Coast redwoods are famous for their size, longevity and ability to store enormous amounts of carbon. Yet the climate that helped them thrive is becoming less predictable.

These trees evolved in the cool, foggy environment of coastal California, where summer temperatures were usually moderate. As heat waves intensify, scientists are beginning to ask whether even some of the world’s most resilient trees have a thermal limit.

Leaves Can Become Hotter Than The Air

Researchers monitored coast redwoods in Mendocino County over two years, measuring leaf temperature and photosynthetic activity throughout different seasons.

One of the clearest findings was that the leaves did not simply match the surrounding air temperature.

During hot periods, redwood foliage sometimes became as much as 10°C warmer than the air.

That matters because leaves normally cool themselves by releasing water through tiny pores called stomata.

When conditions are dry, however, trees may partially close those pores to conserve water. This reduces evaporative cooling and allows leaf temperature to rise.

Photosynthesis Slows Above 30°C

The researchers also exposed redwood leaves to controlled temperature increases.

Photosynthesis began to decline once leaf temperatures rose above roughly 30°C.

Above 35°C, the drop became much steeper.

Importantly, the slowdown was not caused only by stomata closing and limiting carbon dioxide intake. The leaves still contained enough internal carbon dioxide, suggesting that heat was directly interfering with the biochemical machinery responsible for photosynthesis.

That makes extreme heat a problem even when water is not the only limiting factor.

The Trees Try To Protect Themselves

Redwoods do have defensive mechanisms.

When temperatures rise, they can redirect excess light energy away from photosynthesis and release it as heat. This process helps protect sensitive cellular structures from damage.

The study detected increased activity in this protective system during hot summer periods.

That suggests the trees are actively responding to heat stress rather than simply failing.

However, protection comes at a cost. Energy that is safely dissipated cannot be used to capture carbon and support growth.

Carbon Storage Could Be Affected

Coast redwood forests contain some of the highest aboveground carbon stocks of any ecosystem on Earth.

If heat repeatedly reduces photosynthesis, the consequences could therefore extend beyond individual trees.

Slower carbon uptake could eventually reduce growth rates and weaken the forests’ role as long-term carbon sinks.

This concern may be especially important for younger, second-growth forests, which now make up most of the remaining redwood range and can be more vulnerable to drought and heat stress.

The Future Depends On Frequency

The study does not show that redwoods are about to disappear.

Researchers still do not know whether repeated heat stress causes permanent damage or whether trees fully recover once temperatures fall.

The bigger question is how often extreme conditions occur.

Climate projections suggest that parts of coastal California, particularly inland and southern areas of the redwood range, could experience far more extreme heat days by the end of the century.

Even Giants Have Limits

Redwoods are exceptionally durable, but durability does not mean immunity.

Their greatest vulnerability may come from the combination of heat and water stress, which can push leaf temperatures far beyond what the surrounding air alone would suggest.

The new findings reveal that climate change does not need to kill trees outright to alter a forest.

Simply forcing leaves to spend more time above their ideal temperature may be enough to slow growth, reduce carbon uptake and gradually change the role these giant trees play in California’s ecosystems.