Study Reveals Impact of Extreme Heat on Coast Redwoods

<figure> <img alt="A redwood grove in sunlight is seen from below. Many of the trees are tall, with bare trunks that burst into branches filled with vibrant green leaves at their top." src="https://platform.vox.com/wp-content/uploads/sites/2/2026/10/gettyimages-2271228456.jpg?quality=90&amp;strip=all&amp;crop=0,0,100,100" /> <figcaption> A redwood tree grove at Roberts Regional Recreation area in Oakland, California. | Smith Collection/Gado via Getty Images </figcaption> </figure> <p class="wp-block-paragraph">A grove of coast redwood trees is a forest of forests. Individual trees can live for thousands of years, and as they grow, their canopies develop mats of soils and ferns, creating something like a second forest floor hundreds of feet above the ground.</p> <p class="wp-block-paragraph">These elevated ecosystems are home to creatures like the <a href="https://research.fs.usda.gov/treesearch/25045">wandering salamander</a>, which can spend its entire life cycle on top of a single tree, and the marbled murrelet, a seabird whose nesting habits were a mystery until <a href="https://www.savetheredwoods.org/explore/stories/a-seabird-that-lives-in-the-redwoods/">one day in 1974</a>, when a tree-trimmer stumbled upon a nest 150 feet above the ground. Sometimes a stray seed — a sitka spruce or oak or even another redwood — finds its way up a massive redwood and grows into another tree entirely, nestled among the branches of a giant.</p> <p class="wp-block-paragraph">For millennia, redwoods have been the anchors of this incredibly stable ecosystem; they can even survive wildfires that devastate other tree species. They have also contributed significantly to California’s tourist economy. However, they are not immune to warming temperatures, and recent studies indicate that these trees have a specific vulnerability.</p> <p class="wp-block-paragraph">A <a href="https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2026JG010052">new study from researchers at University of California Davis</a>, using measurements taken twice a month for two years, found that coast redwood leaves reduce their photosynthesis during periods of extreme heat. This indicates that trees are slowing their photosynthesis because they cannot cool down sufficiently when temperatures rise. This issue is expected to worsen as climate change continues to increase global temperatures, raising questions about the future of these trees and the ecosystems that depend on them.</p> <h2 class="wp-block-heading">How a redwood fasts</h2> <p class="wp-block-paragraph">To understand the challenges facing redwoods and their dependent species, it is important to examine tree anatomy. Trees utilize stomata in their leaves to take in carbon dioxide and release water. Open stomata allow for photosynthesis but also lead to water loss, while closed stomata conserve water but limit carbon dioxide intake.</p> <p class="wp-block-paragraph">“Because leaves are a physical, light-absorbing object, they heat up just like asphalt would in the sun,” said <a href="https://magneylab.ucdavis.edu/people/lily-klinek">Lily Klinek</a>, a PhD student at UC Davis and lead author of the study.</p> <p class="wp-block-paragraph">“They’re heating up faster than the air around them,” she said. “And one way that they can cope with this is by opening their stomata and allowing water to transpire out of the leaf, which is kind of like the way that humans sweat. When water evaporates off our skin, it cools us down, and it’s the same for leaves. But they can only really do that if they have enough water to lose.”</p> <p class="wp-block-paragraph">Klinek and her colleagues studied coast redwoods in Mendocino County, where they used specialized instruments to measure whether individual leaves were photosynthesizing. During heat waves, they found that coast redwoods reach a point where they cannot cool down quickly enough, leading them to keep their stomata closed. While this conserves water, it also means the tree is not taking in more carbon, which affects its energy stores and growth.</p> <p class="wp-block-paragraph">This physiological response could have significant implications for these forests, potentially creating a cycle where trees lack sufficient carbon to replenish their leaves, leading to further declines in photosynthesis and energy reserves. This phenomenon has been observed in tropical trees, but this study is the first to examine how coast redwoods respond to heat stress.</p> <p class="wp-block-paragraph">“The term people use is carbon starvation,” said <a href="https://directory.forestry.oregonstate.edu/people/still-chris">Chris Still</a>, a forest ecologist at Oregon State University who did not participate in the study. “Leaves can be expensive to build for a plant.”</p> <p class="wp-block-paragraph">Most redwoods in California and the Pacific Northwest are new-growth forests planted in the last couple of centuries. This means the trees are relatively in need of carbon dioxide to grow taller.</p> <p class="wp-block-paragraph">Redwoods are also <a href="https://www.sciencedirect.com/science/article/pii/S0378112716302584">highly effective at storing carbon</a>. Due to their size and longevity, they store more carbon dioxide per acre than any other forest globally. “Not only is it negative feedback on the climate system itself, with less carbon sequestration from these trees and forests, but the trees themselves are going to obviously not grow as big,” said <a href="https://www.anthonyrambrose.com/">Anthony Ambrose</a>, a plant physiological ecologist and co-founder of the <a href="https://ancientforestsociety.org/">Ancient Forest Society</a>. Even if the trees survive, they may not have as much space for ferns and soil mats, which could reduce habitat for the organisms that rely on them.</p> <h2 class="wp-block-heading">The fog of old forests</h2> <p class="wp-block-paragraph">A significant threat to these trees is that, in addition to heat stress, they may also lose a vital source of hydration and protection: Coast redwoods have a complex <a href="https://www.savetheredwoods.org/explore/stories/the-science-of-redwoods-and-fog/">relationship with fog</a> in their native Northern California and the Pacific Northwest. The trees can absorb water directly from fog, which also provides shade and helps create a cooler microclimate. However, warmer oceans and drier air are reducing fog frequency, depriving the trees of a crucial source of relief from increasing heat.</p> <p class="wp-block-paragraph">“It’s kind of a double whammy,” said <a href="https://vcresearch.berkeley.edu/faculty/todd-dawson">Todd Dawson</a>, a plant ecophysiologist at the University of California Berkeley, who also did not participate in the study. “They’re running out of water and also getting this heat stress,” he said. “They’re used to living in a very narrow temperature range of 8 to 27 degrees Celsius (about 46 to 80 degrees Fahrenheit). California, like everywhere, has been experiencing longer periods of much warmer days than ever before, and now the fog, which typically acts like an air conditioner, has declined by 30 to 40 percent.”</p> <p class="wp-block-paragraph">These temperature variations also affect the organisms living in the trees. “If they experience excessive heat throughout the summer,” Ambrose said, “then the soil mats and ferns will probably dry out much faster. And then those salamanders and birds might not be able to live there. So we’ll see cascading effects through the whole ecosystem at that point.”</p> <p class="wp-block-paragraph">Klinek noted that the trees appear to recover after heat events; while the leaves studied did not acclimate to the heat, they did not lose their capacity to photosynthesize once the heat subsided. While this offers some positive news, it does not diminish the urgency to understand the impact of climate change on these trees and to mitigate warming.</p> <p class="wp-block-paragraph">“Right now the temperatures are returning to normal in the fall and winter, allowing the trees to recover,” Klinek said. “But in five or ten years, if temperatures are even hotter and heat waves are more frequent, we don’t know if there would be a recovery.”</p> <p class="wp-block-paragraph">Currently, the salamanders and seabirds still have their homes high in the coast redwoods. However, humans are also part of the redwoods’ ecosystem, and these trees are vital for maintaining a hospitable planet. The trees studied have the potential to live for over a thousand years, raising the question of whether we will allow them to thrive.</p>