Even After 20 Million Years, It’s Never Been Harder to Be the World’s Tallest Tree

Here’s what you’ll learn when you read this story:

• Coast redwoods (Sequoia sempervirens) are the largest trees in the world and exist only in a specific stretch of coastal habitat in northern California and southern Oregon.
• A new study finds that rising temperatures in the area could negatively impact the trees’ ability to perform evapotranspiration, a process where a leaf’s mouth-like pore, called a stomata, releases moisture to cool down.
• By analyzing leaf physiological data, gathered monthly across a two-year period, scientists found that under extreme heat, leaves could be nearly 20 degrees Fahrenheit warmer than the surrounding air temperature due to the breakdown of this crucial biological process.

Millions of years before some distant ancestor of Homo sapiens split off from gibbons, the coast redwood (Sequoia sempervirens) emerged from the mists of evolution. For millions of years, this mighty tree, the tallest in our planet’s history, spread across the northern hemisphere. But eventually, cyclic climatic changes forced it to retreat to the foggy banks of the northern California and southern Oregon coasts. Then humans—that restless ape lineage, now walking upright and wielding saws—pushed this incredible plant to the very brink, leaving only 5 percent of its natural range intact.

And now, thanks to our species’ insatiable desire for fossil fuels, the coast redwood faces its greatest challenge ever. A new study, published in the journal JGR Biogeosciences and led by scientists at the University of California (UC) Davis, discovered that rising temperatures in this coastal region is causing the breakdown of biological systems designed to feed and cool the trees. This new stress only adds to the trees’ growing lists of dangers, including wildfires. Although adapted to survive fearsome blazes, redwood forests are burning longer and hotter than ever before.

In the redwood’s 20 million-year-long history, it’s never been harder to be the world’s tallest tree.

“With this work, we see there’s a significant impact of heat on photosynthesis and physiology,” UC Davis’s Lily Klinek, lead author of the study, said in a press statement. “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.”

To better understand this growing threat, Klinek and her team gathered monthly measurements of redwood leaf physiology spread across two years (from July 2022 until October 2024). What they found is that during periods of heat stress, photosynthesis decreased sharply, and the tree’s ability to decrease the temperature of its leaves declined as well. According to the researchers, when temperatures reached 86 degrees Fahrenheit or higher, redwood leaves could be nearly 20 degrees hotter than the surrounding air temperature.

One of the most important biological components of leaves is the stomata, mouth-like pores that “breathe” by taking in carbon dioxide and releasing oxygen. As air temperatures climb, a redwood’s stomata open wider and “sweat” out moisture, cooling the leaf the way perspiration cools skin. But as higher temperatures burn off the fog banks that the trees use to gather moisture, stomata effectively close up shop to reserve what water remains in the tree itself, causing leaf temperatures to soar. In the lab, researchers kept leaf samples in jars of water so they had plenty to drink and could sweat all they pleased. Photosynthesis still dropped, even with the stomata wide open and taking in plenty of carbon dioxide. “Even if the redwood trees have more water to sweat with, the heat might still impair their breathing and functioning,” Klinek said in a press statement.

Redwood forests flourish in their foggy, coastal habitat due to abundant moisture—crucial when you’re a 300-foot-tall tree—along with average annual temperatures around 59 degrees Fahrenheit. Reports by the Scripps Institution of Oceanography and the U.S. Forest Service both project major increases in regional temperatures, including a 6.8 degree Celsius rise in annual maximum temperatures by the end of the century. This new study—the first to study the effects of heat stress on the photosynthesis of coast redwoods—establishes that these magnificent trees might have a thermal threshold for climate adaptation, and that similar problems could impact other forests in the U.S. and around the world.

“Taken together, these leaf-level responses align with evidence from recent studies suggesting that high temperature can suppress photosynthetic capacity,” the authors write. “The vulnerability observed in redwoods may be part of a broader trend in temperate forests, underscoring the need to consider thermal thresholds in climate resilience assessments.”