A new study from UC Davis and collaborators found that heat directly reduces photosynthesis in California redwoods, even when water is not a limiting factor. Researchers measured leaf temperatures and fluorescence in the Garcia River Forest from 2022 to 2024, showing that sunlit leaves can run up to 10 degrees Celsius hotter than the air on hot days. The study, published in JGR Biogeosciences, suggests that heat itself, not just stomatal closure, limits the trees' ability to take in carbon dioxide. This matters for carbon storage. Redwoods are among the most carbon-dense forests on Earth, and if heat stress lowers their photosynthetic capacity, that could reduce how much carbon these forests absorb under a warming climate. The authors note that the thresholds for irreversible damage are still unclear, so longer-term monitoring will be needed to understand how redwoods respond as heat waves become more frequent.
