Coast redwood groves are forests within forests. Individual trees can live for thousands of years, and as they grow, their canopies develop mats of soil and ferns, forming a second forest floor hundreds of feet above the ground.
These upland ecosystems are home to creatures like the wandering salamander, which spends its entire life cycle in a single tree, and the marbled auk, a seabird whose nesting habits remained a mystery until one day in 1974, when a tree trimmer stumbled upon a nest 150 feet above the ground. Sometimes a stray seed—a Thika spruce or oak, or even another redwood—climbs up a giant redwood and grows into another tree entirely, nestled among the branches of a giant.
For thousands of years, redwoods have been the backbone of this extremely stable ecosystem. They can even survive wildfires that destroy other tree species. They are also a source of immense human inspiration and generate millions of dollars in revenue for California’s tourism economy. But like other living things, they are not immune to rising temperatures, and it turns out these trees have a particular weakness.
A new study by UC Davis researchers, which took measurements twice a month over two years, found that coast redwood leaves essentially reduce their photosynthesis during periods of extreme heat. This means the trees are slowing down their “breathing” and “eating” because they can’t cool down fast enough when temperatures rise. The problem will only get worse as climate change continues to push up global temperatures, raising urgent questions about the future of these iconic trees and the layers of life that live in and around their shadow-dappled lengths.
In order to understand what’s happening to the redwoods, and what that means for the species that depend on them, first we need to take a quick look at tree anatomy. Trees use the stomata (stomata) in their leaves to do two things: take in carbon dioxide and expel water. Open stomata mean the tree is “breathing” carbon dioxide and performing photosynthesis, but may also be releasing water. Closing the stomata means the tree can conserve water, but it also slows down photosynthesis because it doesn’t absorb any carbon dioxide.
“Because leaves are a physical, light-absorbing object, they heat up like asphalt in sunlight,” said Lily Klinek, a doctoral student at UC Davis and lead author of the study.
“They heat up faster than the surrounding air,” she said. “One way they deal with this is by opening their stomata and allowing water to evaporate out of the leaves, a bit like how humans sweat. When water evaporates from our skin, we cool down, and so do the leaves. But they can only really do that if they have enough water to lose.”
Klinek and her colleagues studied coastal redwoods in Mendocino County, toward the southern end of their range, where the researchers used specialized instruments to measure whether individual leaves were photosynthesizing. They found that during heat waves, coast redwoods reach a point where they can’t cool down fast enough, and they simply keep their stomata closed. While this saves water, it also means the tree doesn’t absorb more carbon, meaning it doesn’t “eat” or accumulate energy stores that can be used to keep growing.
This sounds like a technical detail of tree physiology. But this could have a significant impact on these forests, creating a vicious cycle where the trees don’t have enough carbon stores to replenish future leaves, meaning they won’t be able to photosynthesize efficiently, which will starve them further, and so on, until the trees eventually die. While this behavior has been observed in tropical trees, this is the first time anyone has studied how coast redwoods respond to the same heat stress.
“The term people use is carbon starvation,” said Chris Still, a forest ecologist at Oregon State University who was not involved in the study. “Building leaves for plants can be expensive.”
Today, most of the redwoods in California and the Pacific Northwest are new forests planted over the past few centuries. This means that these trees, like growing teenagers, are relatively hungry compared to their older tree cousins, so they need to absorb large amounts of carbon dioxide to grow taller.
Sequoias are also excellent at storing carbon. Because of their size and longevity, they store more carbon dioxide per acre than any other forest in the world. “Not only does this create a negative feedback on the climate system itself, with these trees and forests sequestering less carbon, but the trees themselves obviously don’t grow as big,” said Anthony Ambrose, a plant physiology ecologist and co-founder of the Ancient Forest Society. “So even if the trees survive, they may not have enough room for ferns and soil mats, which means the organisms that live in them may have less habitat in the same way they did before.”
Klinek explains that the main threat to these trees, in addition to heat stress, is that they may lose an important source of hydration and protection: Coast redwoods have a complex relationship with fog in their native Northern California and the Pacific Northwest. These trees are so tall, with their canopies extending high into the fog layer, that they draw water directly from the sky. Fog also shades trees from the sun and helps coast redwoods create their own microclimate, which is why these forests are noticeably cooler than neighboring areas. But warming oceans and drying air are making fog more sporadic, meaning trees lose an important source of relief from hot weather that is only getting more intense.
“It’s kind of a double whammy,” said Todd Dawson, a plant ecophysiologist at the University of California, Berkeley, who was not involved in the study. “They’re running out of water and they’re facing heat stress,” he said. “They’re used to living in a narrow temperature range of 8 to 27 degrees Celsius (about 46 to 80 degrees Fahrenheit). Like elsewhere, California’s weather has been much warmer than we’ve experienced for a long time, and now the fog that’s often a bit like air conditioning is reduced by 30 to 40 percent.”
These temperature fluctuations can also affect organisms that live high in trees. “If they experience very hot weather all summer long,” Ambrose said, “then the soil mats and ferns may dry out faster. And then those salamanders and birds may not be able to live there. So at that point we’ll see knock-on effects throughout the ecosystem.”
Klinek said the trees seemed to be doing well after the hot weather passed; although she and her team observed that the leaves did not adapt to the heat during their two-year study, they did not lose the ability to photosynthesize after the heat passed. While this is hopeful news, it doesn’t diminish the urgency of better understanding the effects of climate change on these trees and reducing warming itself.
“Now that fall and winter temperatures are returning to normal, trees can recover,” Klinek said. “But five or 10 years from now, if temperatures are higher and heat waves are more frequent, we really don’t know if there will be a recovery.”
Currently, the home of salamanders and seabirds remains high in the coastal redwoods. But we’re also part of the larger ecosystem of redwoods, even if we don’t live near them—they, like all trees, are key to keeping our planet livable. The trees Klinek studies have potential of more than a thousand years. The question is whether we will let them do it.