The tropics could get so hot that all leaves on rainforest trees die

A tiny proportion of leaves in the canopies of tropical forests are already passing the critical temperature threshold beyond which they cannot photosynthesise, resulting in their death. Modelling and experiments suggest the proportion of leaves affected in this way will rise exponentially as local temperatures continue to increase.

“We are predicting total leaf death,” says Christopher Doughty at Northern Arizona University. “If it was to occur, this would be a major tipping point.”

However, his team’s findings indicate that this tipping point is likely to be reached only in the worst-case warming scenarios, which are now thought to be implausible. “It doesn’t seem like we’re going to get to that, but it’s possible,” he says.

Lab studies show that when the leaves of rainforest trees reach a temperature of around 47°C (117°F), the cellular machinery that captures energy from sunlight is irreversibly damaged and the leaves usually die.

“It seems high,” says team member Martijn Slot at the Smithsonian Tropical Research Institute in Panama. “But leaf temperature can be a lot higher than air temperature.”

Desert plants can tolerate temperatures above 47°C, but in rainforests there are only small variations in heat tolerance between species, says Slot.

It was thought that no leaves in tropical forests were reaching their tolerance limit. But when they analysed measurements of plant temperatures by the ECOSTRESS instrument on the International Space Station from 2018 to 2020, Doughty and his colleagues found that around 0.01 per cent of leaves in the canopies of rainforests around the world are already reaching this threshold.

To confirm this, the researchers did a number of ground-based studies around the world, including placing temperature sensors on individual leaves in the upper canopy of rainforests. “This is incredibly challenging,” says Slot. “You come back, and a storm has ripped the sensors off or ants have eaten the tape.”

They then created a simple model based on these findings and on experiments involving warming plants. They concluded that the proportion of leaves affected will increase as local temperatures increase, rising more rapidly after reaching a tipping point between 2 and 8°C (3.6 and 14.4°F) of local warming, most likely at 4°C (7.2°F).

There are a number of reasons why the rise might accelerate, says Doughty. For instance, the pores of leaves, called stomata, will close during extreme heat and drought to prevent water loss. Without the cooling effect of evaporation through the stomata, the leaves warm rapidly.

In addition, once the most heat-exposed leaves start dying, others that were previously sheltered become exposed and die as well.

Continuing deforestation will make it more likely that local temperatures could increase to levels where lots of leaves start exceeding the limit, says Doughty. “Where you have fragmentation of forests, the existing forests get quite a bit warmer,” he says.

It is possible that the rising number of trees dying in the Amazon is due in part to this temperature threshold, he says. Recent studies also suggest that the Amazon has started releasing more carbon than it soaks up, leading to further warming.

“This paper is more evidence that we need to stop and reverse climate change as quickly as possible,” says Julia Jones at Bangor University in the UK, who wasn’t involved in the study.

The research shows that the local effects of deforestation, coupled with global climate warming, may already be resulting in regions becoming climatically unsuitable for rainforests, says Iain Hartley at the University of Exeter in the UK. “To preserve tropical forests and the critical ecosystem services they provide, action is needed both locally and globally,” he says.