A misty forest scene

Tropical forests in South America lose their ability to absorb carbon from the atmosphere when conditions become exceptionally hot and dry, according to new research.

For a long time, tropical forests have acted as a carbon sink, taking more carbon out of the air than they release into it, a process that has moderated the impact of climate change.

But research led by Dr Amy Bennett, a Research Fellow at the University of Leeds, found that in 2015-16, when an El Niño climate event resulted in drought and the hottest temperatures ever recorded, South American forests were unable to function as a carbon sink.

El Niño occurs when sea-surface temperatures in the Pacific Ocean increase sharply, triggering a major shift in the world’s climate system. In 2015-16, the result was exceptionally hot weather for South America. A similar event is under way now.

Dr Bennett, from the School of Geography at Leeds, said: “Tropical forests in the Amazon have played a key role in slowing the build-up of carbon dioxide in the atmosphere.

 “Scientists have known that the trees in the Amazon are sensitive to changes in temperature and water availability, but we do not know how individual forests could be changed by future climate change.

“Investigating what happened in the Amazon during this huge El Niño event gave us a window into the future by showing how unprecedented hot and dry weather impacts forests.”

The researchers today report their findings in the journal Nature Climate Change. The study united the RAINFOR and PPBio research networks, with dozens of short-term grants enabling more than 100 scientists to measure forests for decades across 123 experimental plots.

The plots span Amazon and Atlantic forests as well as drier forests in tropical South America.

These direct, tree-by-tree records showed that most forests had acted as a carbon sink for most of the last 30 years, with tree growth exceeding mortality. When the 2015-16 El Niño hit, the sink shut down. This was because tree death increased with the heat and drought. 

Professor Beatriz Marimon, of Brazil’s Mato Grosso State University, said: “Here in the southeastern Amazon on the edge of the rainforest, the trees may have now switched from storing carbon to emitting it. While tree growth rates resisted the higher temperatures, tree mortality jumped when this climate extreme hit.”

Study’s findings

Of the 123 plots studied, 119 of them experienced an average monthly temperature increase of 0.5 degrees Celsius. 99 of the plots also suffered water deficits. Where it was hotter, it was also drier.

Prior to El Niño, the researchers calculated that the plots were storing and sequestering around one third of a tonne of carbon per hectare per year. This declined to zero with the hotter and drier El Niño conditions.

The change was due to biomass being lost through the death of trees.

Writing in the paper, the researchers noted that the greatest relative impact of the El Niño event were in forests where the long-term climate was already relatively dry.

The expectation was that wetter forests would be most vulnerable to the extreme drier weather, as they would be least adapted to such conditions.

However, the opposite was the case. Instead, those forests more used to a drier climate at the dry periphery of the tropical forest biome turned out to be most vulnerable to drought.

This suggested some trees were already operating at the limits of tolerable conditions.

Ted Feldpausch, professor of tropical ecology and global change at the University of Exeter, said: “We have been tracking how trees in Amazonia respond to drought over the past few decades and for each major drought, such as those in 2005 and 2010, we find that the severe water deficits shut down the carbon sink in above-ground vegetation.

“This stops the forest’s capacity to remove CO2 from the atmosphere and slow climate warming.

“While the most recent El Niño 2015-16 drought was the hottest recorded, the impact on forests was not worse than prior drought events.

“Not all forests in Amazonia responded in the same way. For example, the forests that normally have drier conditions, those that could be expected to have strong drought tolerance, were most negatively affected by this El Niño.

“Those drier forests face multiple challenges—they are also often highly degraded and fragmented, which opens the canopy and causes them to be drier and at greater risk of fire, and which can lead to additional carbon emissions.”

Two reports are published in Nature Climate Change related to this research: the scientific paper, “Sensitivity of South American tropical forests to an extreme climate anomaly”, and a research brief titled “Impact of the 2015-2016 El Nino on South American tropical forests”.