anyone has studied how coast redwoods respond to the same heat stress.