- With higher temperatures and increasingly severe droughts resulting from climate change, fires are becoming a more frequent phenomenon in the Amazon.
- New research finds that fires fundamentally change the structure of the forest, leading it to stockpile less carbon even decades after a burn.
- The research also shows that the burning of dead organic matter in the understory can release far more carbon into the atmosphere than previously thought.
The fires closing in on Erika Berenguer’s research site in Brazil’s eastern Amazon were unlike anything she had seen in years working in the forest. Smoke hung heavy the air.
“I couldn’t see 10 meters in front of me,” Berenguer, an ecologist at Lancaster University and the University of Oxford in the U.K., said in an interview. “We were right in the middle of a giant barbecue of the world’s largest tropical rainforest,” helped along by an intense drought during the 2015-2016 El Niño.
As the flames lapped at her boots, she worried that more than five years of fieldwork toward her doctorate was literally going up in the smoke around her.
“I’m not only losing my country’s forests,” the Rio de Janeiro native said she remembers thinking. “I’m also losing my plots and my own work, so it was all very emotional until I got out of the forest.”
Once she’d had a chance to huddle with her colleagues, though, she realized, “We can actually use this data to investigate what are the impacts of the fire into carbon cycling.”
They had data on plots before and after the fires, and on some plots that didn’t burn, leading to new revelations into the Amazon forest’s response to such a destructive force. Publishing their results in a series of three papers Oct. 8 in the journal Philosophical Transactions of the Royal Society B: Biological Sciences, they demonstrate that, while tree growth rates spiked after the fires, the pulse of carbon emissions into the atmosphere was perhaps four times higher than predicted because of the unaccounted-for burning of dead organic matter on the forest floor.
The team also looked at how forests in the Amazon recovered from fire on longer timescales, and they found that carbon stocks still hadn’t returned to pre-fire levels even 31 years after being burned.
Sorting out the relationship between fires and the forests of the Amazon is a relatively new problem for scientists, and the complex suite of repercussions that they bring is still something of a mystery.
“Amazonian forests really don’t burn naturally,” said Jos Barlow, an ecologist at Lancaster University and co-author on the three studies. “Fires happen very rarely, perhaps on millennial cycles.”
That’s why the wildfires in 2015 provided such a unique opportunity. In the study led by Berenguer, the team took monthly measurements of the diameters of trees growing in both burned and unburned plots every month between July 2015 and December 2017. When they compared the growth rates, they found that trees grew faster in the burned plots than in the areas untouched by fire. This result wasn’t unexpected: the charred organic matter likely provided young growing trees in the burned plots with a jolt of nutrients.
What did take the researchers by surprise was that the total amounts of carbon taken up in both sets of plots was about the same — a finding explained by the fact that lower-density trees, which sock away smaller amounts of carbon, were more likely to spring up after the fire.
Berenguer’s study dovetails with research led by her colleague, ecologist Camila Silva. The study that Silva led looked at tree growth and death after fires going back several decades.
“We could build a history of these fire-affected areas,” Silva said.
At first glance, it seemed that the forest had found a way to come back.
“When you go into the forest 30 years after a fire, the forest is in a kind of equilibrium,” Silva said. “You can see that it had recovered most of the trees and the understory is established again.”
But hidden away in that snapshot were fundamental changes to the forest wrought by fires that had happened decades beforehand. The team’s measurements of the trees revealed that those low-wood-density trees were now more dominant. As they drilled into the data, they found that all types of trees were more likely to die after a burn.
“Basically, the forests have no evolutionary history of fire, so they’re not adapted to [it],” Barlow said. “That’s why you get such high levels of tree mortality from very small wildfires.”
Distressingly, they also found that trees with greater biomass — larger and denser-wood trees that hold more carbon — were much more likely to die after fires than they were in unburned plots.
“If we keep losing these big and dense trees,” Silva said, “it will affect the carbon cycle, and that’s something that we really need to avoid.”
Yet another study addressed the issue of carbon budgets, this one led by Lancaster University ecologist Kieran Withey. This research looked at the carbon released immediately by fires into the atmosphere. The team measured the buildup of leaves, woody debris and other organic matter in the forest — what scientists call “necromass” — across a range of forest types, from untouched primary forests to secondary forests growing back after they’d been cleared. They then calculated how much had burned during the fires during the 2015-2016 El Niño.
They found that the carbon these fires released was three to four times the emissions estimates predicted by established databases. In the team’s study area, around 10,000 square kilometers (3,900 square miles) burned, or less than 0.2 percent of the Brazilian Amazon. Yet the burning released 33 million tons of carbon — equivalent to the emissions from fossil fuels and cement production by Denmark for an entire year.
Understanding these dynamics of wildfires in the Amazon is only one aspect of the broader issue. The scientists agree that finding ways to prevent them in the first place is another critical part of the equation.
Every fire in the forest can be traced back to one set by a farmer or rancher to clear the way for crops or livestock. But that belies “a complex set of causes,” Barlow said. “The fire in the smallholder plots is just the trigger.”
“In previous climates, it’s been safe,” Barlow said. But forest degradation and fragmentation as a result of logging and agriculture, shifts in rainfall patterns, higher temperatures and more severe droughts have all fundamentally changed the forest and its response to fires.
“You’re basically in a completely different system where the previous agricultural land uses are no longer safe,” Barlow said.
Berenguer said awareness campaigns that encouraged the creation of fire breaks and taught farmers to set fires outside the windiest or hottest parts of the day, backed by a government willing to address the issue head-on, could help prevent fires in the first place.
Still, she said, an altered reality now faces the world’s largest rainforest: “In the Anthropocene, fire has become the new norm in many parts of the Amazon.”
Banner image of central Amazonian forests burning in the 2015 El Niño event. Image by Adam Ronan.
John Cannon is a Mongabay staff writer based in the Middle East. Find him on Twitter: @johnccannon
Berenguer, E., Malhi, Y., Brando, P., Cardoso Nunes Cordeiro, A., Ferreira, J., França, F., … Barlow, J. (2018). Tree growth and stem carbon accumulation in human-modified Amazonian forests following drought and fire. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1760).
Silva, C. V. J., Aragão, L. E. O. C., Barlow, J., Espirito-Santo, F., Young, P. J., Anderson, L. O., … Xaud, H. A. M. (2018). Drought-induced Amazonian wildfires instigate a decadal-scale disruption of forest carbon dynamics. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1760).
Withey, K., Berenguer, E., Palmeira, A., Espírito-Santo, F. D. B., Aragão, L., Ferreira, J., … Barlow, J. (2018). Quantifying the immediate carbon emissions from ENSO-mediated wildfires in human-modified tropical forests. Proceedings of the Royal Society B: Biological Sciences, 373(1760).
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