When climate change brings the heat, what happens to a mighty redwood?

Towering coast redwoods are known for their resiliency, withstanding fire, fungus and rot to survive for centuries. But these ancient trees now may be vulnerable to a relatively recent foe: heat waves.

As temperatures climb, photosynthesis slows in these iconic trees, researchers report September 22 in the Journal of Geophysical Research: Biogeosciences. Lab experiments show the rate of the sunlight-driven food making process in its needles roughly halves when temperatures rise from 30° to 40° Celsius.

Such a change usually indicates “severe levels of stress,” says Lily Klinek, an ecophysiologist at the University of California, Davis. That vulnerability can cause tree deaths that may jeopardize one of California’s largest stores of carbon, a shield against climate change.

Coast redwoods (Sequoia sempervirens) live only in a narrow strip of coastal habitat from central California to southern Oregon, where temperatures are mild and fog is abundant. But recent heat waves have spiked temperatures in parts of their range to over 40° C.

The impact of heat waves on photosynthesis has been studied in other tree species, with some faring better than others. But this is the first study to investigate how coast redwoods respond to the heat, says Troy Magney, an ecophysiologist at the University of Montana in Missoula.

From July 2022 to October 2024, Klinek, Magney and colleagues repeatedly measured the performance of needles from redwoods in California’s Mendocino County. That period included two heat waves.

Typically, trees stay cool when small porelike openings called stomata allow water to evaporate, taking heat with it, similar to how sweat cools humans. But when conditions get too hot and dry, stomata close, retaining a plant’s moisture. Under extreme heat, the team found that needles reached up to 10 degrees C hotter than the surrounding air due to the dark needles absorbing ample sunlight. That suggests that the stomata were at least partially closed.

Those same pores are crucial for photosynthesis; it’s where the trees take in carbon dioxide — used to make food — and release oxygen. “They could essentially starve if they don’t have access to enough carbon,” Klinek says.

It’s hard to investigate the impact of heat on photosynthesis in the field, though. That would take building a “heat chamber around a real-life branch or wait[ing] for a heat event to happen,” Klinek says. Instead, she climbed onto the roof of her car with a pole pruner and collected samples from about five meters high, in the lower canopy.

In the lab, the researchers subjected the samples to increasing heat to match the heat waves seen in recent summers and measured the resulting photosynthesis rates. Even though the samples were submerged in water, so had their stomata open and had full access to the ingredients for photosynthesis, the rate still declined. “They weren’t slowing their photosynthesis because of carbon starvation,” Klinek says.

For practical reasons, the study focused on younger coast redwoods, not the old growth giants. It’s difficult to reach even the lowest branches on trees more than 100 meters tall. “There are just fundamental limitations on what you can sample if you don’t have a jetpack,” says Christopher Still, an ecophysiologist at Oregon State University in Corvallis who was not associated with the study. Even so, the team’s results show that coast redwoods are “really temperature sensitive. You don’t see this as much with other species necessarily,” Still says.

In the field, Klinek observed trees closing their stomata to preserve water. This suggests that in more realistic conditions, where a heat wave and drought hit simultaneously, lack of heat and water could make the slowdown “even more intense.” But more work needs to be done, she notes. “We still don’t know what the threshold is for irreversible damage.”