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 reported September 22 in the Journal of Geophysical Research: Biogeosciences. Laboratory experiments show that the rate of the sunlight‑driven food‑making process in the 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. This vulnerability can lead to tree deaths that may jeopardize one of California’s largest carbon stores, a natural buffer against climate change.
Coast redwoods (Sequoia sempervirens) inhabit a narrow coastal strip from central California to southern Oregon, where temperatures are mild and fog is abundant. Yet recent heat waves have pushed temperatures in parts of their range above 40° Celsius.
The impact of heat waves on photosynthesis has been examined in other tree species, with varying tolerance, but this is the first study to investigate how coast redwoods respond to heat, notes Troy Magney, an ecophysiologist at the University of Montana in Missoula.
A coast redwood sample sits in water during a laboratory experiment; a gray instrument clamped onto some needles measures gases that indicate photosynthesis rates (Lily Klinek/UC Davis).
From July 2022 through October 2024, Klinek, Magney and colleagues repeatedly measured needle performance from redwoods in Mendocino County, a period that included two heat waves.
Normally, trees stay cool as stomata—tiny pore‑like openings—allow water to evaporate, carrying heat away much like human sweat. When conditions become excessively hot and dry, stomata close to conserve moisture. Under extreme heat, the team found that needle temperatures can reach up to 10° Celsius higher than the surrounding air because the dark foliage absorbs ample sunlight, indicating at least partial stomatal closure.
Those same pores are essential for photosynthesis; they are where the trees take in carbon dioxide to make food and release oxygen. “They could essentially starve if they don’t have access to enough carbon,” Klinek says.
Studying heat’s effect on photosynthesis in the field is challenging; it would require building a heat chamber around a live branch or waiting for a natural heat event. Instead, Klinek climbed onto her car’s roof with a pole pruner and collected samples from about five meters up, in the lower canopy.
In the lab, the researchers exposed the samples to increasing heat to match recent summer heat waves and measured the resulting photosynthesis rates. Even though the samples were submerged in water—keeping stomata open and providing full access to photosynthetic ingredients—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 rather than the old‑growth giants. Sampling the lowest branches on trees taller than 100 meters is difficult. “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 involved in the study. Nevertheless, the findings show that coast redwoods are “really temperature sensitive—you don’t see this as much with other species necessarily,” Still adds.
In the field, Klinek observed trees closing their stomata to preserve water. This suggests that when a heat wave coincides with drought, the combined lack of heat and water could intensify the slowdown further. More work is needed, she notes. “We still don’t know what the threshold is for irreversible damage.”