Global warming could push tropical forest leaves past a ‘critical temperature’

Temperatures high enough to cause “irreversible damage” to a plant’s ability to photosynthesise are already impacting a small portion of tropical forest leaves, new research finds. 

The study, published in Nature, combines ground, satellite and modelling data to assess how close the leaves in tropical forests around the world are to reaching their heat limits. 

The research finds that 0.01% of upper canopy leaves already exceed a “critical temperature” for photosynthesis at least once per year – potentially resulting in a “tipping point” in their ability to function. 

This could increase to 1.4% under future warming conditions, the researchers say. 

Although these percentages are small, the researchers write that more global warming could “tip the forest towards the death of all leaves and possible tree mortality”, if leaves die and other knock-on effects occur at high temperatures.

The “alarming” findings are tempered by the fact that “we still have the opportunity” to tackle rising temperatures, an expert who was not involved in the study tells Carbon Brief. 

Absorbing CO2 

Tropical forests perform the vital roles of absorbing billions of tonnes of carbon dioxide (CO2) and supporting at least two-thirds of the world’s biodiversity.

Climate change and deforestation already negatively impact these ecosystems in a number of different ways. 

One 2020 study found that tropical forests are losing their ability to remove CO2 from the atmosphere. The forests face “substantial carbon loss” by the end of this century, separate research found earlier this year. 

Global warming could also make it more difficult for plants to absorb CO2, slowing rates of photosynthesis. 

Through photosynthesis, plants convert CO2, water and sunlight into the fuel they need to grow. They lock up carbon in their leaves, stems and roots in the process.

Previous research has suggested that as more CO2 is added to the atmosphere, plants will be able to grow more quickly, an effect known as “CO2 fertilisation”. 

However, numerous other studies have called the current strength of this effect into question. And climate change negatively impacts plants in other ways, such as increasing drought, heat and risk of disease. 

Dr Martijn Slot, an author of the new study and a scientist at the Smithsonian Tropical Research Institute in Panama, says the researchers wanted to determine whether temperature increases bring tropical leaves closer to a “hard upper limit”. He told a press briefing: 

“We know that when leaves start to exceed that critical threshold, then the leaves start to die. They turn brown, they can’t contribute to photosynthesis anymore, they can’t transpire anymore. They’re basically not sweating, therefore not cooling themselves.” 

The new study finds that tropical plant species could be greatly affected by even a small temperature change, as the tropics are already hot and experience little temperature variation throughout the year. 

Temperatures rising further could close leaf pores, reduce transpirational cooling and expose leaves to harmful temperatures, the study adds. 

Recording extremes 

The study uses ground data, satellite imagery and modelling to assess current leaf temperatures and predict future global warming impacts. 

They measure the average temperature of leaves in the upper canopy – the leaves, branches and organisms at the top of a forest that are exposed to direct sunlight – of tropical forests across a range of countries, such as Brazil, Puerto Rico, Panama and Australia. 

They compare peak temperature results on a three-year, 30-minute averaged canopy temperature dataset using satellite data and ground information. 

The study homes in on the observed extreme temperatures that leaves are exposed to. The authors say that these extremes are “low-probability, high-impact” events that may have a “catastrophic effect” on a leaf’s ability to function.

Dr Greg Goldsmith, a study author and associate professor at Chapman University in California, told a press briefing that “for leaf temperatures, it’s really not the averages that are important…it’s the extremes”. 

The researchers take 46.7C as the average “critical temperature” beyond which a plant’s ability to photosynthesise begins to run into difficulty, according to previous studies.

The findings show that 0.01% of upper canopy leaves in tropical forests rose above this temperature at least once per year. 

Through leaf warming experiments, the researchers find that this percentage could increase to 1.4% under future warming conditions.

This is based on three warming experiments – conducted in Brazil, Australia and Puerto Rico – where leaves and branches were heated up to assess how many leaves surpassed the critical temperature. 

