The world’s hottest rainforest is located not in the Amazon or anywhere else you might expect, but inside Biosphere 2, the experimental scientific research facility in the desert outside Tucson, Arizona. A recent study of tropical trees planted there in the early 1990s reported a surprising result: They have withstood temperatures higher than any likely to be experienced by tropical forests this century.
The study adds to a growing tally of findings that are giving forest scientists something that’s been in short supply lately: hope. Plants may have unexpected resources that could help them survive—and perhaps even thrive—in a hotter, more carbon-rich future. And while tropical forests still face both human and natural threats, some researchers believe dire reports of their impending decline due to climate change may have been exaggerated.
“Biology is ingenious,” says Scott Saleska, an ecologist at the University of Arizona in Tucson and co-leader of the Biosphere 2 study. “It’s a lot more ingenious than our models yet represent.”
The last few years have seen a flood of alarming reports about forests and climate change’s effects on them. Scientists have announced that the Amazon forest is no longer a reliable carbon sink; the Amazon rainforest may be nearing a tipping point; tropical forests globally are already close to the hottest temperatures they can tolerate and climate change is killing off old trees.
One thing is certainly true: Our fossil fuel emissions are creating a climate that humans have never seen and trees haven’t experienced in a very long time. “We’re pushing tropical forests into temperatures they’ve never seen since the Cretaceous—since there were dinosaurs,” says Abigail Swann, an ecologist and climate scientist at the University of Washington in Seattle.
But predicting how trees will respond is a tough task. Subjecting entire forests to an experimental simulation of a hotter future is costly and logistically daunting. Scientists have mostly been forced to extrapolate from small-scale experiments or field observations, often using computer models to project to the coming decades.
A unique facility
Biosphere 2 provided a rare opportunity to put a full-sized forest to a climate test. Though best known for the crews of people who were sequestered inside between 1991 and 1994, the facility also houses artificial ecosystems. Among them is a roughly half-acre tropical rainforest inside a glass pyramid-shaped structure whose apex rises nearly 100 feet (30 meters) above the desert floor. The tops of trees planted there in the early 1990s now push against the ceiling.
Temperatures inside the structure exceed what even the Amazon—the world’s hottest tropical rainforest—is expected to see this century. In such sweltering conditions, plants in previous open-air studies have almost shut down photosynthesis, the biochemical process plants use to turn carbon dioxide into simple sugars they use for energy.
Data on the trees’ growth under different environmental conditions were logged in the early 2000s and stored on servers and hard drives. Ecologist Marielle Smith, a postdoctoral researcher at Michigan State University, saw in those records a rare chance to study a forest under a future climate.
She wanted to parse the effects of two related variables: temperature and vapor pressure deficit, or VPD—essentially, the difference between how much water the air can hold and how much it does hold at a given location and time. When VPD is high, plants lose water more quickly.
Normally, VPD rises nearly in lock step with temperature, because hot air can hold more moisture. But in the Biosphere, misters kept the air moist, creating a rare combination of high heat and low VPD. The CO2 level was stable at just over 400 parts per million, only slightly higher than the air outside at the time.
The Biosphere trees photosynthesized at the same rate until temperatures topped about 100 degrees Fahrenheit (38 degrees Celsius), Smith and her colleagues reported last month in Nature Plants. In natural forests in Brazil and Mexico, on the other hand, photosynthesis rates plummeted starting at just 82 degrees F (28 degrees C).
The result strikes a powerful blow, Smith and other experts say, against one popular hypothesis for why high heat shuts down photosynthesis—the idea that it directly disables the process.
Instead, high temperatures seem to harm plants indirectly, by raising VPD and thus the dryness of the air. Plant leaves take in carbon dioxide via mouth-like leaf cells called stomata, but those cells also let out water—up to 300 water molecules out for every CO2 molecule in. When VPD rises in response to a temperature rise, plants close stomata to hold onto life-sustaining water, even if it means forgoing food.
In our real world, it’s not just temperatures that are rising: Carbon dioxide is also going up, fast. That may help shield plants from the heat, Smith says: In the hot, high-CO2 future, stomata may be able to gobble up carbon dioxide, then slam shut to hold in water.
“It’s a somewhat hopeful result, which is not always what we see,” says Laura Meredith, a University of Arizona ecologist who leads research on the Biosphere 2 rainforest but wasn’t involved in the study. “It’s encouraging and hopeful that forests have strategies to help adapt and maintain efficiency.”
