California’s Redwoods Face Heatstroke as Climate Warming Turns Up the Heat

When you stand beneath a coast redwood, you’re basically under a forest of forests. These trees can live for millennia, and their canopies become airborne gardens of soil, ferns, and even tiny seedlings that cling to branches like drifters on a wind‑blown sea.

That airborne world isn’t just scenery. It’s home to the wandering salamander, which can spend its whole life clambering up a single trunk, and the marbled murrelet—a seabird that, back in 1974, surprised a tree‑trimmer by nesting 150 feet above ground. Sometimes a stray seed—maybe a Sitka spruce or an oak—gets caught in the leafy maze and sprouts into a new tree perched among the giants.

For ages, redwoods have been the rock‑steady anchors of this delicate system. They shrug off wildfires that would raze other forests, they pull in tourists by the millions, and they lock away more carbon per acre than any other forest on Earth. But they’re not invincible. A fresh study from UC Davis shows they have a surprisingly weak spot when the thermometer spikes.

Researchers visited a Mendocino County stand—near the southern edge of the species’ range—and measured individual leaves twice a month for two years. What they found was unsettling: during hot spells, the trees essentially turn off their “breathing” by closing stomata, the tiny pores that let carbon dioxide in and water vapor out. It’s a bit like us shutting the windows during a sweltering day to keep the cool air in.

“Leaves heat up like asphalt in the sun,” explains Lily Klinek, the study’s lead author and a PhD student at UC Davis. “They can only cool themselves by letting water evaporate—like sweating. But if there isn’t enough water to lose, they just keep the pores shut.” The result? Photosynthesis slows, carbon intake drops, and the tree can’t “eat” enough to fuel growth.

That sounds like a small physiological tweak, but it could spiral. Less carbon means fewer resources to build new leaves, which in turn means even less photosynthesis—a feedback loop that may eventually starve the tree. Scientists call this “carbon starvation,” a term usually reserved for tropical trees, now observed in these temperate giants for the first time.

Most of the redwoods we see today are relatively young, planted over the last couple of centuries. They’re still in a growth‑hungry phase, needing lots of carbon to reach sky‑piercing heights. If heat stress curtails that intake, the whole forest could shrink, storing less carbon and offering less habitat for the myriad creatures that rely on the cool, fern‑covered understory.

Adding to the dilemma is the fog that has long been a lifeline for these trees. Their towering crowns plunge into low‑lying fog banks, sipping water straight from the mist and enjoying a natural shade that keeps the forest cooler than surrounding lands. But warming oceans and drier air are turning fog into a fickle guest, showing up less often and for shorter periods.

“It’s a double whammy,” notes Todd Dawson, a plant ecophysiologist at UC Berkeley who wasn’t involved in the study. “Redwoods are running out of water and getting hammered by heat at the same time. They evolved for a narrow temperature window—roughly 8 °C to 27 °C (46 °F‑80 °F). Anything outside that is stressful.”

If these trends continue, we could see a future where the iconic redwoods aren’t as tall, their canopy‑soil mats thin out, and the whole ecosystem feels the squeeze. The trees might survive, but they’ll be less effective carbon sinks and less hospitable homes for the salamanders, murrelets, and the countless other species that make these forests extraordinary.

In short, the giants of the West Coast are catching a heatstroke, and the cure will likely require more than just wishful thinking—it will need serious climate action, fog‑preserving strategies, and a renewed commitment to protecting the water that keeps these ancient sentinels alive.