The study focuses on the upper canopy of the tree, which experiences strong, direct sunlight. Slot says that if this upper layer dies due to increased temperatures, it risks pushing a forest past a tipping point. He told a press briefing: 

“If the critical temperature gets exceeded in the forest, that top layer is replaced by a layer underneath that has a lower heat tolerance.” 

The researchers write that more global warming could “tip the forest towards the death of all leaves and possible tree mortality”, if leaves die and other knock-on effects occur at temperatures above 46.7C. 

‘Non-linear’ rise  

The findings show a non-linear relationship between increased air and leaf temperatures. 

For example, air temperature increases of 2C, 3C and 4C show a rise in maximum leaf temperatures by 8.1C, 6.1C and 8C respectively, based on warming experiments in three different countries.  

Most tropical forests can withstand around 3.9C of air temperature increase before reaching the potential tipping point, the model finds. 

Such a rise is consistent with the RCP8.5 very high-emissions future pathway by the end of the century, the paper says. It also falls within the RCP6.0 scenario, a medium stabilisation pathway, towards the end of the 22nd century. In contrast, it is “out of the range” of the low-emissions RCP2.6 pathway. As a result, this study notes that it is “still within our power” to determine the future outcome for trees. 

The branch warming experiments show noticeable signs of leaf damage and a 27% reduction in transpiration when leaf temperatures rise above 46.7C for extended periods, the study says. 

Dr Christopher Doughty, the lead study author and assistant professor at Northern Arizona University, highlights that there is a lot of “uncertainty” and variability with these temperature findings. He told a press briefing: 

“There’s a lot we don’t know and what we’re saying in this paper is that we need more research to understand a lot of these key issues.” 

Dr Viola Heinrich, a postdoctoral researcher in remote sensing at the German Research Centre for Geosciences, who was not involved in the research, says the findings offer an “in-depth analysis” of the effect of extreme temperatures on tropical forest leaves. She tells Carbon Brief: 

“Leaves are the fundamental components of trees and forests that we rely on so heavily for forests to act as carbon sinks at large scales.”

She notes that the remote sensing and ground data “seems very robust, thus putting confidence in the overall results and their implications”.

‘Concerning’ findings

The researchers conducted three upper canopy leaf and branch warming experiments at a rise of 2C, 3C and 4C above pre-industrial levels to validate their model results. 

The study authors conclude that climate change and local deforestation may already be putting the hottest tropical forest regions, such as the Amazon, “close to, or even beyond” a critical temperature limit. 

Doughty says that these effects are exacerbated by deforestation. He told the press briefing that “where you have fragmentation of forests, the existing forest gets quite a bit warmer”. 

There are “all sorts of potential feedbacks once you start losing bits of forest [or] even leaves on individual trees”, Doughty says. 

Heinrich adds that “it would be interesting to see how results of critical temperature experienced play out in the observations in the coming years”, considering the emerging El Niño event. 

The warmer conditions associated with an El Niño event can cause drought, high temperatures and increased risk of forest fires to the tropics. 

Dr Stephanie Pau, an associate professor at Florida State University, who was not involved in the research, says the work is “innovative and valuable”, with a robust methodology. 

She tells Carbon Brief that the “alarming” findings are tempered by the fact that 4C of warming “is on the high end” of projections and “we still have the opportunity” to avert this outcome. She adds: 

“The study raises the wider question of the ability for tropical trees to acclimate to warming temperatures. There is evidence that tropical trees can acclimate, but that will have a limit as well.”

Dr Cat Scott, a Natural Environment Research Council independent research fellow at the University of Leeds, who was also not involved in the study, tells Carbon Brief: 

“The fact that a fraction of the leaves in tropical forests are already regularly experiencing temperatures above which they can continue to function is concerning, particularly as temperature extremes are occurring more frequently due to climate change.”

Scott adds that there are still many unknowns about how tropical forests will respond to rising temperatures and changing rainfall patterns. She says: 

“For me, this [study] emphasises that the only way to ensure that we minimise negative impacts on the biosphere is to limit future warming to as low a temperature rise as possible.”