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New field measurements show California coast redwoods slow their photosynthetic activity when air temperatures climb — a response that could weaken these forests’ ability to absorb and store carbon as heat waves become more frequent. The result comes from a multi-year study that tracked leaf responses to high temperatures in Mendocino County.
How the team measured stress in redwood leaves
Researchers visited the Garcia River Forest between 2022 and 2024, sampling redwood foliage once or twice a month to record leaf temperature and fluorescence. Fluorescence, a faint light re-emitted by leaves, serves as a practical indicator of how efficiently a plant is converting light into chemical energy.
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For each sampling event the team measured three sun‑exposed leaves and used a fourth, wrapped in foil for 20 minutes, to establish a rested baseline. That rested leaf was then exposed to a pulse of light to estimate the tree’s maximum photosynthetic capacity.
Heat pushes leaves beyond their limits
On hot days the exposed leaves were often far warmer than the surrounding air — sometimes by as much as 18°F (10°C). “That’s a huge difference and indicates that the leaves are stressed to the point where they can’t thermoregulate effectively,” study first author Lily Klinek, a doctoral candidate in the Plant Optics Lab at the University of California, Davis, told Live Science.
Field observations suggested the leaves were closing their pores to prevent water loss. Those pores, known as stomata, normally balance carbon dioxide intake with water loss. When stomata close, less CO2 enters the leaf and photosynthesis drops.
Direct heat damage, not just stomatal closure
To separate effects of water stress from heat itself, the team brought leaf samples into the lab and kept them hydrated while exposing them to increasing temperatures. Even with stomata open and water available, the leaves’ photosynthetic performance fell as temperature rose.
The authors conclude that heat imposes a direct limitation on the leaves’ photochemistry — not only an indirect effect through stomatal closure. As University of Exeter ecologist Lucy Rowland, who was not involved in the work, summarized in an email to Live Science: “It is not only stomatal closure that is causing reduced photosynthesis.”
Why this matters for redwoods and the climate
Coast redwood (Sequoia sempervirens) forests rank among the planet’s largest aboveground stores of carbon and evolved under historically mild coastal climates in central and northern California. The region has warmed in recent decades and now experiences more frequent extreme‑heat days, including temperatures over 100°F (37°C).
If redwoods lose photosynthetic capacity during heat events, their ability to sequester carbon could decline — with consequences for forest health and for the role these ecosystems play in mitigating atmospheric CO2.
Unanswered questions and next steps
Klinek told Live Science the next phase of research will look beyond individual leaves to understand whether the stress responses are reversible and at what point damage becomes permanent. “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.
The study, published Sept. 22 in Journal of Geophysical Research: Biogeosciences, is listed as: Klinek, L., Au, J., Rodriguez‑Caton, M., Rao, M. P., Fety, L., Koch, G. W., & Magney, T. S. (2026). Summer heat stress downregulates photochemistry in Coast Redwoods. JGR Biogeosciences, 131(9). https://doi.org/10.1029/2026jg010052
