The carbon sink in South American subtropical forests is in danger

During the El Niño climate phenomenon, extreme conditions occurred that negatively affected the forest carbon sink in South America, causing it to decline or shut down.

Recent research has revealed that tropical forests in South America experience a decline in their ability to absorb carbon from the atmosphere when conditions become extremely hot and dry. This raises concerns about the impact of climate change on these key ecosystems for global balance.

Tropical forests have long played a crucial role as carbon sinks. This means they have absorbed more carbon from the air than they release, which has been essential in mitigating the effects of climate change.

El Niño and its impact on the carbon sink function of South American forests

According to Amy Bennett, a researcher at the University of Leeds, during the period 2015-2016, South American forests were affected by a climate event known as El Niño. As a result of this phenomenon, there were droughts and extremely high temperatures. These adverse conditions prevented forests from fulfilling their role as carbon sinks and efficiently capturing greenhouse gas emissions.

The El Niño phenomenon occurs when sea surface temperatures in the Pacific Ocean experience a rapid increase, causing a significant change in the global climate system. During the years 2015-2016, South America experienced unusually warm weather. Today, we find ourselves facing a similar event unfolding.

Dr Bennett, from the Leeds School of Geography, has pointed out that tropical forests in the Amazon have played a crucial role in reducing the rise of carbon dioxide in the atmosphere. This is further evidence of the vital importance of preserving and protecting these ecosystems to combat climate change.

Amazonian trees, sensitive to climate change

"Scientists have known that trees in the Amazon are sensitive to changes in temperature and water availability, but we don't know how individual forests might be changed by future climate change."

"Investigating what happened in the Amazon during this major El Niño event gave us a window into the future by showing how the unprecedented hot and dry climate affects forests.".

So the researchers have published their findings in the prestigious journal Nature Climate Change. This significant study was possible thanks to the collaboration between the RAINFOR and PPBio research networks. Thanks to dozens of short-term grants, more than 100 scientists were able to carefully measure forests in 123 experimental plots over several decades.

Study methodology

The plots cover a variety of forests in South America, both in the Amazon and Atlantic regions. They also include drier forests found in the tropical part of the continent.

According to detailed records, examining tree by tree, most forests have been found to have functioned as carbon sinks for most of the last 30 years. This means that the growth of the trees has exceeded the mortality rate. During the 2015-2016 El Niño phenomenon, a closure of the sink occurred due to increased tree mortality associated with heat and drought.

Professor Beatriz Marimon, from the State University of Mato Grosso in Brazil, added her contribution to the debate. Her experience and knowledge of the field are valuable and enrich the academic discussion: "Here in the southeast of the Amazon, at the edge of the rainforest, trees may have switched from storing carbon to emitting it. While tree growth rates withstood the higher temperatures, tree mortality increased when this extreme weather hit.".

lower layer of Amazon forest

Results of the study on the impact on the carbon sink

In a study of 123 plots, it was found that 119 of them experienced an average monthly temperature increase of 0.5 degrees Celsius. In addition, it was observed that in 99 of these plots there were also water deficits. These results indicate a relationship between the increase in temperature and the lack of precipitation, since the hottest areas also presented greater dryness.

Before the climate phenomenon known as El Niño, scientists estimated that the plots were capable of storing and capturing about a third of a ton of carbon per hectare per year. However, during the warmer and drier conditions associated with El Niño, this capacity was reduced to zero.

The reason behind the change was the decrease in biomass caused by the death of trees.

In the paper, the researchers highlighted that the El Niño event had a significant impact on forests that were already relatively dry in the long term.

Surprise for researchers

Contrary to expectations, wetter forests have been found to be less vulnerable to extremely dry climates than previously thought. This has surprised researchers, since it was believed that these forests would be less adapted to such adverse conditions. However, forests located in drier areas on the outskirts of the rainforest biome are the most susceptible to drought. This is because they are accustomed to dry climates and do not have the same ability to cope with long periods without rain as more humid tropical forests.

This finding suggests that some trees are already functioning under tolerable borderline conditions.

Professor Oliver Phillips, a leading ecologist at the University of Leeds and leader of the global ForestPlots initiative, oversaw research that offered hope for the resilience of tropical nature in South America.

He added: "The full 30-year outlook provided by our diverse team shows that this El Niño had no worse effect on intact forests than previous droughts. However, this was the hottest drought in history” 

"Where tree mortality increased was in the drier areas of the Amazon periphery, where forests were already fragmented. Knowing these risks, conservationists and resource managers can take steps to protect them.

"“Through the complex dynamics that occur in forest environments, land clearing makes the environment drier and warmer, further stressing the remaining trees.”

"So the big challenge is keeping the forests standing in the first place. If we can do that, then our evidence on the ground shows that they can continue to help lock in carbon and slow climate change."

Note:

Two reports were published in Nature Climate Change related to this research. The scientific article, "Sensitivity of South American tropical forests to an extreme climate anomaly", and a research report titled "Impact of El Niño 2015-2016 on South American tropical forests".

Ecoportal.net

With information of: https://www.eurekalert.org/