Coast redwoods have stood as sentinels along California’s northern coast for centuries. Their massive trunks rise hundreds of feet into the air. Canopies thick with ferns and soil create miniature worlds high above the forest floor. Yet these ancient giants now show signs of strain. A new study reveals they slow their core functions when temperatures climb too high.
Researchers at the University of California, Davis spent two years measuring individual leaves on young coast redwoods in Mendocino County’s Garcia River Forest. They took readings twice a month. The results paint a troubling picture. When air temperatures topped 86 degrees Fahrenheit during the hot summers of 2022 and 2024, some leaves reached nearly 20 degrees hotter than the surrounding air. UC Davis reported the findings in late September.
Leaves that hot signal serious trouble. The trees close their stomata. These tiny pores normally pull in carbon dioxide while releasing water vapor to cool the plant. Like sweating in humans. But with the pores shut, cooling stops. Photosynthesis drops sharply. The trees essentially stop breathing and eating to avoid losing more moisture. Lily Klinek, the study’s lead author and a doctoral candidate in UC Davis’s Plant Optics Lab, called it the first direct look at heat’s impact on redwood photosynthesis.
“The leaves aren’t able to cool down.” Klinek said in the UC Davis release. Her team followed three trees from July 2022 through October 2024. They recorded 1,527 individual leaf measurements. Leaf temperatures averaged just under one degree warmer than air. During extreme heat, the gap widened dramatically. Photosystem II efficiency, a key marker of photosynthetic health, plunged below 0.4 in the worst conditions. Healthy leaves typically sit around 0.82.
The lab experiments proved even more revealing. Branches placed in jars of water still showed reduced photosynthesis at temperatures up to 110 degrees. Even with open stomata and ample carbon dioxide. Heat itself damages the machinery. Extra water may not solve the problem. This finding carries weight for forests already parched by changing rainfall patterns.
And the threats compound. Redwoods evolved in a narrow comfort zone. Average temperatures around 59 degrees. Daily highs between 46 and 80 degrees. Coastal fog once acted as a natural air conditioner. It provided moisture and kept things cool. But fog has declined 30 to 40 percent in recent decades. Warmer oceans and drier air make it sporadic. “It’s kind of a double whammy,” said Todd Dawson, a plant ecophysiologist at the University of California, Berkeley, who was not involved in the study. WIRED quoted him on October 11. “They’re running out of water and also getting this heat stress.”
The study site sits near the southern edge of the redwoods’ current range. Young trees dominate here. Most California redwoods today grew in the past few hundred years. They act like hungry teenagers. They need plenty of carbon dioxide to fuel rapid height gains. Reduced photosynthesis hits them harder. Older giants might fare differently. No one knows the exact temperature where photosynthesis collapses entirely or trees start dying. Klinek noted the trees appeared to recover after heat waves passed during the study period. Their leaves didn’t acclimate to higher temperatures. They simply bounced back once conditions cooled.
Yet future summers promise more extremes. California has already seen record heat. The 2025-2026 winter ranked among the warmest on record in the Sierra Nevada. Snowpack, vital for water supply, ran far below average. Scientists warn that such patterns will intensify. A March 2026 study in Fire Ecology examined giant sequoias, the redwoods’ massive inland cousins. It found 17.6 percent of large sequoias killed by wildfire since 1984. Most losses came in the 2020 and 2021 megafires. Save the Redwoods League covered the coalition’s findings in April.
Giant sequoias face their own heat and drought pressures. Thick bark protects them from fire under normal conditions. But decades of fire suppression built up fuels. Recent blazes burned hotter and killed mature trees at unprecedented rates. From 2015 to 2024, 82 percent of sequoia grove area burned. Only 24 percent had burned in the previous century. The Giant Sequoia Lands Coalition now pushes prescribed burns to restore resilience. They treated over 4,500 acres in 2025 alone. Early October 2026 reports from Comstock’s magazine highlighted ongoing efforts at sites like Calaveras Big Trees State Park.
Coast redwoods and giant sequoias tell parallel stories. Both depend on specific climates. Both now confront conditions outside their evolutionary experience. Heat waves scorch leaves. Drought tightens water access. Fog disappears. Wildfires grow more destructive. A September 2026 piece in Active NorCal summarized the redwood study’s implications. The central, southern and eastern edges of the redwood range look most vulnerable.
Foresters and conservation groups watch closely. The National Science Foundation and Cal Fire funded Klinek’s work. It appeared in the Journal of Geophysical Research: Biogeosciences. Results align with earlier observations of redwood stress during the 2012-2016 drought. Foliage dieback appeared then in sequoias too. Beetle attacks followed in some cases. Yet redwoods have shown remarkable resistance to disease and typical fires. Their poor water regulation becomes a liability in extreme heat.
Scientists emphasize the need for more data. What happens over five or ten years of repeated heat spikes? Will younger trees reach maturity if carbon uptake stays limited? Can forests adapt through natural selection or assisted migration? These questions lack clear answers. The redwood study offers a starting point. It quantifies physiological responses at the leaf level. Such detail was missing before.
Public attention has grown. Recent social media posts on X highlighted the WIRED article. Users expressed concern over the trees’ future. Similar discussions surround sequoia regeneration debates. Some researchers report strong natural regrowth after fires. Others call for replanting programs. A August 2026 Earth Island Journal article captured the tension between those views.
California’s iconic trees have survived centuries of change. Indigenous peoples managed these forests with fire for generations. European settlement brought logging and suppression policies. Now climate shifts test their limits once more. The tallest living things on Earth face a test unlike any in their long history. Their response will shape entire ecosystems. Salamanders living in canopy soils. Birds nesting in high branches. Carbon storage measured in millions of tons. All connected to the health of these giants.
Recovery after individual heat events offers some hope. Trees in the study resumed normal function when temperatures dropped. Autumn and winter still bring relief. But the frequency and intensity of hot spells continue to rise. Fog’s return remains uncertain. Ocean temperatures drive much of that pattern. Broader climate trends suggest challenges ahead.
Land managers already act on sequoia groves. Prescribed fire reduces fuels and prepares forests for inevitable blazes. Similar strategies could help coastal redwoods by maintaining overall ecosystem balance. Yet direct intervention for heat stress proves harder. You cannot easily shade a 300-foot tree or increase fog. Protection starts with addressing the root causes of warming.
The Davis team’s work stands out for its rigor. Specialized instruments tracked real-time photosynthesis. Lab simulations isolated heat from drought. Combined field and controlled data strengthen the conclusions. Other studies have documented forest stress across California. This one zeros in on the state’s most famous species. It delivers specific numbers. Leaf temperature differentials. Efficiency drops. Threshold behaviors.
So the redwoods endure. For now. Their leaves close against the heat. Growth slows. Observers wonder how long that strategy holds. Another record heat wave could arrive any summer. The trees have no escape. They stand rooted in place as conditions shift around them. Their story becomes a measure of larger environmental change. One that industry professionals in forestry, conservation and climate science track with growing urgency.