Redwoods are natural magic kingdoms. Extreme heat is breaking their spell.
Redwoods are natural magic kingdoms. Extreme heat is breaking their spell.
A grove of coast redwood trees is a forest of forests. Individual trees can live for thousands of years, and as they grow, their canopies develop mats of soils and ferns, creating something like a second forest floor hundreds of feet above the ground.
These elevated ecosystems are home to creatures like the wandering salamander, which can spend its entire life cycle on top of a single tree, and the marbled murrelet, a seabird whose nesting habits were 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 sitka spruce or oak or even another redwood — finds its way up a massive redwood and grows into another tree entirely, nestled among the branches of a giant.
For millennia, redwoods have been the anchors of this incredibly stable ecosystem; they can even survive wildfires that devastate other tree species. They’ve also been the source of plenty of human inspiration and drive millions of dollars to California’s tourist economy. But like other living things, they aren’t immune to warming temperatures, and it turns out these trees have a particular weak spot.
A new study from researchers at University of California Davis, using measurements taken twice a month for two years, found that coast redwood leaves essentially dial down their photosynthesis during periods of extreme heat. This means trees are slowing their “breathing” and “eating” because they can’t cool down fast enough when temperatures rise. It’s a problem that’s only going to get worse as climate change continues to drive global temperatures up, and it raises urgent questions about the future of these iconic trees and the layers of life that live on and around their shadow-dappled lengths.
How a redwood fasts
To understand what’s happening to the redwoods — and what that may mean for the species that rely on them — first we need to take a quick foray into tree anatomy. Trees use the pores in their leaves, stomata, to do two things: take carbon dioxide in, and let water out. Open stomata mean the tree is “breathing” in carbon dioxide and photosynthesizing, but probably also letting water out. Closed stomata mean the tree is conserving water but also slowing down its photosynthesis, since it isn’t taking in any carbon dioxide.
“Because leaves are a physical, light-absorbing object, they heat up just like asphalt would in the sun,” said Lily Klinek, a PhD student at UC Davis who was the lead author on the study.
“They’re heating up faster than the air around them,” she said. “And one way that they can cope with this is by opening their stomata and allowing water to transpire out of the leaf, which is kind of like the way that humans sweat. When water evaporates off our skin, it cools us down, and it’s the same for leaves. But they can only really do that if they have enough water to lose.”
Klinek and her colleagues studied coast redwoods in Mendocino County, near the southern part of their range, where the researchers used specialized instruments to measure whether or not individual leaves were photosynthesizing. During heat waves, they found, coast redwoods hit a point where they can’t cool down fast enough, and they just keep their stomata closed. While that saves water, it also means the tree isn’t taking in more carbon — which means it’s not “eating” or building up energy stores it can use to keep growing.
This may sound like a technicality of tree physiology. But it could have a big impact on these forests, creating a vicious cycle where trees don’t have enough carbon stored to replenish their leaves in the future, which means they won’t be able to photosynthesize as efficiently, which will starve them even further, repeating until trees eventually die. And while this sort of behavior has been observed in tropical trees, it’s 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 didn’t participate in the study. “Leaves can be expensive to build for a plant.”
Most of the redwoods in California and the Pacific Northwest nowadays are new-growth forests planted in the last couple of centuries. That means the trees are relatively hungry, like growing adolescents, compared to their old-growth cousins, so they need to suck in lots of carbon dioxide to grow taller.
Redwoods are also incredibly good at storing carbon. Thanks to their bulk and long lives, they store more carbon dioxide per acre than any other forest in the world. “Not only is it negative feedback on the climate system itself, with less carbon sequestration from these trees and forests, but the trees themselves are going to obviously not grow as big,” said Anthony Ambrose, a plant physiological ecologist and co-founder of the Ancient Forest Society. So even if the trees survive, they might not have as much space for fern and soil mats, which means less habitat for the creatures that live in them, and they may not grow as tall as they once used to, which could hurt them in another way.
The fog of old forests
A central threat to these trees, Klinek explained, is that in addition to the heat stress, they may also lose a crucial source of hydration and protection: Coast redwoods have an intricate relationship with the fogs of their native Northern California and the Pacific Northwest. The trees are so tall that their canopies reach high into fog layers, where they can sip water directly from the sky. The fog also shades the trees from the sun and helps coast redwoods create their own microclimate, which is why these forests are noticeably cooler than the areas adjacent to them. But warmer oceans and drier air are making fog more sporadic, which means the trees are missing a crucial source of relief from heat 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 also didn’t participate in the study. “They’re running out of water and also getting this heat stress,” he said. “They’re used to living in a very narrow temperature range of 8 to 27 degrees Celsius (about 46 to 80 degrees Fahrenheit). California, like everywhere, has been getting long periods of much warmer days than we’ve ever experienced, and now the fog, which is normally kind of like an air conditioner, has also declined between 30 and 40 percent.”
These temperature swings also impact the organisms living high up in the trees. “If they experience really excessive heat throughout the summer,” Ambrose said, “then the soil mats and ferns will probably dry out much faster. And then those salamanders and birds might not be able to live there. So we’ll see cascading effects through the whole ecosystem at that point.”
Klinek said, the trees seem to be doing okay after the heat passes; while the leaves she and her team looked at over their two-year study didn’t acclimate to the heat, they also didn’t lose their capacity to photosynthesize after the heat passed. While that’s a bit of hopeful news, it doesn’t reduce the urgency of the need to better understand the impact of climate change on these trees, and to reduce the warming itself.
“Right now the temperatures are coming back to normal in the fall and winter and letting the trees recover,” Klinek said. “But in five or 10 years, if temperatures are even hotter and heat waves are even more frequent, we don’t really know if there would be a recovery.”
For now, the salamanders and seabirds still have their homes high atop the coast redwoods. But we’re a part of the redwoods’ larger ecosystem too, even if we don’t live near them — they, like all trees, are key to keeping our planet hospitable. The trees Klinek studied have the potential to live for more than a thousand years. The question is if we’ll let them.
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