Coast redwoods are iconic giants of the Pacific Coast, found only in a narrow band of land stretching from central California to southern Oregon. These towering trees—the tallest in the world—are adapted to the region’s relatively cool, moist climate, particularly the thick summer fog that blankets their canopies during the dry season. But in recent years, the area has been getting hotter, and the fog has been dwindling, raising questions about how these ancient trees will fare in a warming world.
Now, scientists say they have an answer. A study published September 22 in the Journal of Geophysical Research: Biogeosciences finds that coast redwoods slow photosynthesis when temperatures spike, a discovery that suggests climate change could be threatening the trees’ long-term health and survival. Through photosynthesis, plants convert sunlight, water and carbon dioxide into oxygen and sugars, which provide the energy and building blocks they need to grow.
As trees build trunks, branches, roots and leaves, they store some of that carbon in their tissues. Because of their large size, thick bark and long life spans, coast redwoods act as massive carbon sinks, keeping it out of the atmosphere for decades or even centuries. To better understand how rising temperatures might be affecting these processes in coast redwoods, researchers studied trees growing in Garcia River Forest in Mendocino County, California, near the southern part of the species’ range.
Once or twice a month from July 2022 through October 2024—a span that included two heat waves—they took measurements for individual leaves, colloquially called needles. Their analyses revealed that, during bouts of extreme heat, the needles were up to 18 degrees Fahrenheit hotter than the surrounding air. Based on the temperature difference, the team suspects that the leaves’ stomata—small, pore-like openings that help release heat via water vapor—were at least partially closed to help the tree retain moisture.
But stomata also play an important role in photosynthesis, taking in the carbon dioxide needed for the process, so the results also suggest that the higher temperatures could be hampering photosynthesis. The world’s tallest living tree The world’s tallest living tree is a roughly 380-foot-tall coast redwood named Hyperion, located in California’s Redwood National Park. Discovered in 2006, Hyperion has been officially off-limits to visitors since 2022 because tourists kept veering off established trails and damaging the delicate forest ecosystem.
The towering plant is estimated to be about 600 years old. To measure the impact of heat on photosynthesis more directly, Klinek and colleagues collected samples from the lower canopies of younger coast redwoods, whose branches are easier to reach than those of massive old-growth trees. Back in the laboratory, the team submerged the leaves in water and exposed them to a range of temperatures between 72 degrees and 110 degrees.
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