California‘s coast redwoods like cool weather. They grow best along the foggy coast, where the average temperature is about 59°F. But the climate is changing. Yearly averages are going up, and heatwaves can now pass 100°F. A new study from the University of California, Davis, shows that the trees react to this heat by slowing down photosynthesis.
Photosynthesis is how plants make food. Trees take sunlight, water, and carbon dioxide from the air and turn them into carbon. They use that carbon to grow and to stay alive. When redwoods get too hot, this process slows. You could say the trees breathe and eat more slowly. The results worry scientists. Redwoods in the central, southern, and eastern parts of their range in California are already under pressure, so they may be at greater risk.
The study appeared in JGR Biogeosciences. It is the first to look at how heat affects photosynthesis in coast redwoods. Lily Klinek led the work. She is a Ph.D. student in the Plant Optics Lab at UC Davis, in the Department of Plant Sciences. Her team collected many kinds of measurements during three summers in a redwood forest in Mendocino County.
In the summers of 2022 and 2024, air temperatures rose above 86°F. On those days, some leaves were almost 20 degrees hotter than the air around them. That is a clear sign of stress. Klinek said the leaves were unable to cool down. She explained that they probably closed their stomata. Stomata are tiny pores that work like mouths. They let in carbon dioxide from the air. The team then ran lab tests at temperatures up to 110°F. Photosynthesis dropped there too. It dropped even when the stomata were open and plenty of carbon dioxide was coming in.
Klinek said scientists do not yet know the temperature limits at which redwood photosynthesis fails or the trees die. But the study indicates that heat has a strong effect on how the trees function. She called this an important first step. It can help researchers find which parts of the redwood range face the most danger, predict what will happen, and plan ways to protect the forests.
Redwoods Sweat, Unless They Are Thirsty
Trees can cool themselves through a process called evapotranspiration. They release some of the water inside them through their stomata. This is much like human sweating. Klinek made that same comparison.
Heat spikes do more than warm the air. They also burn away the coastal fog, and redwoods rely on that fog for much of their water. The big gap between leaf and air temperature suggests the leaves shut their stomata to save water. Without sweating, they overheated.
In the lab, the team placed leaf samples in jars of water. The leaves had all the water they needed and could sweat freely. Even so, photosynthesis went down while the stomata stayed open. Klinek said the finding means that extra water may not be enough. Heat alone could still harm how redwoods work.
Other authors include Troy Magney, Jessie Au, and Milagros Rodriguez-Caton of UC Davis. Mukund Palat Rao of Columbia University, Lauren Fety of The Conservation Fund, and George Koch of Northern Arizona University also contributed. The study was funded by the National Science Foundation and California Forestry and Fire Protection. It was carried out with the Conservation Fund.
These findings convey a clear message. Redwoods depend on cool air and fog, and both are becoming less common during heatwaves. If summers keep getting hotter, the tallest trees may struggle to grow and remain healthy. More research is needed to know how much heat they can take.
