In the Brazilian Amazon deforestation is a serious problem. But a new study suggests that there is another threat to the rainforest that has been largely ignored: fire.



As an ecosystem, the Amazon region is not typically fire-prone. “This is a landscape where natural fire occurrences from lightning strikes are super, super rare,” says Jennifer Balch, a professor of geography at The Pennsylvania State University and one of the lead authors of the study, which was published in the April 14 Proceedings of the National Academy of Sciences. Instead, people light fires to intentionally clear areas for agriculture or to rejuvenate pasturelands, Balch says.



These fires are not well managed and can extend further than intended. This is not usually a problem, because the forests are often moist enough to prevent the fires from spreading too widely or from doing too much damage, according to the study. But in years of severe drought, the researchers found, these fires can grow much more intense, killing large numbers of trees.



From 2004 to 2010 Balch and her colleagues set up three adjacent forest plots. One was burned yearly, one every three years and the third was left unburned as a control. During most of these controlled burns, there was low tree mortality. But in 2007 a severe drought struck the region. “Four times more trees died during that drought year compared to any other year,” Balch says. Forest edges were also more susceptible to fires during the study, as they are exposed to warmer, drier air from neighboring clearings.



Climate change models (pdf) indicate that the Amazon Basin will get warmer and dryer over the coming decades. Balch says that this will leave the region even more susceptible to widespread damage from forest fires, considering the damage from burns they observed in the study. “It is worrisome if you’re going to lay on top of that current situation a warmer and drier climate scenario,” she says.



Balch notes that one possible solution is better management of fires in Brazil. “There’s a lot of room for improvement,” she says. Whereas the U.S. spends billions of dollars every year fighting wildfires, “Brazil spends virtually nothing, compared to that,” she says. Some efforts, however, are underway there, including new federal- and state-level policies to control when and how fires can be used, along with the creation of volunteer brigades to fight infernos that have spread.



Jos Barlow, a researcher at Lancaster University’s Environment Centre in England who was not involved in the study, thinks that the new findings are an important demonstration of the damage that fires can do. “It’s great to have this long-term data from experimental fires, especially in that region of the Amazon, where our knowledge is relatively poor,” he says. And whereas predicting severe weather events, such as droughts, is always difficult, he adds, there is evidence to suggest that as climate change continues, there may be more such droughts in the Amazon.



Barlow also notes that dealing with the problem of escaped fires is more complex than it may seem. In a 2013 paper he and his co-authors wrote that there was a discrepancy between Brazilian laws regulating the use of land management fires and what actually happens on the ground. “Those laws were made without any consideration of what local people do,” he says.



Despite the intricacies of dealing with Amazon forest fires, Barlow does see some possible solutions. For example, selective logging of the Amazon can increase forest flammability; he recommends zoning such logging so that it is farther from areas likely to burn. He also suggests that some fire management laws could be in effect only in years when there is a severe drought, as those are the times that the forest is most susceptible to escaped fires.