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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Coast redwoods are measurably losing photosynthetic performance in extreme heat, and their leaves can become far hotter than the surrounding air. Scientists have not yet converted those leaf-level responses into a reliable estimate of how many California redwoods are dying statewide.
What the latest evidence shows
A peer-reviewed study first published on September 22, 2026, tracked coast redwood (Sequoia sempervirens) leaves for two years in the greater Mendocino County area. The researchers found that heat reduced photosynthetic yield, capacity and performance. Their temperature-response curves showed carbon uptake declining above approximately 30°C and falling more steeply above 35°C.
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Those temperatures are thresholds for measured leaf photosynthesis, not tree-death limits. A redwood can survive a period of reduced carbon uptake, and the severity and duration of stress vary with water supply, canopy position, stand history and subsequent weather.
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Leaves can substantially outheat the air
During hot periods, leaf temperatures consistently exceeded air temperatures, with differences reaching 10°C. A UC Davis report also found that some leaves were nearly 20°F hotter than the air when air temperatures exceeded 86°F in 2022 and 2024. Sun exposure, leaf orientation and limited evaporative cooling can all widen this leaf-to-air gap.
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Heat limits photosynthesis directly
The temperature experiments kept internal carbon dioxide relatively high, yet photosynthesis still declined. That result points to non-stomatal, biochemical limits in the photosynthetic machinery rather than a response caused only by stomata closing to conserve water. In practical terms, heat can reduce a redwood’s carbon gain even when the tree is not simply shutting its pores because of drought.
“Our findings demonstrate that coast redwoods exhibit clear declines in photosynthetic yield, capacity, and performance under elevated temperatures, and that redwoods may have limited capability for thermal acclimation,” lead author Lily Klinek and colleagues wrote.
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Why fog can determine how severe a hot summer becomes
Coast redwoods evolved in a summer-dry coastal climate where fog moderates leaf temperatures and helps maintain water availability. 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.
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What the measurements do—and do not—tell us
| Evidence | Measured result | What it establishes |
|---|---|---|
| Mendocino County leaf study, 2024 growing season | Photosynthesis declined above about 30°C and dropped more sharply above 35°C | Heat suppresses leaf carbon uptake under the tested conditions |
| Mendocino County heat observations | Leaves reached temperatures up to 10°C above air temperature | Air-temperature readings can understate the heat experienced by foliage |
| Caspar Creek fog monitoring, 2020–2022 | Fog reduced soil-moisture drawdown and transpiration and slowed streamflow recession | Fog can buffer summer water stress |
| Big Basin post-fire assessment, July 2022 | 24% of burned forest cover showed moderate regrowth two years after the fire | Severe compound disturbance can leave limited early regrowth in a burned stand |
The strongest new findings are physiological and leaf-level. They show a mechanism for reduced carbon uptake, but they do not count dead trees across the coast-redwood range. The 30°C and 35°C values therefore should not be used as universal mortality thresholds. Scaling from leaves to whole trees, plots and entire forests remains an active scientific problem.
How fire, drought and heat compound one another
Big Basin Redwoods State Park provides a documented example of interacting stresses rather than a statewide average. 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, and field surveys reported severe damage to most old-growth trees. Satellite and field-based analysis found that only 24% of burned forest cover was alive and regrowing to a moderate level by July 2022.
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This result describes that burned landscape after an unusually severe sequence of fire, drought and heat. It is not a mortality rate for intact redwood forests, for unburned stands or for every region where coast redwoods grow. A stand’s condition depends heavily on whether it burned, how intensely it burned, its age structure and whether it can access fog and soil water.
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Where conditions differ across California’s redwood forests
Risk cannot be inferred from air temperature alone. Useful comparisons include:
- Heat exposure: air temperature, leaf temperature, heat duration and vapor-pressure deficit.
- Water access: soil moisture, fog frequency, transpiration and streamflow response.
- Physiological response: short-term downregulation versus evidence of sustained photosynthetic injury.
- Disturbance history: intact, drought-stressed, burned or post-fire restoration sites.
- Forest context: northern versus central, southern or inland-edge locations; old-growth versus second-growth; and managed versus unmanaged stands.
- Evidence scale: leaf measurements, plot-level carbon, remote-sensing regrowth or whole-forest mortality.
Coast redwoods should also be distinguished from giant sequoias (Sequoiadendron giganteum). They are different species with different ranges and ecological settings, so findings about one should not automatically be presented as findings about the other.
How scientists are tracking longer-term 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 run in five-year increments from 2024 through 2064 and compare aboveground live-tree carbon under different restoration-treatment conditions. Plot projections can reveal how stand carbon may change, but they are not a direct count of statewide tree deaths.
Management programs are also targeting the stresses that can compound heat. CAL FIRE’s Forest Health Program addresses drought, wildfire and pest mortality. Through 2025, CAL FIRE reported more than $700 million awarded to 148 forest-health projects, including a $4.9 million Sonoma County project aimed at restoring a forest with old-growth redwoods. Project funding and treatment goals indicate active intervention; they do not by themselves establish how much heat damage has already occurred.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsSo, are California redwoods dying from heat?
Some redwoods are clearly experiencing heat stress and reduced carbon uptake, especially when hot conditions coincide with dry soils, reduced fog or prior fire. The 2026 physiology study demonstrates a direct heat effect on photosynthesis, while the Caspar Creek observations show why fog can moderate drought pressure. Big Basin demonstrates how fire followed by drought and heat can produce severe damage.
What remains unresolved is the statewide total: scientists do not yet have a defensible number for how many coast redwoods have died primarily because of heat, how many will recover after a hot season, or how those outcomes differ among regions and stand types. Answering that requires linking leaf physiology to repeated tree-level measurements, plot carbon trends, remote sensing and long-term mortality records.
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