GEOGRAPHY

The tropics get hot, and the leaves of the rainforest trees wither


If it’s too hot, the leaves of rainforest trees may wither Image Credit: Ghislain & Marie David de Lossy/Getty Images

A small percentage of the leaves of the rainforest have been subjected to temperatures above the critical threshold, which causes them to wither without photosynthesis. Modelling and experiments show that as local temperatures continue to rise, the proportion of leaves affected by this will increase exponentially.

Christopher Doughty of Northern Arizona University said: “We are predicting whole-leaf death. “If this happens, it will be a major turning point.”

However, his team’s findings suggest that reaching this tipping point is only possible under worst-case warming, which is now considered unbelievable. “Things don’t seem going to get out that way, but it’s possible,” he said. ”

Laboratory studies have shown that when the leaf temperature of rainforest trees reaches around 47°C, the cellular machinery for harvesting energy from sunlight is irreversibly disrupted, and the leaves usually wither.

Martijn Slot, a team member at the Smithsonian Institute of Tropical Research in Panama, said: “It looks tall. “But the temperature of the leaves can be much higher than the temperature of the air.”

Desert plants can tolerate temperatures above 47°C, but in tropical rainforests, there is little difference in heat tolerance between species, Slot said.

It is currently believed that not a single leaf in a tropical forest reaches its tolerance limit. But when the researchers analyzed measurements of plant temperatures by the International Space Station’s ECOSTRESS instrument from 2018 to 2020, Doughty and colleagues found that about 0.01 percent of leaves in rainforest canopies around the world had reached that threshold.

To confirm this, the researchers conducted ground-based studies around the world, including placing temperature sensors on individual leaves in the upper canopy of the rainforest. “It’s extremely challenging,” says Slot. “You come back and a storm ripped the sensor off, or the ants ate the tape.”

They then created a simple model based on these findings and related plant experiments, concluding that the proportion of affected leaves would increase with local temperature, rising faster after reaching the local warming cut-off point of 2°C to 8°C, which is likely to be around 4°C.

Doughty said there are many possible reasons for the acceleration of the rate increase. For example, the stomata of the blades close during extreme heat and drought to prevent water loss. If there is no cooling action by stomatal evaporation, the leaves warm up quickly.

In addition, once leaves exposed to high temperatures begin to die, other leaves that were previously shaded will also be exposed to high temperatures and die.

Doughty said continued deforestation would make it more likely that local temperatures would rise above the temperature threshold for large amounts of leaves. “Where forests are fragmented, existing forests become warmer,” he said. ”

The increase in the number of dead trees in the Amazon may be partly due to this temperature threshold, he said. Recent studies have also shown that the Amazon has begun to release more carbon than it absorbs, leading to further warming.

Julia Jones of Bangor University in the UK, who was not involved in the study, said: “This paper provides further evidence that we need to stop and reverse climate change as quickly as possible. ”

Iain Hartley, of the University of Exeter in the UK, said the research showed that local effects of deforestation, as well as global warming, may have made some areas climatetically unsuitable for tropical rainforests. “Protecting tropical forests and the key ecosystem services they provide requires action at local and global scales.”

The researchers published the findings in the August 23 issue of Nature. (Source: China Science News Guo Yueying)

Related paper information:https://doi.org/10.1038/s41586-023-06391-z



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