In the world’s hottest forests, scientists are probing how plants cope with rising temperatures

It was an experiment ahead of its time. Working in his laboratory in Breslau, Prussia, botanist Julius von Sachs placed a large glass bell jar over a flowerpot and heated it from below with alcohol burners. The pioneering 19th century physiologist measured the rising temperature and watched for signs that the plant was in distress, repeating the process with various species. It was the first effort to determine the hottest temperature plants could survive. His finding—50°C—must have seemed unimaginably hot.

Sachs, now best known for his work on photosynthesis and plant nutrition, was simply curious. But today, as global temperatures climb and ecosystems are hit by heat waves, the question of how plants respond to extreme heat has taken on practical importance. Johan Uddling, a plant ecophysiologist at the University of Gothenburg, says a global experiment is underway, with “potentially devastating” consequences. “You could lose a lot of species that are on the edge,” adds Lina Mercado, a plant ecophysiologist at the University of Exeter.

Perhaps the biggest challenge confronting plants will be in the tropics, where temperatures are already high—and are projected to rise as much as 4°C by the end of the century if more isn’t done to curb climate change. “They’re the hottest forests,” says ecologist Kenneth Feeley of the University of Miami. “So, the question is: What happens when we go into unprecedented heat?”

Scientists already see signs that plants are responding to the roughly 1.5°C of global warming so far. Some tree species in Mexico, for example, have shifted to higher elevations, where temperatures are cooler, a recent paper in Science reported). But across the Americas, tropical species aren’t moving fast enough to keep pace with warming, and researchers are skeptical that they will be able to cope by migrating upslope or away from the equator. Nor is evolution likely to be the answer. Trees can take decades to start reproducing, making it unlikely they can evolve new genetic adaptations for heat tolerance fast enough to withstand rising temperatures.

That means that over the coming decades, the fate of tropical forests may hinge in part on how well individual plants can acclimate to warmer temperatures by adjusting aspects of their structure or physiology. The question is especially urgent for the tallest species. “The trees that make up the canopy right now, during their lifetimes, they’re going to see changes in temperature that are quite significant,” says Martijn Slot, a plant ecologist at the Smithsonian Tropical Research Institute.

To find out what plants are capable of, researchers are going far beyond Sachs’s simple experiment. They are manipulating temperatures in plots of forest to push tropical species beyond their comfort zone. They are probing the traits of plants in a naturally superheated forest. And they are transplanting trees into warmer habitats to see how they fare.

The work has already revealed hints of resilience. “We know that plants can handle a little bit of warming,” says Slot, who has measured how temperature affects photosynthesis in dozens of species. “They have the machinery to make adjustments that allows them to acclimate to gradual changes.” But whether tropical forests can withstand the surge of warming likely in the coming decades remains a focus of feverish research.

THE STAKES extend well beyond the plants themselves. In the world’s megadiverse tropical forests, many species depend on the shade and microhabitats created by gargantuan trees that can grow 100 meters tall. The planet, too, depends on tropical forests; they hold as much as half of all the living carbon on land and continually suck in carbon dioxide (CO₂) emitted into the atmosphere by human activities. “If you start to see trees die, that’s going to matter hugely,” says Christopher Doughty, an ecologist at Northern Arizona University.

Plants rely on photosynthesis for energy, and one worry is that as temperatures ramp up, their photosynthetic machinery won’t be able to cope. The enzymes and membranes that are key to capturing photons and creating sugar are generally tuned to the range of temperatures a species experiences. If the plants get much hotter or colder, photosynthesis becomes less efficient.

In tropical species, photosynthesis starts to decline around 31°C. By 47°C, the photosynthetic machinery begins to be permanently damaged, according to measurements by Slot and colleagues. Eventually, cellular membranes start to leak and enzyme activity grinds to a halt. Chlorophyll, the green photosynthetic pigment, breaks down and leaves turn brown and die. Upper canopy leaves in tropical forests currently exceed 47°C only 0.01% of the time, but that number could jump to 1.3% with only a few degrees of warming, according to a 2023 paper published in Nature.

Another problem is that as temperatures increase, cellular concentrations of highly reactive oxygen-bearing molecules begin to build up, which can cause oxidative damage and disrupt cellular function. One reason for the proliferation is that heat increases the speed of the chemical reactions that generate these troublesome molecules.