Smith admits, however, there’s still “a big if”: The Biosphere experiment didn’t include high CO2, so it couldn’t prove that plants will actually use it to conserve water. “The jury’s still out on whether this mechanism could actually happen,” she says.
More CO2? Not a problem.
Researchers in Panama are taking the next step, by testing whether high carbon dioxide levels really do protect plants from heat. So far, the answer seems to be a qualified yes.
Botanist Klaus Winter has built a half-dozen geodesic domes at the Smithsonian Tropical Research Institute’s research station near the Panama Canal. Winter’s domes are far smaller than Biosphere 2’s and hold only small trees, but they give him control of both carbon dioxide and temperature. In work he has presented at meetings but not yet published, he has found that at temperatures above those likely to be seen this century, well-watered plants bathed in carbon dioxide grew just fine. The growth of one species, the balsa wood tree, even skyrocketed.
While not a direct test of Smith’s mechanism, the experiment confirms that some trees can endure high temperatures if they get lots of CO2—and water, Winter says. “They are less susceptible than I thought.”
Winter’s colleague Martijn Slot has investigated a parallel question: whether plants can adapt to warmer conditions. Every plant has a preferred temperature range, which researchers map out using gas-sensing devices to measure photosynthesis at the leaf level while cranking up the heat.
Slot found that seedlings grown at 77 degrees F (25 degrees C) photosynthesized optimally. But when he grew the same plants at 95 degrees F (35 degrees C), that optimal point shifted up to around 86 degrees F (30 degrees C). The plants’ ability to change their internal physiology is an example of “plasticity,” increasingly seen as a botanical bulwark against changing conditions.
“Treating plants as kind of static and rigid in how they respond to environmental conditions leads to inaccurate or probably wrong predictions,” Slot says. “Plasticity should be accounted for” in computer models seeking to forecast the climate’s future.
Yet another recent hint of hidden resilience comes from the field. Flavia Costa, of the National Institute for Amazonian Research in Manaus, Brazil, analyzed 20 years of data from Brazilian forest monitoring plots. They included low-lying forests with ready access to groundwater, making them, like Winter’s plants, well watered. Costa’s team found that such “shallow-water-table” forests, which have been estimated to make up more than a third of the entire Amazon, grew unperturbed and continued socking away carbon through punishing droughts in 2005, 2010 and 2015.
Previous papers had sounded alarms that climate-driven droughts and accelerated growth and death rates were killing trees and hamstringing the Amazon forest’s ability to continue serving as a carbon sink. If wet forests throughout the Amazon are as resilient as those in the research plots, “the loss of productivity [and] the increase in mortality is probably overestimated,” Costa says.
Oliver Phillips, an environmental scientist at the University of Leeds who heads one of the major Amazon research networks, agrees that wet, low-lying forests appear to be more drought resilient than others. But his studies include such forests and he is unsure whether adding more would dramatically change their conclusions. He and Costa are now jointly analyzing their plot data to generate a more complete representation of Amazonian forests.
There’s a catch
All of these studies come with caveats and cautions.
Forests could face future droughts even more severe than any seen to date, which might stress even low-lying wet forests that have held up so far, says Costa. Studies that simulate forests, on the other hand, struggle to replicate the stunning diversity of real tropical forests, which could harbor both especially vulnerable trees and as-yet-undiscovered resilience mechanisms, she adds. The Amazon alone contains an estimated 16,000 tree species, far more than are represented in Biosphere 2, Winter’s domes or any computer model.
Winter’s plants, in addition, are still saplings, and he has kept them well watered. They may not do equally well in droughts—something Winter plans to study in his domes once coronavirus restrictions are lifted.
To Nate McDowell, an earth scientist at the Pacific Northwest National Laboratory in Richland, Washington who earlier this year warned in Science that climate change is already curtailing tree growth and carbon storage, Smith’s results are “encouraging,” but a key question remains unanswered: Can elevated carbon dioxide actually help plants cope with the drier air they will face in the future? “That’s a great science question,” McDowell says—“a burning science question.”
And even if high CO2 keeps plants alive, they may respond to heat by growing shorter but tougher, Smith adds, making her and McDowell’s studies potentially complementary rather than at odds. Indeed, the Biosphere forest has changed over its three decades, possibly due to the extreme conditions it has faced. Trees in the facility that produce a chemical called isoprene, which seems to help plants photosynthesize at high temperatures, survived better than those that don’t—a shift whose full implications are still being worked out.
“We might be inadvertently building a more resilient Amazon,” Smith says, “but one that will not necessarily be able to store the same amount of carbon.”