Source Attribution
This adapted feature originally appeared on Vox, as part of the collaborative Climate Desk journalism network.
The Hidden Heat Threat to Coast Redwoods, Earth’s Most Multi-Tiered Forest Ecosystem
A grove of coast redwoods is functionally a forest stacked inside other forests. Individual redwood specimens can live for thousands of years, and as they mature, their thick, interwoven canopies accumulate dense mats of soil, moss and fern colonies, forming a fully functional second forest floor hundreds of feet above the terrestrial ground. These elevated, self-contained ecosystems support a wide range of highly specialized life: the wandering salamander, for example, can complete its entire life cycle without ever leaving the top of a single redwood, while the nesting habits of the seabird known as the marbled murrelet remained a total scientific mystery until 1974, when a tree trimmer stumbled on one of their nests perched 150 feet up a redwood trunk. Even stray seeds from Sitka spruces, oaks, or other redwood trees often sprout high up on the massive redwood structure, growing into full, separate trees tucked between the giant’s thick boughs.
For millennia, redwoods have served as the unshakable anchors of this extraordinarily stable ecosystem, even surviving high-intensity wildfires that wipe out nearly all competing tree species. They have long been a source of profound cultural inspiration for humans, and today drive hundreds of millions of dollars into California’s tourism economy. But for all their legendary resilience, these ancient giants are far from immune to climate-driven rising temperatures — and researchers have just documented a little-known critical weak point in their biology.
The Newly Uncovered Heat Vulnerability
A 2-year study led by University of California, Davis researchers, which collected biometric measurements from redwoods twice per month, found that coast redwood foliage effectively shuts down most photosynthetic activity during extreme heat events. That means the trees slow down their core “breathing” and “feeding” processes, because they cannot cool themselves fast enough when temperatures spike. The problem will only worsen as uncurbed climate change pushes global average temperatures higher, and it raises urgent, unanswered questions about the long-term survival of these iconic trees, and the entire tiers of life that make their home on, under, and hundreds of feet above the redwood trunks.
To understand this dynamic, it helps to break down basic tree physiology. Redwood leaves, like those of nearly all plant species, use tiny pores called stomata for two core functions: pulling in carbon dioxide to fuel photosynthesis, and releasing excess water as vapor. When stomata are open, the tree is actively “breathing in” CO₂ and generating energy, but it is also losing large volumes of water to the surrounding air. When stomata close, the tree conserves its limited internal water stores, but it also cuts off its carbon dioxide intake, grinding photosynthesis nearly to a halt.
“Leaves are physical, light-absorbing structures, so they heat up exactly the same way a stretch of asphalt does sitting in full sun,” explained Lily Klinek, lead study author and UC Davis PhD candidate in plant biology. “They warm up far faster than the surrounding air. One of their key coping mechanisms is to open their stomata and let water transpire out of the leaf — this is the plant equivalent of human sweating. When water evaporates off our skin, it cools us down, and the exact same cooling effect works for leaves. But that cooling trick only works if the tree has enough stored water to spare.”
Klinek’s team studied coast redwoods in Mendocino County, near the southern edge of their native range, using specialized high-sensitivity instruments to track real-time photosynthesis rates in individual leaves. They observed that during heatwaves, redwoods quickly hit a threshold where they can no longer cool themselves fast enough, and lock their stomata shut entirely. While this adaptation keeps the trees from drying out completely, it also cuts off their new carbon intake — meaning they cannot “feed” or build up the energy reserves they need to keep growing.
A Cascading, Vicious Cycle for Redwood Forests
This might sound like a trivial, niche detail of plant physiology, but it could trigger cascading harms that reshape entire redwood ecosystems. The resulting vicious cycle would see trees run out of enough stored carbon to regrow their full set of leaves the following season, leaving them with less foliage to carry out photosynthesis, which cuts their energy reserves even further, until the tree eventually dies. This carbon starvation dynamic has been documented in tropical tree species before, but this is the first study to confirm the same response in coast redwoods.
