Tropical forests may no longer be carbon sinks with El Niño

Tropical forests absorb CO? through photosynthesis and convert it into biomass. But the balance between photosynthesis and respiration is delicate and depends on two factors: temperature and water availability, explains Amy Bennett, associate researcher at the School of Earth and Environmental Sciences at the University of Leeds, in an article in The Conversation, published by Terra e Brasil 247. Amy is one of the authors of the 2023 study that verified the impacts of the 2015-2016 El Niño on tropical vegetation.

In hotter, drier conditions, such as those created by El Niño, plants close the pores in their leaves to prevent water loss. But it is through these pores that they absorb CO?. In other words, this deprives plants of the carbon necessary for photosynthesis and growth.

Thus, in El Niño years, characterized by high temperature and drought in tropical regions, prolonged climate stress leads to reduced forest growth and increased tree mortality. The effects of this are felt for decades, as carbon is released back into the atmosphere when dead trees decay.

In the 2015-2016 El Niño, when terrestrial temperatures were, on average, at least 1oC above usual conditions, some tropical forests in South America became net carbon emitters. This raises concerns about the possible impact of the current phenomenon on the Amazon and the global climate, highlights Amy.

However, “Super El Niño” is the “icing on the cake” of the stress that tropical forests have been suffering due to climate change. According to a study published in the Proceedings of the National Academy of Sciences (PNAS), heat waves led 57 million hectares of tropical forests – an area equivalent to the state of Bahia – to exceed the limits of photosynthesis, affecting their carbon absorption capacity. This number could almost double by 2100, reaching 160 million hectares, an area larger than South Africa, highlight Down to Earth and Economic Times.

To arrive at such numbers, researchers compared cataloged critical temperatures for 200 species with satellite-derived temperature measurements collected in tropical forests between 2001 and 2020. “In two decades, the area of tropical forest with treetop temperatures above the average critical threshold increased from 43 million to 57 million hectares,” they noted.

The study authors say heat stress affects biochemical reactions in leaves in a similar way to how extreme heat affects biological functions in humans. "When leaves get too hot, the proteins that drive photosynthesis begin to degrade. As a result, trees absorb less carbon dioxide and grow less efficiently," says Charlotte Grossiord, assistant professor at the Plant Ecology Research Laboratory at École Polytechnique Fédérale de Lausanne, Switzerland, who led the research.