Tropical forests are threatened by temperatures incompatible with photosynthesis

Tropical forests are threatened by temperatures incompatible with photosynthesis

Tropical forests aren’t just threatened by deforestation or drought, according to a study published on Thursday: Global warming could also expose them to temperatures that are too high to allow for photosynthesis.

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So far, only a tiny percentage (0.01%) of leaves in the upper canopy have ever exceeded the temperature threshold of 47°C, blocking this mechanism that converts sunlight and CO2 into vital energy for plants, according to the study, published in Nature .

But that percentage could increase quickly as the leaves warm faster than the air.

“If you warm the air two to three degrees, the temperature of the surface of these leaves increases by eight,” said Christopher Doughty of the University of Northern Arizona during a news conference.

With an average temperature increase of 4°C compared to the current climate – the trajectory commonly referred to as the worst-case scenario – “we expect complete necrosis of these leaves,” he said.

This necrosis could be an additional factor in the announced conversion of part of the tropical forests into a savannah landscape under the influence of climate change and deforestation.

If air temperatures continued to rise by 0.03°C per year, this massive dieback could occur in just over a century, the researchers say. These were based on NASA satellite data measuring plant-cover temperatures, as well as observations on the ground, thanks in particular to sensors attached to the leaves.

45% of the forests

However, there remains uncertainty about the impact this overheating of the top leaves will have on the forest as a whole, they warn.

“Believe it or not, we don’t know much about what causes trees to die off,” said co-author Gregory Goldsmith of Chapman University (California).

You don’t have to be a scientist to know that a tree dies when it loses its roots, he noted. However, the health effects of temperature or humidity are less obvious and leaf necrosis does not necessarily mean death of the entire tree.

However, there are already worrying signs. In the Amazon, which is warmer than other forests, tree mortality rates have increased in recent decades. And the increasing fragmentation of forests due to deforestation makes them even hotter.

However, tropical ecosystems host 45% of the world’s forests, which play an important role in absorbing humanity’s carbon emissions, and host at least half of the world’s plant biodiversity, with probably more than 40,000 tree species.

The discovery of a small proportion of overheated tree canopies is “a warning sign” that prompts action, said co-author Joshua Fisher of Chapman University.