Tropical forests are being increasingly pushed beyond their threshold limits, affecting their ability to perform photosynthesis and thus their capacity to absorb carbon dioxide, a new study has found.
The study, published in the Proceedings of the National Academy of Sciences, noted that plants have a safety margin of about 15 degrees Celsius where they can perform photosynthesis. But extreme weather events along with droughts and global warming narrow down their safety margins drastically.
Human bodies get affected by heat waves, making them unable to function efficiently. Similarly, trees reach their limits as temperatures exceed beyond tolerance capacity.
“Above a certain critical temperature, photosynthesis, the process by which plants produce their own nutrients, release oxygen and absorb CO2 from the atmosphere, starts declining, reducing growth and increasing the risk of plant mortality,” said a statement issued by the researchers.
In the study by École Polytechnique Fédérale de Lausanne (translated as the Swiss Federal Institute of Technology in Lausanne), EPFL, researchers compared the critical temperatures cataloged for 200 species with satellite-derived temperature measurements collected across tropical forests between 2001 and 2020.
“Over two decades, the area of tropical forest experiencing treetop temperatures above the average critical threshold increased from 43 million hectares to 57 million hectares, an area larger than France,” they noted.
Indicating a grimmer situation, they pointed out that by 2050, it is estimated that the area of tropical forests exposed to exceed the critical temperature could be over 93 million hectares, while by 2100, this numbers could nearly double to 160 million hectares - an area larger than South Africa.
The study is among the first to combine species-specific thermal thresholds with large-scale satellite observations to assess heat stress across tropical forests.
The authors said though the study focused on tropical forests, recent heat wave events reportedly pushed some Swiss trees and crops to exceed their thermal tolerance, suggesting that heat stress is not limited to tropical areas.
“We can detect those areas that are at risk and prepare for the coming decades,” said Charlotte Grossiord, tenure-track assistant professor at the Plant Ecology Research Laboratory, who led the study.
The authors said thermal stress impacts biochemical reactions in leaves in a way similar to how extreme heat affects biological functions in humans. “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 says.
Rising temperatures cause more dramatic loss of some plant species, the authors noted, adding that alteration in forest composition can disturb ecosystem functioning and reduce animal biodiversity overall. This can deteriorate resilience to future heat waves and droughts.
“Tropical forests are among the world’s most important carbon sinks. As their capacity to absorb CO2 declines, global warming could accelerate. At the same time, tropical forests will also release less water vapour into the atmosphere, thus increasing the risk of droughts and extreme events worldwide,” the authors warn.
However, they noted that while climate change is accelerating, it allows room for some species to adapt to changing conditions.
Within the same forest, high heat tolerant species may adapt and eventually replace the others which disappear, Grossiord explained.
It however, remains largely unknown as to how quickly these shifts are achieved and the temperatures beyond which the plants can no longer adapt to survive.