Mendocino, Calif. — Redwoods Dial Back Photosynthesis as Heat Rises

A new study from the University of California, Davis, published via Wired on Oct. 11, 2026, finds that coast redwood leaves cut back photosynthesis when temperatures spike, a response researchers describe as a biological bottleneck for one of California's most climate-sensitive tree species.

The finding comes from leaf-level measurements taken twice a month for two years in Mendocino County, near the southern edge of the redwood range, according to the study cited by Wired. Lead author Lily Klinek, a Ph.D. student at UC Davis, says the trees physically can't shed heat fast enough once temperatures climb, a mechanism with implications for carbon uptake across the species' range.

What did the UC Davis study actually measure?

Researchers tracked stomata — the microscopic pores on redwood leaves that regulate gas exchange — across two years of fieldwork in Mendocino County, per the study described by Wired. Twice-monthly sampling let the team compare leaf behavior during cool, foggy stretches against periods of extreme heat. The two-year duration matters because it captures seasonal swings rather than a single hot spell, giving the dataset enough range to show a consistent pattern: photosynthesis activity drops when heat rises, not just occasionally but as a repeatable response.

How do redwood leaves cope with heat?

Stomata serve two functions at once, taking in carbon dioxide for photosynthesis and releasing water vapor that cools the leaf, according to Klinek's explanation to Wired. Open stomata speed up both processes; closed stomata conserve water but throttle photosynthesis. Klinek compares it to human sweating: "Because leaves are a physical, light-absorbing object, they heat up just like asphalt would in the sun," she told Wired. "They're heating up faster than the air around them. 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."

The catch, Klinek says, is water supply. Redwoods can only cool themselves by transpiring if they have water to spare — and during extended heat, many don't.

Data points from the study

• Measurement frequency: twice monthly, sustained for two years, per the UC Davis research described by Wired (Oct. 11, 2026).
• Study location: Mendocino County, described as near the southern end of the coast redwood's natural range.

Why does the southern range matter for this data?

Mendocino County sits at the warmer, drier edge of where coast redwoods naturally grow, which makes it a useful early-warning site. Trees there already face more heat stress than counterparts farther north in cooler, foggier stretches of the California coast. A pattern detected at the range's southern margin offers a preview of what could spread northward if warming trends continue, though the study itself is confined to measurements from this one region over the two-year window.

What's at stake for the ecosystem redwoods support?

Coast redwoods function as what Wired calls "a forest of forests": individual trees can live for thousands of years, and their canopies accumulate mats of soil and ferns that form elevated habitat hundreds of feet up. That canopy layer supports species including the wandering salamander, which can complete its entire life cycle atop a single tree, and the marbled murrelet, a seabird whose canopy-nesting behavior wasn't documented until a tree-trimmer found a nest 150 feet above the ground in 1974, per Wired's account. If reduced photosynthesis slows redwood growth or canopy development over time, the knock-on effects could reach species that depend entirely on those elevated microhabitats.

What happens next as temperatures keep rising?

"They're heating up faster than the air around them... it's the same for leaves. But they can only really do that if they have enough water to lose." — Lily Klinek, UC Davis Ph.D. student, via Wired

The study frames the stomata response as a near-term physiological limit rather than a one-off anomaly, meaning the photosynthesis slowdown documented across the two-year Mendocino County sampling period should recur whenever heat extremes return. Wired's report does not provide a projected timeline for range-wide impact or a numerical threshold at which redwoods face long-term decline, and no such figures are included in the available source material. Researchers' broader point, as relayed by Wired, is that a species long considered resilient — redwoods survive wildfires that kill other trees — has a specific vulnerability to sustained heat that intensifying climate trends are expected to test more often in the years ahead.

Timeline

• 1974: A tree-trimmer discovers a marbled murrelet nest 150 feet up a redwood, solving a long-standing mystery about the seabird's nesting habits, per Wired.
• Two-year study period (concluding before publication): UC Davis researchers collect twice-monthly leaf measurements in Mendocino County.
• Oct. 11, 2026: Wired publishes the findings, via Vox and the Climate Desk collaboration, with Klinek as lead author.

For the full study context and researcher commentary, see the original Wired report.