In 2019, hundreds of fires across the Amazon burned through enough rain forest to fill the state of New Jersey. At the peak of the fires in August, smoke plunged São Paulo, hundreds of kilometers away, into midday darkness. The 2020 fire season was even more severe. Many of the blazes were traced to deforestation, which hit a 10-year high in the Brazilian Amazon in 2020. There, fire is a tool used by farmers and ranchers to clear land, but when conditions are hot and dry, the burns can escape uncontrolled into the rain forest. Fires linked to deforestation were up 50% in 2019 and another 23% in 2020.
Fires can have profound impacts on the landscape; converting rain forest to pasture or agricultural land leads to shifts in temperature, water cycling, and carbon storage. But what’s less clear is what happens to burned forest that’s not converted. In a new study, researchers used satellite data to answer the question, If a burn scar is left alone, can the forest recover from fire, and how long will it take?
“We were able to see, on a small scale, what actually happens in the forest if they are not converted to agricultural land or pasture after a fire,” said Gabriel de Oliveira, a postdoc at the University of Toronto and lead author on the new study.
A Glimmer of Hope
The Amazon is home to the largest rain forest in the world, and the biodiversity hot spot provides countless benefits to those living near and far: The vast rain forest sequesters huge amounts of carbon dioxide and recycles water. Indeed, deforestation in the region could influence the water supply as far away as the United States or Africa.
“As these [deforestation] fires start escaping from open, deforested areas and entering more standing forests, we need to know what they’re going to do to this system,” said Paulo Brando, an assistant professor at the University of California, Irvine, who was not involved with the study. The new study provides a glimmer of hope to experts worried about the future of the region: It shows that some key parameters of forest health can recover in as little as 3 years.
“[The findings] show that if we leave the forests alone, they are capable of restoring very important functions, and very fast,” Brando said.
To find out how resilient the forest is to fire, the researchers selected two plots to study in Pará, Brazil, within the “Arc of Fire” (the strip of land with the highest annual fire ignitions in the Amazon). Both plots had burned in 2010, a year in which the Amazon was enduring one of the worst droughts in its recorded history.
Using images collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard NASA’s Terra satellite, the team was able to track air and land surface temperature, evapotranspiration, and gross primary production in the 3 years before and 3 years after the fire.
The data revealed that land surface temperature increased by roughly 2°C in the month after the fire. Three years after the blaze, the land surface and air temperature remained nearly 1°C above the 3-year average from before the fire.
By then, however, the rates of evapotranspiration and gross primary productivity had nearly returned to the prefire levels. But that doesn’t mean the forest looked the same as it did before.
Different Kinds of Diversity
Some species thrive after fire, allowing productivity and water cycling rates to rebound, but they may not be the same species that thrived before. That can change both the composition and the structure of the forest.
“The productivity of the system increased, and the capacity to cycle water increased, so you get to the same outcome with a burned forest and an unburned forest, but the characteristics of the systems are completely different,” said Brando. “If you care about species diversity, and the structure of the forest, and carbon storage, then you should be still worried.”
Indeed, the authors did note a shift in the structure of the forested plots, according to de Oliveira: After the fire, the canopy opened up. This change led to an intensification of the seasonal cycles of temperature and water and carbon fluxes. The open canopy allowed more solar energy to reach the soil, which over longer timescales could lead to higher temperatures and more evaporation, de Oliveira said.
He cautions that the current study also doesn’t account for the severity of the fire, which could have significant impacts on forest structure and recovery. Future studies are needed to understand the long-term effects of seasonal shifts and how fire severity might affect recovery.
The new study was published in the Journal of Geophysical Research: Biogeosciences in March.
How Much of a Beating Can the Rain Forest Take?
There is little time to waste. The Amazon is becoming more flammable as it becomes more fragmented, and the rain forest that is left is being exposed to more sources of ignition.
“The fires are getting worse and worse every year. 2019 was the worst in a decade, and 2020 was even worse than we expected,” de Oliveira said. “With more deforestation, more fires will happen, because fires are part of the deforestation process. We’re just trying to understand what happens to the forest, but someone actually has to do something to stop this.”
And fire is only one factor that’s reshaping the Amazon rain forest; logging, a loss of biodiversity, and a longer, more intense dry season are all eating away at the forest’s resiliency. “If the system was adaptive, it should bounce back,” said Brando. “It’s just the question is, How much of a beating can it take before it cannot bounce back?”
—Kate Wheeling (@katewheeling), Science Writer