“The term ecologists use for this is carbon starvation,” noted Chris Still, a forest ecologist at Oregon State University who was not involved in the UC Davis research. “Producing new foliage is extremely energy-intensive for a plant.”
Nearly all the coast redwood forests growing in California and the Pacific Northwest today are young new-growth stands planted over the last 200 years. Comparable to growing human adolescents, these younger trees have far higher energy demands than old-growth redwoods that have stood for millennia, requiring massive volumes of carbon dioxide to fuel their upward growth. Redwoods also have unmatched carbon storage capacity: thanks to their massive size and multi-millennial lifespans, they sequester more carbon per acre than any other forest ecosystem on the planet.
“If these trees slow their growth or stop storing as much carbon, that creates a negative feedback loop for the entire climate system, reducing one of our most powerful natural carbon sinks,” explained Anthony Ambrose, plant physiological ecologist and co-founder of the Ancient Forest Society. “On top of that, the trees simply won’t grow to their full legendary sizes anymore.”
Even if individual redwoods do not die directly from heat stress, their stunted growth will leave far less space on their canopies for the soil and fern mats that support the wandering salamanders, marbled murrelets and dozens of other unique canopy species. That stunted size also amplifies a second, overlapping climate threat.
The Double Blow of Heat and Vanishing Fog
Coast redwoods have co-evolved over millions of years to rely on a one-of-a-kind climate feature of their Northern California and Pacific Northwest range: regular coastal fog. Their towering heights let their canopies reach straight up into low-lying fog banks, where they can sip water directly out of the air. The fog also filters harsh direct sunlight and creates a cool, self-sustaining microclimate that makes old-growth redwood groves noticeably far cooler than the surrounding unforested areas. But rapidly warming ocean temperatures and drier atmospheric conditions have made this life-giving fog far more rare.
“It’s a perfect double whammy,” said Todd Dawson, plant ecophysiologist at the University of California, Berkeley, who also did not participate in the 2-year study. “They’re running out of available soil water, while also facing unprecedented heat stress. These trees evolved to live in an extremely narrow temperature band, between 8 and 27 degrees Celsius (46 to 80 degrees Fahrenheit). Like every other part of the world, California is now seeing extended stretches of far hotter days than any recorded in modern history, and the fog that used to act as their natural air conditioner has declined by 30 to 40 percent in recent decades.”
That dual stress ripples all the way up to the canopy ecosystems. “If they face repeated, extreme summer heat events,” Ambrose noted, “the canopy soil mats and fern colonies will dry out much more quickly. That leaves no viable habitat for the salamanders, seabirds and other specialized species that live hundreds of feet off the ground. At that point, you see cascading collapse across every layer of the whole ecosystem.”
The Uncertain Future of These Ancient Giants
There is one small, hopeful detail from the UC Davis study: the redwoods appear to rebound fully once heatwaves end. Over the two years of monitoring, Klinek’s team did not observe leaves that lost their photosynthetic capacity permanently after high heat exposure, though none of the foliage adjusted or acclimated to higher temperatures over the study period.
For now, the cool fall and winter seasons still bring temperatures back to baseline levels, letting the trees rebuild their energy reserves and recover from summer heat stress, Klinek explained. “But we have no idea if that recovery will still be possible 5 or 10 years from now, if summer temperatures keep climbing higher, and heatwaves become longer, more frequent, and more intense.”
Right now, the wandering salamanders still have their mossy, fern-covered canopy homes hundreds of feet above the ground, and the marbled murrelets still nest undisturbed on redwood boughs. These forests belong to all of us, even people who live thousands of miles from the Pacific coast: like all old-growth tree species, redwoods are a critical part of the planetary systems that keep Earth habitable for humans and every other living thing. The redwoods Klinek studied have the biological potential to live for more than 1,000 years. The only open question now is whether we will choose to let them.
