Drought and fire increase Amazon tree deaths and CO2 emissions

El Niño-driven extreme drought and forest fires in 2015-16 caused the loss of 3 billion trees and emissions of 495 million metric tons of carbon dioxide in the ensuing three years, destroying more than deforestation throughout the Brazilian Amazon.

By Elton Alisson | Agência FAPESP – Extreme droughts are becoming more and more frequent and intense owing to climate change, and this could have significant effects on the Amazon Rainforest. In the summer season between late 2015 and early 2016, severe drought and forest fires associated with El Niño ravaged parts of the region. The effects of this climate-driven event lasted at least three years, resulting in the death of 3 billion trees by 2018, and in the emission of 495 million metric tons of carbon dioxide (CO₂), more than the annual average for the entire Brazilian Amazon.

These are among the findings of a study conducted by researchers affiliated with institutions in Brazil and the United Kingdom. The study was supported by FAPESP under its Biodiversity Characterization, Conservation, Restoration and Sustainable Use Program (BIOTA-FAPESP), and reported in an article published in Proceedings of the National Academy of Sciences of the United States of America (PNAS).

“We saw that trees located in forest areas that had been disturbed by past human activity such as burning or logging were more vulnerable to the combined effects of drought and fire associated with the 2015 El Niño than trees located in more conserved areas of Amazonia,” Erika Berenguer, first author of the article, told Agência FAPESP. Berenguer is a Brazilian-born researcher affiliated with Lancaster University and Oxford University (UK).

The researchers had conducted research in the Lower Tapajós area since 2010. The Tapajós is one of the main tributaries of the Amazon River. With 6.5 million hectares, the Lower Tapajós is about twice the size of Belgium. In 2015 it was the epicenter of the El Niño event in Amazonia. To measure the impact of the El Niño-driven drought and fires on undisturbed and human-modified forests, they conducted quarterly surveys from October 2015 to October 2018, monitoring the fate of 6,117 trees in 21 long-term plots, each with an area of 2,500 square meters.

Under normal conditions, the Amazon Rainforest does not catch fire because of high humidity. In this case, however, extreme drought made the forest temporarily flammable, and burning to clear areas after logging for pasture spun out of control, allowing wildfires to spread through huge swathes of jungle as well as secondary forest.

“Fires that started elsewhere spread to the plots we’d been studying since 2010 and burned everything. A series of experiments we were conducting literally melted down because they involved devices made of plastic,” Berenguer said.

Tree losses

This was the bleak setting for the research project. Some of the plots had consisted of undisturbed primary forest, others of primary forest that had been selectively logged or ruined by illegal logging and burning, while yet others consisted of secondary forest that had grown back in completely cleared areas.

The analysis showed that the El Niño-driven combination of extreme drought and huge fires caused the death of some 3 billion trees in the area, which corresponds to 1.2% of Brazilian Amazonia and 1% of the entire biome.

About 15% of these trees were large, with a diameter at breast height (DBH) of more than 10 cm. Some 2.5 billion were smaller, with a DBH of less than 10 cm, according to the researchers’ estimates.

“Some areas lost 75% of their trees. The forest was completely changed and became fully open,” Berenguer said.

Tree losses were far worse in secondary forest areas and other areas of forest affected by human activity. Trees with lower wood density and thinner bark were more likely to be killed by drought and fire. Such trees are more common in forests affected by human activity.

The researchers also compared the effects of drought on different types of forest and the combined stresses of drought and fire exacerbated by El Niño. Tree mortality due to drought was much higher in secondary than primary forest areas. The impact was greatest in areas of forest modified by human activity and damaged by both drought and fire.

“Although previous research had shown that forests affected by disturbances due to human interference are more susceptible to fire, it wasn’t known whether there was any difference in tree vulnerability and resilience due to a combination of drought and forest fires,” Berenguer said.

The researchers found that trees in forests affected by drought and burned forest areas continued to die at an above-normal rate for three years after the El Niño-driven drought, releasing more CO₂ into the atmosphere.

Tree deaths in the Lower Tapajós caused the emission of 495 million tons of CO₂, more than that caused by an entire year of deforestation in all of the Amazon. Drought and fire released an amount of CO₂ in a three-year period equivalent to annual greenhouse gas emissions in some of the world’s most polluted countries.

“The amount of CO₂ emitted was greater than annual emissions in Australia or the UK, for example,” Berenguer said.

CO₂ emissions from burned areas were almost six times those of areas only affected by drought.

After three years, only 37% of the emissions had been reabsorbed by tree growth in the areas concerned.

“The results of the study are compatible with those recently published by other groups showing that the Amazon could cease to be a carbon sink and become a carbon source,” said Carlos Joly, a professor at the University of Campinas’s Institute of Biology (IB-UNICAMP) and a member of BIOTA-FAPESP’s steering committee.

“All these studies show that the frequency of human disturbances in the Amazon is accelerating and could take forest losses to the point of irreversibility. If so, the Amazon would cease to be a closed forest formation and become an open forest, far less dense and exuberant than it is now,” said Joly, also a co-author of the article.

Part of the study was conducted under the aegis of the Thematic Project “ECOFOR: Biodiversity and ecosystem functioning in degraded and recovering Amazonian and Atlantic Forests”, supported by FAPESP, with Joly and Jos Barlow, a professor at Lancaster University, as co-principal investigators.

“In the Atlantic Rainforest we don’t see the same correlation of factors as in the Amazon Rainforest,” Joly said.

The article “Tracking the impacts of El Niño drought and fire in human-modified Amazonian forests” (doi: 10.1073/pnas.2019377118) by Erika Berenguer, Gareth D. Lennox, Joice Ferreira, Yadvinder Malhi, Luiz E. O. C. Aragão, Julia R. Barreto, Fernando Espírito-Santo, Axa Figueredo, Filipe França, Toby Alan Gardner, Carlos A. Joly, Alessandro F. Palmeira, Carlos Alberto Quesada, Liana Chesini Rossi, Marina Maria Moraes de Seixas, Charlotte C. Smith, Kieran Withey and Jos Barlow is at: https://www.pnas.org/content/118/30/e2019377118

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