El Niño caused tropical tree deaths in South America

Study finds a repetition of the 2015-2016 El Niño event unfolding in tropical forests, rendering trees more susceptible to drought conditions.
Shubhangi Dua
Rainforests in South America are drying out severely affecting climate change
Rainforests in South America are drying out severely affecting climate change

Lena Trindade/Brazil Photos/LightRocket via Getty Images 

NASA recently declared July as the hottest month ever recorded since 1880, and it appears that global temperatures have had global repercussions including their impact on depriving tropical forests.

A recent study found that tropical forests in South America lost their ability to absorb carbon from the atmosphere during the 2015-2016 El Niño occurrence as a result of severely hot and dry conditions.

According to National Geographic, El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern equatorial Pacific Ocean. Trade winds and the atmosphere are also impacted by El Niño.

El Niño causing droughts

The National Oceanic and Atmospheric Administration stated that El Niño is interpreted as a Little Boy in Spanish. In the 1600s, South American fishermen first noticed periods of unusually warm water in the Pacific Ocean. They began calling it – El Niño de Navidad, because the condition typically peaks around December.

The study led by Dr Amy Bennett, a research fellow at the University of Leeds focused on observing tropical forests in South America including the Amazon, Atlantic forests, and other drought-sensitive forests in the area.

It has long been established that tropical forests perform as carbon sinks – a situation when tropical trees consume more carbon out of the air rather than releasing it. This process has moderated the impact of climate change.

The forests are not functioning as carbon sinks anymore similar to the case in 2015-2016 El Niño, scientists find. “Tropical forests in the Amazon have played a key role in slowing the build-up of carbon dioxide in the atmosphere,” said Bennett.

“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,” Bennet added.

Bennet further emphasized the research provided the scientists with a view of what the future would entail after ascertaining the unprecedented hot and dry weather impacts forests.

Decades of research tagging vulnerable regions

The study employed nearly a hundred scientists under the RAINFOR and PPBio research networks and measured forests for decades across 123 experimental plots in the Amazon and Atlantic forests, a statement by the scientists noted.

Researchers documented that 119 of the plots experienced an average monthly temperature increase of 0.5 degrees Celsius while 99 plots faced water deficits.

The study noted that the plots were able to store and sequester almost one-third of a tonne of carbon per hectare per year before El Niño developed but declined to zero when hotter weather conditions came about. 

The statement said that the change occurred due to biomass being lost through the death of trees. The impact of extreme conditions affected the forests so deeply that the forests that were least expected to be vulnerable to drier weather conditions have now become the victims of drought.

Additionally, it was uncovered that certain trees within tropical forests were already functioning under conditions close to their maximum tolerance levels.

Professor Oliver Phillips, an ecologist at the University of Leeds sheds light on the possible resilience of the South American tropical nature. 

He stated: “The full 30-year perspective that our diverse team provides shows that this El Niño had no worse effect on intact forests than earlier droughts. Yet this was the hottest drought ever.”

Phillips further said that tree mortality increased in the drier areas on the Amazon periphery where forests were already fragmented. “Knowing these risks, conservationists and resource managers can take steps to protect them.”

He proposed salvaging the remaining forests as challenging as it may be as the tropical forests will continue to help lock up carbon and slow climate change.

The study was published on September 4 in the journal – Nature.

Study abstract:

The tropical forest carbon sink is known to be drought sensitive, but it is unclear which forests are the most vulnerable to extreme events. Forests with hotter and drier baseline conditions may be protected by prior adaptation, or more vulnerable because they operate closer to physiological limits. Here we report that forests in drier South American climates experienced the greatest impacts of the 2015–2016 El Niño, indicating greater vulnerability to extreme temperatures and drought. The long-term, ground-measured tree-by-tree responses of 123 forest plots across tropical South America show that the biomass carbon sink ceased during the event with carbon balance becoming indistinguishable from zero (−0.02 ± 0.37 Mg C ha−1per year). However, intact tropical South American forests overall were no more sensitive to the extreme 2015–2016 El Niño than to previous less intense events, remaining a key defence against climate change as long as they are protected.

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