Tropical forests are not only threatened by deforestation or drought: global warming may also subject them to temperatures too high to allow photosynthesis, according to a study published Thursday.
To date, only a tiny percentage (0.01%) of leaves in the upper canopy have ever crossed the 47°C threshold blocking this mechanism that transforms sunlight and CO2 into vital energy for plants, says the study published in Nature.
But this percentage could quickly increase because the leaves heat up 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 press briefing.
With an average temperature rise of 4°C compared to the current climate — the trajectory generally referred to as a worst-case scenario — “we expect a total necrosis of these leaves,” he said.
This necrosis could become an additional factor in the announced transformation of part of the tropical forests into a savannah landscape, under the effect of climate change and deforestation.
If air temperatures were to continuously rise by 0.03°C per year, this massive leaf die-off could occur in just over a century, the researchers say. These were based on NASA satellite data measuring the temperatures of the plant cover coupled with observations on the ground, in particular thanks to sensors attached to the leaves.
However, there remains uncertainty about the effect of this overheating of the uppermost leaves on the forest as a whole, they warn.
“Believe it or not, we don’t know much about what causes trees to die,” said co-author Gregory Goldsmith, of Chapman University (California).
You don’t have to be a scientist to know that a tree dies if it loses its roots, he noted. But the effects of temperature or humidity on its health are less obvious and leaf necrosis does not necessarily mean the death of the whole tree.
However, worrying signs are already present. In the Amazon, which is warmer than other forests, the tree mortality rate has increased in recent decades. And the increased fragmentation of forests, due to deforestation, makes them even hotter.
However, tropical ecosystems are home to 45% of the world’s forests, which play a major role in absorbing humanity’s carbon emissions and are home to at least half of the world’s plant biodiversity, with probably more than 40,000 species of trees.
The discovery of a small proportion of overheated canopy is “a warning sign” that must prompt action, insisted co-author Joshua Fisher of Chapman University.
08/23/2023 21:41:01 – Paris (AFP) – © 2023 AFP