- Areas of the Amazon forest with higher concentration of soil pyrogenic carbon, a material produced by the burning of vegetation centuries or millennia ago, show an enhanced resistance to droughts, a new study says.
- Regions richer in pyrogenic carbon appear to have higher soil fertility and water-holding capacity, helping the forest to get through dry periods without enduring the usual damage.
- While the underlying mechanisms aren’t yet understood, the authors hypothesize that species substitution after ancient fires, which brings in trees with lower wood density, might play a role.
- Modern fires, which have become more intense and frequent in recent decades, are unlikely to produce similar effects.
Ancient forest fires seem to have played a role in enhancing resistance to drought in the Amazon, a recent study suggests. The research, published in Frontiers in Forests and Global Change, focused on areas without known recent fires, but with high concentrations of pyrogenic carbon (PyC), a material found in the soil that’s produced by burning vegetation. A higher presence of PyC in these areas indicates a record of forest fires that happened a long time ago. Where its concentrations in the soil were higher, the researchers found, characteristics related to drought resistance, such as higher soil fertility and water-holding capacity, were also more noticeable.
“Recent works have shown that the Amazonian forests already had fire events centuries or millennia ago, although on a much smaller scale than what is seen today,” says study lead author Laura Vedovato, a researcher at the University of Exeter, U.K.
This discovery against the commonplace notion that this humid region only started to experience fires in the past few decades, because of a combination of intentional burning by farmers, deforestation and climate change. However, the long-term consequences of these ancient fires are barely understood. “This is an unprecedented study, it has never been carried out in other tropical forests,” Vedovato told Mongabay.
The researchers looked at forest dynamics — accounting for taxonomy, forest growth and tree mortality, among others — and collected soil samples in 95 different sites across the Amazon Basin. After analyzing them for fertility, texture and concentration of pyrogenic carbon, they combined the findings with data on extreme drought periods in the past four decades. “Our results indicated that areas with a higher concentration of soil PyC, that is, with evidence of greater occurrences of ancestral fires, maintained their ability to gain carbon at the same rates when compared to years without drought,” Vedovato says.
While the actual mechanism behind this phenomenon remains unknown, the study points to three possible explanations. Because the higher presence of PyC is intertwined with enhanced soil fertility, trees in these areas could be dying and growing at a higher rate, led by species with lower wood density.
“Thus, even with tree mortality in drought periods, there is also a recruitment of new trees in such a short period, without altering the balance between carbon loss and gain,” Vedovato says.
This hypothesis also relates to the second one, regarding the changing of tree species when an area is disturbed by fires, which enables species with a faster growth period to take over. The third theory relates to trees’ enhanced water-holding capacity for surviving dry periods, which also correlates to the presence of pyrogenic carbon. “One or more hypotheses may be correct,” Vedovato says, adding that further studies of the underlying mechanisms are needed.
Modern fires are different
This historical response to natural fires, however, isn’t an indication of how the Amazon might adapt to current threats. “The regime of occurrence of ancestral fires cannot be compared to the intense and frequent fires that occur today,” says Ted Feldpausch, a professor at the University of Exeter who supervised the study. “The forest fires recorded in recent years have occurred under different conditions than the ones of centuries ago, being heavily controlled by droughts and increased deforestation and degradation.”
While ancient fires occurred every few hundred or even thousand years, nowadays, some areas of the Amazon experience multiple fires in the same decade. According to data from INPE, the Brazilian National Institute for Space Research, fires in the Brazilian Amazon increased by 74% in 2020 since the turn of the century.
Sonaira Silva, a professor at the Federal University of Acre in Northern Brazil, who specializes in forest fires in the Amazon and analyzed the paper at Mongabay’s request, agrees.
“The study brings attention to the fact that, contrary to popular belief, the fires are not a new phenomenon,” she says. “But the current context is a lot more difficult because we have a synergy of threats: deforestation, more sources of ignition, warmer temperatures in general, and areas of fragmented forest that get more dry even if the drought isn’t as intense as in the past.”
The result, Silva says, is forest fires that not only have become more frequent, but can also spread at a faster pace because of the lack of humidity.
Human action plays a central role in making the forest more vulnerable, according to Philip Fearnside, a biologist and researcher at the National Institute for Amazonian Research (INPA) in Brazil.
“It is a vicious cycle: first comes the wood extraction, then a fire, then a second fire brings even more destruction, and after three or four fires we cannot talk about there being a proper forest anymore,” Fearnside says.
Human-driven deforestation contributes to forest fragmentation and adds more flammable material to the environment: when a tree is alive, it can preserve humidity and act as a natural barrier against fires. But after it’s felled, the branches and stumps left behind dry out and become just another combustible in the path of destruction.
“It can be even more dangerous than deforestation itself because with deforestation the government can still act preventively,” Fearnside tells Mongabay. “But if a forest fire grows beyond control, there is nothing that can feasibly be done to stop it.”
Reviewing the study, Fearnside says it’s necessary to put the data in context to avoid misinterpreting its conclusions.
“Those who read the title might get the wrong idea” about what exactly the fires might do to enhance drought resistance, he says. This position is shared by the authors, who say the results need to be considered only in regards to ancient fires, and most likely will not be replicable in the future, with how the nature of the fires has changed in recent decades.
“The mechanisms we discuss are unlikely to be valid when considering modern fires, but are relevant to help understand the impacts of ancestral fires in the Amazon,” Vedovato says.
“The effects of the fires remain in the forests for more than two decades after the fact. The increased frequency of these fires does not give the forest enough time to recover and show any benefits during short-term droughts.”
Vedovato, L. B., Carvalho, L., Aragão, L. E., Bird, M., Phillips, O. L., Alvarez, P., … Feldpausch, T. R. (2023). Ancient fires enhance Amazon forest drought resistance. Frontiers in Forests and Global Change, 6. doi:10.3389/ffgc.2023.1024101
Banner image: Photo by Cristian Braga/Greenpeace.