Plants do have ways to cope with heat stress, however (see graphic, below). Increasing the production of isoprene, a volatile organic chemical found in many plants, for example, can protect cells against reactive oxygen molecules. Plants can also tune their photosynthetic machinery to perform better at higher temperatures. One way is for leaf cells to make more saturated fatty acids to stabilize parts of the chloroplast, the organelle in photosynthetic cells where sugar is produced, that would otherwise become less efficient.

Plants can lower their own temperature as well. One strategy is to grow smaller, thinner leaves, which can dissipate heat more effectively than larger, thicker ones. Another involves opening tiny pores in their leaves, called stomata, which allows water to evaporate. That can be an effective strategy if the plant isn’t deprived of water. But during droughts, plants often keep their stomata closed to prevent water loss. “When you combine the higher temperature with droughts, then things get really stressful,” Slot says.

To find out whether any of those strategies might help tropical forests survive a hotter future, scientists have turned to a spot that offers a preview: the Boiling River in eastern Peru, locally known as the Shanay-timpishka. Across a few square kilometers, superheated groundwater rising from deep faults pushes temperatures as much as 11°C above those of nearby areas. The air is so hot and humid near the river, which drains the scalding groundwater, that visitors are at risk of heat stroke.

Walking into the hottest part of the forest for the first time, ecophysiologist Alyssa Kullberg, now a postdoc at the Swiss Federal Institute of Technology, was struck by how oddly uniform it looked. Far fewer tree species grew there than in nearby, cooler parts of the forest, she thought—an impression that was later confirmed by a study in Global Change Biology. And the soil was strangely bare, lacking lush plants and wildlife. “You don’t see a lot of animals, and you barely hear birds,” she says. Sometimes dead frogs and snakes could be seen through the steam wafting off the river, floating by after presumably falling in and dying from the heat. “It’s a weird forest, but very interesting.”

Over the past several years, Feeley, Kullberg, and colleagues have studied six tree species that are able to live near the river. They didn’t see obvious adjustments for heat. Compared with individuals of the same species growing elsewhere, the trees in the hottest locations don’t have smaller, thinner leaves or other physical traits that might help them dissipate heat, such as a greater density of stomata, they reported in New Phytologist in 2023.

But the team did find signs of heat tolerance after clipping fingernail-size disks from leaves and probing their photosynthetic ability. At the tourist lodge where they were staying, Kullberg and colleagues immersed each disk in water ranging from 28°C to 68°C. Then they probed each sample with a handheld fluorometer, which shines light on the leaf and detects the photons emitted in response by its chlorophyll.

The team found that for three of the six species, photosynthesis in leaves from trees growing near the Boiling River could tolerate higher temperatures. For example, in Cecropia membranacea, a small tree known locally as the yarumo, they found an increase in a measure known as T50—the point at which photosynthesis is impaired by 50%. Leaf samples taken from a cooler part of the forest could tolerate only 47°C before reaching T50, but a leaf from the hottest area had a higher T50, 51°C. That’s a good sign, because it means the species has the ability to respond to future warming. “Maybe it won’t have a significant impact on their vulnerability,” Kullberg says.

Feeley’s team is continuing to study the forest, trying to figure out exactly how the toughest species tolerate the extreme environment and what their resilience bodes for the same species elsewhere as temperatures rise. “I think we’re just really scratching the surface,” he says.

The forest near the Boiling River has had thousands of years to adjust to the geothermal heat. Tropical forests elsewhere are warming on a far faster timescale. What happens to species that are suddenly thrust into heat? Researchers in several countries are seeking answers by planting trees adapted to cooler, high-elevation conditions in lower elevation areas. Uddling and a large team did that in Rwanda, adding rain shields and irrigation pipes to ensure that moisture in the new sites matched the species’ normal habitat.

When the trees were a few years old, the team measured their rate of photosynthesis in the field under a range of temperatures. The key data came from a portable gas analyzer, which when clamped onto a living leaf determines the amount of CO₂ flowing into its tissue. Two higher elevation species showed no increase in their optimal temperature for photosynthesis after growing in the lower plots, which are 5°C warmer on average, the researchers reported in Global Change Biology in 2021—a result that is contrary to the hints of photosynthetic acclimation seen along the Boiling River. “That’s not that good news,” says Myriam Mujawamariya, a plant ecophysiologist at the University of Rwanda who works on the project.

