Tropical forests are approaching critical temperature thresholds

Paris (AFP) – Global warming is driving the tropical canopies’ leafy greens to temperatures close to where they can no longer convert sunlight and carbon dioxide into energy, a study published Thursday reported, threatening total collapse if the thermometer continues to rise.

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The study, published in the journal Nature, reports that a small percentage of upper canopy leaves actually exceed this threshold, reaching temperatures so high — more than 47 degrees Celsius — that they inhibit photosynthesis.

The researchers said some leaves currently exceed critical temperatures only 0.01% of the time, but the effects could build up quickly because leaves heat up faster than air.

“You’re heating the air by two to three degrees and the actual top temperature of these leaves goes up by eight degrees,” lead author Christopher Doughty of Northern Arizona University told reporters.

He said that if the average surface temperature of tropical forests rises by 4 degrees Celsius above current levels – which is widely considered a worst-case scenario – “we anticipate the potential for complete leaf death”.

The new research indicates that leaf death could become a new factor in the projected “tipping point” where tropical forests due to climate change and deforestation transition to savanna-like landscapes.

The study predicted that if air temperatures increased inexorably by 0.03°C annually, mass deaths among parachutes could occur in just over a century.

Doughty and his team used data from NASA’s ECOSTRESS satellite — designed to measure plant temperatures — that has been validated by ground-based observations, which rely in part on sensors attached to individual leaves.

Increased tree death

Scientists cautioned that there is still uncertainty about how high leaf temperatures will affect the forest as a whole.

“Believe it or not, we don’t know a lot about why trees die,” said co-author Gregory Goldsmith of Chapman University.

He said it didn’t take a scientist to know that when a tree loses its roots, it dies.

But the interactions and feedbacks between heat and drought – and water and temperature – on tree health in general are not clear.

Total leaf death may not necessarily mean the complete death of the tree.

The critical temperature at which leaves turn brown and die may also vary across species, depending on the size and thickness of their leaves and the breadth of their canopy.

But there are already worrying signs. And in the Amazon, where temperatures are higher than in other tropical forests, the rate of tree death has increased in recent decades.

“The Amazon region is currently experiencing higher levels of deaths than central Africa, and that may be due to the high temperatures we have seen there,” Doughty said.

It has also been shown that increased forest fragmentation due to deforestation makes remaining forest areas warmer.

Deforestation in the Amazon
Deforestation in the Amazon © Julia Han-Janicki, Jean-Michel Corneau/AFP

Tropical biomes contain 45% of the Earth’s forests, and play a large role in absorbing human-caused carbon pollution.

It also harbors half or more of the world’s plant biodiversity, with at least 40,000 different tree species, according to the Intergovernmental Panel on Climate Change (IPCC).

The fact that some leaves are overheating at current temperatures is “a canary in a coal mine,” said lead researcher Joshua Fisher, from Chapman University.

“You want to be able to detect something that’s going on before it becomes widespread,” he said.

“The fact that we can do that now gives us the ability to do something as a collective community.”

Scientists not involved in the study said it should serve as a warning that nature’s ability to adapt to climate change has limits.

“It is true that trees and other plant species can absorb emissions and provide cooling,” said Leslie Mabon, Lecturer in Ecosystems at the Open University.

“However, this study demonstrates that without concerted action by humans to reduce emissions and limit global warming at the same time as protecting and enhancing nature, some of nature’s functions may begin to break down at higher temperatures.”

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