Leaves, like people, have limits to how hot they can get.
Scientists first reported in 1864 that the leaves of some plants could survive up to 50° Celsius, only to perish beyond that threshold. Researchers are still making similar discoveries more than 150 years after the first report. In 2021, researchers found in a study of tropical tree species that the average temperature above which photosynthesis is unable to occur was 46.7° C.
Now, in the upper canopies of Earth’s tropical forests, roughly One leaf in every 10,000 is a spruce.Researchers say that the temperature in August is too high to allow photosynthesis. Nature.
Scientists warn that while this amount may seem small, if climate changes continue, it could cause a breakdown of photosynthetic processes, which would harm entire forests. Simulations indicate that a rise in temperature of 4 degrees C could cause large swathes of tropical leaves to die. The researchers admit that their prediction is not without uncertainties.
“One small possibility that we’re suggesting … is an incredibly dire tipping point” beyond which tropical forests perish, Christopher Doughty, an ecologist at Northern Arizona University in Flagstaff, said at an August 21 news briefing. But “there’s a lot we don’t know.”
When leaves get too hot, their photosynthetic machinery — proteins that convert light energy into sugars — breaks down. Doughty, along with his colleagues, wanted to know if tropical forests were nearing this threshold. They obtained data from ECOSTRESS, a thermal sensor aboard the International Space Station, which captures vegetation temperatures on Earth’s surface in 70-meter pixels. That’s about the area that two large tropical trees could fill.
The team compared the data with measurements from devices on the planet’s surface. The team compared the data with measurements from devices on the planet’s surface.
The analysis revealed that forest canopy temperatures were a mosaic. During periods when forests were hot and their soils were dry, temperatures across the canopy could reach an average peak of 34° C. But there was variability; some tracts exceeded 40° C.
The comparison also revealed a detail unseen by ECOSTRESS — a scatter within the mosaic. In a single forest tract, leaf temperatures varied. Some leaves reached temperatures far above the tract average. About 0.01 percent of the time, upper canopy leaves sweltered at temperatures above the 46.7° C threshold, the team found.
Researchers also analysed data from leaf warming experiments conducted in Brazil, Puerto Rico, and Australia. These experiments showed that every degree of ambient warming has a disproportionate effect on leaf temperatures. For example, when Amazon leaves were subjected to an additional 2 degrees C of ambient warming, maximum leaf temperatures rose from 42.8° to 50.9° C.
Using satellite and earth-based data along with experimental data, the team simulated the future of tropical rainforests under climate changes. Simulations indicate that trees could lose their leaves before they die if the temperature rises by 4 degrees C. The researchers claim that under a worst case scenario where greenhouse gas emission continues to rise throughout the century, this level of warming may be possible as early as 2100.
Still, there’s a lot of uncertainty. That’s in part because the adaptive capabilities of different tree species and how the deaths of individual leaves impact a tree’s mortality aren’t well understood.
The study may even overestimate vulnerability by “assuming that when leaves hit this critical temperature, they die,” says ecologist Christopher Still of Oregon State University in Corvallis. That’s possible, he says, but we don’t fully understand how long it takes various temperatures to kill different species’ leaves.
Predicting the future of these forests will also require more insights into what’s unfolding beneath the canopy, says ecologist Marielle Smith of Bangor University in Wales. “There is still a question mark over the role of small trees and understory leaves, which aren’t going to be as hot.”
The Amazon rainforest may be the most vulnerable of all tropical forests to the reckoning predicted by researchers. “There’s more trees dying [there]There are more people now than there were ten years or twenty years ago. We don’t see that in Africa,” Doughty said. That could be because “temperatures are a bit hotter … in the Amazon than in Africa.”
Many researchers have warned that climate change, deforestation and terrorism could be triggered by the actions of some scientists for many years. Amazonia is a large part of the AmazonTo transform into savannas and shrublandSN: 6/16/23).
“This is a glimpse into a potential tipping point. It’s not saying that the tropical forests are now going to be savannas tomorrow,” study coauthor and ecologist Joshua Fisher of Chapman University in Orange, Calif., said at the briefing. “We can now see this insight … and because we can see that, it means we can act.”
