This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: California's iconic coast redwoods are used to growing in cool, coastal areas where the average temperature is around 59 degrees F (15 degrees Celsius). But as average yearly temperatures rise and heat waves push temperatures above 100 degrees F (38 degrees Celsius), the trees are responding by slowing the very process that nourishes them, according to a study from the University of California, Davis.
When under heat stress, the world's tallest trees slow photosynthesis, the chemical reaction that turns sunlight, water and carbon dioxide from the air into carbon the trees use to grow and perform other essential processes. It's as if the trees slow their "breathing" and, as a result, their "eating." The findings raise alarm about the fate of redwood forests, especially in their already vulnerable central, southern and eastern habitats in the state. The study, published in JGR Biogeosciences, is the first to examine the effects of heat on coast redwood photosynthesis, said lead author Lily Klinek, a Ph.D. candidate with the Plant Optics Lab in the UC Davis Department of Plant Sciences.
The team used an array of measurements taken over three summers in a redwood forest in Mendocino County. When air temperatures topped 86 degrees F (30 degrees Celsius), as they did during the summers of 2022 and 2024, some leaves were nearly 20 degrees F warmer than the air. "The leaves aren't able to cool down," Klinek said, which means they probably closed their stomata—the tiny, mouthlike pores that "breathe" in carbon dioxide from the air.
Further tests in the lab at temperatures ranging up to 110 degrees F (43 degrees Celsius) confirmed that photosynthesis was reduced, even when the leaves' "mouths" were open and taking in plenty of carbon dioxide. "We don't know what the upper temperature thresholds are for redwood photosynthesis and mortality," Klinek said. "With this work, we see there's a significant impact of heat on photosynthesis and physiology.
That's an important start for figuring out what parts of the redwood range are at risk, knowing what to expect and trying to intervene for the health of redwood ecosystems." Under the right circumstances, trees can regulate their temperature through a process called evapotranspiration. They use those tiny mouths, or stomata, to "sweat" out some of the moisture they hold inside to cool off. "It's similar to the way we sweat to stay cool," Klinek said.
Heat spikes not only raise air temperatures but also burn off the coastal fog that redwoods depend on for much of their water. The large gap between air and leaf temperatures indicated that the leaves likely closed their stomata to avoid losing precious water and overheated as a result, Klinek explained. In the lab, the researchers put their leaf samples in jars of water, so they had access to plenty of water and could sweat all they pleased.
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