When temperatures spike, California redwoods slow down photosynthesis, new research has found. This finding has serious implications for the long-term survival of the iconic trees in a warming world, and their role in storing carbon from the atmosphere.

California redwood (Sequoia sempervirens) forests are among the largest aboveground carbon stores on the planet. These towering trees are native to central and northern California, and evolved to take advantage of the region's historically mild weather. In recent decades, their range has contracted significantly. But climate change is threatening the health and function of these forests.

The region has seen significant warming in recent decades, and faces more frequent extreme heat days, with temperatures spiking above 100 degrees Fahrenheit (37 degrees Celsius). Now, in a new study, published Sept. 22 in the journal JGR Biogeosciences, a team of researchers has shown that California's renowned redwoods are struggling to photosynthesize in the heat.

Photosynthesis is the process used by plants, algae and some bacteria to turn sunlight into energy. It chemically converts carbon dioxide and water into sugars and oxygen.

"These trees adapted to grow in really temperate environments, so no one had really looked at the impact of heat on photosynthesis," study first author Lily Klinek, a doctoral candidate in the Plant Optics Lab at the University of California, Davis, told Live Science.

From 2022 to 2024, Klinek and colleagues measured how the leaves of redwoods in Garcia River Forest in Mendocino County responded to the heat. Once or twice a month, the researchers took three different California redwood leaf samples and measured their fluorescence and temperature.

A plant's molecules can use absorbed light in three ways: for photosynthesis, dissipated as heat, or reflected as light (florescence). Fluorescence is a proxy measurement to understand photosynthesis in the plant, Klinek said.

The researchers wrapped a fourth leaf in tinfoil for 20 minutes to put it in a state of "rest" before taking the same measurements. "When they're in a state of rest, we expose them to a pulse of light that’s in the wavelength they would use for photosynthesis to see what would be their maximum photosynthetic rate," Klinek said.

They found that on hot days, the leaves exposed to sunlight were up to 18 degrees Fahrenheit (10 degrees Celsius) hotter than the air temperature. "That's a huge difference and indicates that the leaves are stressed to the point where they can't thermoregulate effectively," Klinek said.

Many plants' leaves manage their temperature and photosynthesis by opening or closing their stomata, which are tiny pores on their surface. These stomata (from the Greek word for mouth) regulate the amount of carbon dioxide entering the plant, and how much oxygen and water vapor it releases. Lower carbon dioxide intake leads to reduced photosynthesis, meaning the plant has less energy.

The large differences between air and redwood leaf temperatures suggest that the leaves had closed their stomata to avoid losing water from evaporation during hot conditions and consequently overheated, Klinek said.

To further investigate the impact of heat on leaf physiology, the researchers took California redwood leaf samples back to the laboratory and exposed them to a range of temperatures while they were in water. Despite their access to water, the leaves still photosynthesized less as they got hotter, even though their stomata were open, Klinek said.

This shows that heat imposes direct limitations on redwood photosynthetical capacity, rather than indirectly decreasing that capacity by causing stomatal closure, the researchers wrote. The finding indicates that redwoods' ability to photosynthesise and store carbon will decrease as temperatures rise.

​​"It is not only stomatal closure that is causing reduced photosynthesis," Lucy Rowland, an ecologist at the University of Exeter in the U.K. who was not involved in the study, told Live Science in an email. "These trees are already suffering from the direct effects of heat on their photosynthetic systems."

The finding has serious implications for the future of redwoods and similar plants. "The points at which this damage becomes irreversible remains really poorly understood, but is something which is really essential to understand if we want to predict how our future climate is going to impact plants," she said.

The next step in Klinek's research is to move beyond leaf physiology to understand these thresholds on the trees and their ecosystems as a whole.

"We're not sure to what extent the effect that we saw is a reversible stress response and if they are sustaining damage from this constant stress," she said. Researchers need more data to "really get a better sense, beyond the leaf scale, how these ecosystems are responding to heat and drought," she added.