In tropical forests it can become too hot for photosynthesis

Photosynthesis is today an essential process for life on Earth. Plants and algae convert light energy into chemical energy using light-absorbing substances such as chlorophyll. With this energy, they build organic substances – especially carbohydrates such as glucose – from carbon dioxide (CO₂) and water, producing oxygen as a waste product.

Living things that are incapable of photosynthesis themselves, such as us humans, depend indirectly on this process because they owe their food and the oxygen they need for life. However, this process only works under certain temperature conditions.

In light of these facts, the results of a new study published in the journal Nature are significant. The research team led by ecologist Christopher Doughty of the University of Northern Arizona used new methods to measure the temperature of leaves and foliage in tropical forests. The scientists combined satellite data from thermal sensors on the International Space Station (ISS) with observations from field experiments to create a comprehensive picture of current leaf temperatures in tropical forests in South America, Africa and Southeast Asia.

The research team concluded that at temperatures above 46.7 degrees, the leaves there begin to lose their ability to photosynthesize. This is not a sharply defined boundary, but an average: when the sun shines on the leaves, they can become very hot, with individual leaves becoming much hotter than the average temperature of the entire canopy. Some leaves even get so hot that they can no longer perform photosynthesis. If they are exposed to extreme temperatures for more than eight minutes, they will begin to die.

Not surprisingly, the researchers are concerned about global warming; there is a risk that more and more leaves will lose their ability to photosynthesize and die. According to the study’s findings, only 0.01 percent of leaves in tropical forests currently exceed this critical temperature.

However, the researchers used a simulation to model what might happen if air temperatures were to rise due to global warming. To do this, they exposed leaves in treetops to temperature rises of two, three and four degrees in tests in Brazil, Puerto Rico and Australia. “We were very surprised to see the temperature of the leaves rise up to eight degrees Celsius when it warms up just a few degrees,” notes Doughty. “This shows a worrying non-linear trend that we didn’t expect.”

The scientists’ models indicate that as climate change progresses, entire treetops could die. According to the simulations, the tipping point is a global warming of almost four degrees. Although the world is currently heading for a warming of 2.4 degrees, such a temperature increase of four degrees seems quite realistic if global mitigation measures are not introduced to reduce CO₂ emissions.

Four degrees of warming could cause the proportion of leaves that overheat and die to rise to 1.4 percent. This, in turn, would reduce the cooling effect of the canopy and put additional stress on the trees. If the number of dead leaves increases, the trees may eventually die as well.

According to the study, the Amazon rainforest is most at risk, where temperatures are already slightly higher than in the Congo basin. Previous studies have shown that trees in the Amazon are dying faster than in Central Africa. This is an indication that higher temperatures are indeed to blame for the increased mortality, at least in part.

According to Christopher Still, a professor of forest ecosystems at Oregon State University, the research – which he was not involved in – offers some new insights. Usually the effect of drought on tree loss is in the foreground, but the research shows that it is not just about drought. You also have to think about the temperatures the leaves are exposed to, Still told CNN.

While acknowledging that the numbers from the study are low, he added: “I would focus less on the percentages and more on the idea of ​​leaves fighting those really high temperatures and how often that happens, how long it takes and what it means. means real.”

The Amazon rainforest ecosystem is once again proving to be more fragile than previously thought. If this huge tropical forest could eventually turn into a savannah or even an uninhabitable area, it would not be so devastating because much less oxygen would be produced here. The oft-stated 20 percent of global oxygen production is vastly exaggerated.

According to calculations by the Potsdam Institute for Climate Impact Research (PIK), the Brazilian rainforest alone stores between 290 and 440 billion tons of CO₂ in biomass and soil. When trees die, much of this carbon is released into the atmosphere in the form of greenhouse gases. The tropical forests, which cover about nine percent of the Earth’s land surface, are important not only as carbon sinks. They are also very important climate regulators – and they are home to more than half of all the species on Earth.