A later study on 16 species showed that in slower growing species, which typically make up the larger trees in older forests, photosynthesis was 30% lower for plants in the hotter plots compared with those at higher elevation. The finding, reported in Tree Physiology in 2023, doesn’t bode well for the future of old-growth tropical forests, which store more carbon than any other forest type.

Elsewhere, scientists have been actively applying heat to natural forests. In the Luquillo Experimental Forest in Puerto Rico, Tana Wood, an ecologist with the U.S. Forest Service, and colleagues set up infrared panels to warm 4-meter-wide plots of soil and understory plants by 4°C. The experiment began in 2016; since then, the team has monitored the growth of seedlings to see whether the composition of species changes in the heated plots. They’re also periodically collecting leaf samples to look for evidence that the plants are adjusting to the heat.

So far, they’ve seen mixed signs of acclimation. In one species the optimum temperature for photosynthesis increased somewhat, but in another it decreased, the team reported in Frontiers in Forests and Global Change in 2020. The researchers also extended the experiment to the canopy, which is too high to be warmed by panels near the ground, by adding small heaters to individual leaves near the treetops. They found a similar lack of photosynthetic acclimation there, as they reported in 2021 in Plant, Cell & Environment.

Kristine Crous, a plant ecophysiologist at Western Sydney University, has made similar measurements with leaf heaters in tropical forests in Australia. In 2024, she reported in New Phytologist that after 8 months of being heated 4°C above ambient temperatures, the leaves of four species had decreased their photosynthesis by as much as 35%. “Unfortunately, the tree species that are longest lived in old-growth rainforest, they’re the sensitive ones,” she says.

More encouraging findings come from Biosphere 2, a massive set of greenhouses completed in 1991 outside Tucson, Arizona. Inside the largest space, tropical trees grow up to 27 meters high.

In a 2020 paper in Nature Plants, a team led by Marielle Smith, an ecosystem ecologist now at Bangor University, examined changing CO₂ levels inside the greenhouse under a range of temperatures. The researchers found that even when temperatures reached 43°C, photosynthesis was not impaired. They concluded that the biosphere’s plants could withstand the heat because they had plenty of water, allowing them to cool their leaves by opening their stomata. It’s unclear whether the plants would fare as well in natural environments, where heat often brings water stress.

Still, Joost van Haren of the University of Arizona, who directs rainforest research at Biosphere 2, sees “a positive story so far.” Last summer, he and colleagues raised temperatures inside the greenhouse even more by turning off the cooling system. After clipping tiny fluorometers onto leaves and measuring photosynthesis at various air temperatures through the canopy, they found that species appear to be adjusting their photosynthetic capacity to withstand higher temperatures, van Haren says.

As researchers sift through evidence from various studies that have challenged the heat tolerance of tropical trees, a key question looms large: “How much is too much and when does the tree die?” Crous asks.

Several species appear to have the ability to bounce back, even after a severe heat wave. During van Haren’s experiment in Biosphere 2, one part of the canopy reached 55°C, much hotter than during the conditions examined by Smith and colleagues. That was more than the leaves could handle, and they died. But they have since grown back. “We’re seeing that there might be more resilience at Biosphere 2 than people have given tropical forests credit for,” van Haren says.

Duration is probably an important factor. Wood suspects tropical species can cope with heat in the short term. The plants under the heating panels in Puerto Rico, she notes, don’t die more often. But problems could mount over time if photosynthesis remains depressed. “It’s like you run out of fuel to keep managing,” she says. In her plots, she and her colleagues found that after a few years of warming, the understory plants weren’t growing as tall.

Researchers also point out that heat doesn’t strike in isolation. More intense storms and drought are also stressing the forests, which could compound the effects of warming. In Puerto Rico, for example, plant roots in the hotter plots recovered more slowly after a hurricane ravaged the forest. “That was a light bulb moment for me,” Wood says, because it suggests warming makes plants less able to cope with additional disturbances.

The fate of tropical forests in a warmer future remains uncertain. One thing is not in doubt: The planet’s lushest ecosystems are all now in Sachs’s bell jar, with humanity, an unwitting experimenter, pumping in the heat.

Cover photo:  Some experiments have employed heating devices to push plants beyond their comfort zone.