Extreme Heat Threatens Ancient Redwood Ecosystems

Ancient groves of coast redwood trees, long considered resilient natural wonders, are showing vulnerability to extreme heat, according to new research. These towering forests, some living for thousands of years, create unique ecosystems within their canopies, providing habitat for specialized species like the wandering salamander and the marbled murrelet. However, a recent study from the University of California, Davis, indicates that prolonged periods of extreme heat are forcing these iconic trees to significantly reduce their photosynthetic activity.

How Redwoods Respond to Heat

Trees absorb carbon dioxide and release water vapor through pores in their leaves called stomata. This process, known as photosynthesis, is essential for growth and energy production. When leaves heat up beyond a certain point, they risk drying out. To prevent this, trees can close their stomata, conserving water but also halting carbon dioxide intake. Researchers found that during heat waves, coast redwoods close their stomata to an extent that slows photosynthesis.

"Because leaves are a physical, light-absorbing object, they heat up just like asphalt would in the sun," explained Lily Klinek, a PhD student at UC Davis and lead author of the study. "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. But they can only really do that if they have enough water to lose."

During extreme heat, redwoods reach a threshold where they cannot cool down sufficiently and keep their stomata closed. This conservation of water comes at the cost of energy production. The study, which involved bi-monthly measurements over two years, is the first to document this response in coast redwoods.

This slowdown in photosynthesis can lead to "carbon starvation," a condition where trees do not accumulate enough carbon to replenish their leaves or support future growth. This is particularly concerning for younger, second-growth forests that are still actively growing and require significant carbon uptake. Redwoods are known for their capacity to store carbon, making them vital in the global climate system. A reduction in their carbon sequestration ability not only affects the trees' growth but also diminishes their capacity to mitigate climate change.

The Impact of Declining Fog and Rising Temperatures

Adding to the stress is the declining frequency of coastal fog, a critical hydration and cooling source for redwoods. The dense fog layers allow the tall trees to absorb moisture directly and provide shade, creating a cooler microclimate. Warmer oceans and drier air are making fog more sporadic, leaving redwoods more exposed to heat.

"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. "They’re running out of water and also getting this heat stress. They’re used to living in a very narrow temperature range of 8 to 27 degrees Celsius (about 46 to 80 degrees Fahrenheit). California, like everywhere, has been getting long periods of much warmer days than we’ve ever experienced, and now the fog, which is normally kind of like an air conditioner, has also declined between 30 and 40 percent."

The cascading effects of this heat stress extend to the unique ecosystems within the redwood canopies. If the soil mats and ferns dry out due to excessive heat, the specialized creatures that inhabit these elevated environments may not be able to survive.

While the trees studied showed signs of recovery after heat waves passed, the long-term implications remain uncertain. "Right now the temperatures are coming back to normal in the fall and winter and letting the trees recover," Klinek noted. "But in five or 10 years, if temperatures are even hotter and heat waves are even more frequent, we don’t really know if there would be a recovery."

The future of these ancient forests, and the myriad life they support, hinges on understanding and addressing the impacts of climate change. As vital components of the planet's ecosystem, the health of redwood forests has implications far beyond their immediate geographic range.