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California’s coast redwoods are measurably stressed by high temperatures, but scientists cannot yet translate those leaf-level effects into a reliable statewide count of dead or seriously damaged trees. A two-year study in Mendocino County found that photosynthesis declines as redwood leaves heat up, with sharper losses at temperatures above about 30°C and especially above 35°C. Leaves can also run as much as 10°C hotter than the surrounding air. The findings show a direct heat problem, not simply drought-induced water conservation.
What the latest heat study found
The peer-reviewed paper Summer Heat Stress Downregulates Photochemistry in Coast Redwoods, by Lily Klinek and colleagues, was first published on September 22, 2026. Scientists collected leaf-level measurements over two years in the greater Mendocino County area and tracked how temperature affected redwood photosynthesis during the 2024 growing season.
Photosynthetic performance began falling above approximately 30°C. The decline became steeper above 35°C. These are temperatures measured in leaf-response experiments, not tree-death thresholds: a leaf crossing one of those values does not mean the whole tree will die.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesLeaf temperature was consistently higher than air temperature, with differences reaching 10°C. UC Davis observations similarly found that some leaves were nearly 20°F hotter than the air when air temperatures exceeded 86°F during 2022 and 2024. Sun exposure, humidity, wind and the leaf’s ability to shed heat all affect that gap.
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Why the mechanism matters
In the temperature experiments, internal carbon dioxide remained relatively high even as photosynthesis weakened. That pattern points to non-stomatal, biochemical limits in the photosynthetic machinery. Redwoods were not merely closing their stomata to conserve water; heat itself reduced their ability to convert light and carbon dioxide into usable carbon.
The study’s authors concluded that coast redwoods show clear declines in photosynthetic yield, capacity and performance under elevated temperatures and may have limited ability to acclimate to sustained warming. Lower carbon uptake can reduce growth and reserves, but the measurements do not establish how many trees will die or how quickly a forest will lose canopy.
Why fog can determine how much heat a tree can tolerate
Coast redwoods (Sequoia sempervirens) evolved in a summer-dry coastal climate where fog moderates heat and helps maintain water availability. Fog can wet foliage directly and, by reducing atmospheric dryness, lower the amount of water a tree loses through transpiration.
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A 2025 USDA Forest Service study at the Caspar Creek Experimental Watersheds monitored fog, soil moisture, sap flow and streamflow during the 2020, 2021 and 2022 fog seasons. Fog periods reduced soil-moisture drawdown and transpiration and were associated with slower streamflow recession. In practical terms, fog acted as a buffer during the dry season.
That buffer becomes more important as temperatures rise. A hot atmosphere increases evaporative demand, while drought reduces the water available for cooling. If fog is less frequent or arrives for shorter periods, a tree can face hotter leaves, faster water loss and less soil moisture at the same time. The resulting stress can be greater than the effect of heat or drought considered separately.
What the Big Basin fire shows—and what it does not
Heat stress is only one part of the threat. Big Basin Redwoods State Park provides a documented example of several disturbances arriving in sequence.
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The 2020 CZU Lightning Complex Fire burned more than 35,000 hectares. Two successive years of extreme drought and summer heat followed in 2021 and 2022. Satellite imagery and field surveys found severe damage in most old-growth trees. By July 2022, only 24% of burned forest cover met the study’s moderate level of living, regrowing vegetation.
That 24% figure describes regrowth of burned forest cover in a particular park and time window. It is not a coast-wide mortality rate, and it should not be applied to unburned stands or to every redwood region. Fire injury, drought stress, heat exposure, tree age and site conditions all vary across the range.
Why a statewide damage total is not available yet
The strongest new evidence measures physiology at the leaf scale. It establishes that high temperatures reduce carbon uptake and identifies a likely biochemical mechanism, but it does not track every tree from heat exposure to long-term survival.
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| Evidence source | What it measures | What it can and cannot establish |
|---|---|---|
| 2026 Mendocino County heat study | Leaf temperature and photosynthetic response over two years | Shows direct heat-related reductions in photosynthesis; does not provide a statewide death count |
| Caspar Creek fog monitoring | Fog, soil moisture, sap flow and streamflow during 2020–2022 fog seasons | Shows how fog reduces drought stress; does not define a universal survival threshold |
| Big Basin post-fire assessment | Satellite and field measures after the 2020 fire and later drought and heat | Documents compound disturbance and limited regrowth in one burned landscape; is not representative of every stand |
| USGS second-growth plot dataset | Field observations and Forest Vegetation Simulator projections for 151 plots | Projects aboveground live-tree carbon under treatment scenarios from 2024 through 2064; does not directly count all coast-redwood mortality |
Connecting these scales requires long-term measurements of whole trees, canopy condition, carbon storage, regeneration and mortality across different parts of the range. The Mendocino study itself identifies that scaling as a next step.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which redwoods are most exposed?
Risk is not uniform across California. A useful assessment considers several variables together:
- Heat exposure: maximum air temperature, actual leaf temperature, the duration of hot periods and vapor-pressure deficit.
- Water access: soil moisture, fog frequency, transpiration rates and how quickly streams recede.
- Forest setting: northern versus central, southern or inland locations; old-growth versus second-growth; and managed versus unmanaged stands.
- Disturbance history: intact forest, drought-stressed forest, burned forest or a site undergoing post-fire restoration.
- Physiological response: temporary downregulation that reverses after cooler weather versus evidence of persistent damage to photosynthetic tissues.
These factors also distinguish coast redwoods from giant sequoias (Sequoiadendron giganteum), a different species with different habitats and fire histories. Findings about coast-redwood leaves should not automatically be treated as findings about giant-sequoia groves.
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How scientists are tracking future change
USGS released a 2025 dataset combining field observations with Forest Vegetation Simulator projections for 151 second-growth plots in Redwood National and State Parks. The projections are made in five-year increments from 2024 through 2064 and compare aboveground live-tree carbon under different restoration-treatment conditions. Those scenarios can reveal how management changes carbon trajectories, but they are projections for sampled plots rather than a census of every redwood forest.
California’s Forest Health Program, administered by CAL FIRE, addresses drought, wildfire and pest-related mortality. CAL FIRE reported more than $700 million awarded to 148 forest-health projects through 2025, including a $4.9 million Sonoma County project focused on restoring a forest containing old-growth redwoods. Funding and project availability can change, so individual awards should be checked against current state program information.
What the evidence means for the headline question
California redwoods are not uniformly dying from heat, but they are already showing measurable heat stress and reduced carbon uptake. Hotter leaves can directly impair photosynthesis even when internal carbon dioxide remains available, while drought and reduced fog remove the water needed for cooling. Fire can then compound both stresses.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThe responsible conclusion is therefore narrower than a statewide mortality claim: recent heat is harming redwood physiology, the severity depends strongly on water and disturbance context, and the total number of trees that will suffer lasting damage remains unresolved.
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