How heatwaves are shutting down photosynthesis in forests

Trees have a breaking point, and a new study from the Swiss research center EPFL suggests the world's tropical forests are hitting it more and more often. And the consequences are impacting the entire planet's climate.

Just as heatwaves make it harder for the human body to function, extreme heat can also shut down the biochemical machinery inside leaves. When temperatures climb too high, the process of photosynthesis — through which plants make their own food, release oxygen, and pull carbon dioxide out of the air — starts to break down. That means slower growth and a higher risk that the plant dies altogether.

The new research, published in the Proceedings of the National Academy of Sciences and led by EPFL's Charlotte Grossiord and Devis Tuia, is among the first studies to combine species-specific heat thresholds with large-scale satellite data to measure heat stress across tropical forests.

A shrinking safety margin

Plants aren't as fragile as they might seem — but their cushions are thinner than they used to be. “There is a safety margin of about 15 degrees in which plants can still do photosynthesis. Due to global warming, droughts, and extreme events, this margin has narrowed significantly,” says Grossiord, who leads EPFL's Plant Ecology Research Laboratory.

The problem starts at the molecular level. "When leaves become too hot, the proteins that drive photosynthesis begin to break down. As a result, trees absorb less carbon dioxide and grow less efficiently," Grossiord explains.

Bigger than France — and getting bigger

To measure the scale of the problem, the team compared known heat-tolerance thresholds for 200 tree species with satellite temperature readings collected across tropical forests between 2001 and 2020.

The results are striking. Over those two decades, the area of tropical forest where treetop temperatures exceeded the average critical threshold grew from 43 million to 57 million hectares — an area larger than France.

And it's set to get worse. The researchers project that by 2050, 83 million hectares of tropical forest could be pushed beyond the critical temperature threshold. By the end of the century, that figure could reach 160 million hectares — an area bigger than South Africa.

Why this matters far beyond the tropics

Losing individual trees is only part of the story. As heat-sensitive species die off, forest composition shifts, which can ripple through ecosystems, reducing animal biodiversity and leaving forests less able to withstand future heatwaves and droughts.

The stakes are global because tropical forests are among the planet's most important carbon sinks. If their capacity to absorb CO_₂ continues to decline, global warming could accelerate. At the same time, these forests release less water vapor into the atmosphere as they struggle, which raises the risk of droughts and extreme weather elsewhere in the world.

There is a glimmer of adaptability, though. Climate change, however fast, is still unfolding on a timescale that allows some species room to adjust. "Within the same forest, more heat-tolerant species may adapt and gradually replace others that disappear," Grossiord notes — though scientists still don't know how fast that turnover can happen, or what temperature would be too much for even the toughest species.

Not just a tropical problem

While the study zeroed in on the tropics, recent heatwaves have already pushed some Swiss trees and crops past their own thermal limits — a sign that heat stress on plants isn't confined to the equator. The upside is that the same satellite-based method could help identify vulnerable areas closer to home. "We can detect those areas that are at risk and prepare for the coming decades," Grossiord says, pointing to measures like targeted irrigation.

The team now wants to expand their analysis well beyond the roughly 200 species studied so far — out of more than 15,000 species living in tropical forests — since, as Tuia points out, "rare species can play disproportionately important roles in how ecosystems work." They also plan to extend the approach to other regions, including Europe's Mediterranean forests, which have already been hit hard by heatwaves and drought.