If the Amazonian climate becomes drier and fires become more frequent, floodplain forests in the Amazon basin may change to new ecosystems, according to new research.

Scientists studied 40 years of wildfire history in the seasonally flooded forests of the middle course of the Rio Negro, an area of about 4,100 square kilometers in the central Amazon.

They concluded that when these forests are repeatedly disturbed by wildfires, the soil gradually loses both clay and nutrients and becomes increasingly sandy. Simultaneously, the paper notes, “native herbaceous cover expands, forest tree species disappear and white-sand savanna tree species become dominant.” The research was published in March in Ecosystems.

Floodplain Forests Already at Risk

Floodplain forests like those in the Rio Negro area are seasonally flooded ecosystems. The river mitigates damage from wildfires and contributes to the development of root mats—well-aerated plant roots and leaf litter that lie above the soil—which reduce water infiltration. As droughts become more frequent and severe, seasonal rains become less dependable, and root mats become a fire hazard. In fact, said Bruce Walker Nelson, a staff researcher at the National Institute for Amazon Research, Brazil, the single biggest reason floodplain forests burn so easily is the fact that during dry periods, root mats become “a fine fuel that dries quickly and burns easily.” Nelson was not involved in the new research.

“Studies have suggested that these [regrown] forests hold 25% less carbon than primary forests which have never been affected by fires,” said Liana O. Anderson, a researcher at the National Centre for Monitoring and Early Warning of Natural Disasters in Brazil not involved in the new study. This degraded capacity to store carbon persists for a long time. “Even after 30 years of fires, these forests don’t recover,” she added.

Transition to Savanna

The researchers analyzed Landsat satellite images of forested areas for wildfires between 1973 and 2014. Bernardo M. Flores, a researcher at the Federal University of Santa Catarina, Brazil, and lead author of the new paper, explained that over those 40 years, around 100 square kilometers of the Rio Negro region burned at least once. Additionally, in 2015–2016, data reflected an anomaly: the strongest El Niño in 100 years. This weather phenomenon brought extreme droughts to the region and is associated with the burning of around 700 square kilometers.

“Long-term, progressive conversion to nonforest has already occurred in the fertile floodplain of the more densely occupied parts of the Amazon main stem,” Nelson said. Now, he added, “Bernardo has shown that even the remote floodplains of nutrient-poor blackwater rivers, with very low population density, can go the same route.”

The conversion of forest to savanna has atmospheric and biological implications: In addition to being greater carbon sinks than grasslands, forests also control soil erosion and maintain water quality. These ecosystem services “increase the abundance of fish and other resources for local communities,” Flores said.

Climate Change and Declining Resilience to Wildfires

By bringing warmer temperatures and more severe weather, climate emergency “is changing fire regimes across the world. Wildfires are becoming more severe. And in the tropics, forests are shrinking because of deforestation and wildfires,” Flores said.

Anderson, too, stressed the need to incorporate climate data when modeling the future of the rain forest. “We need to think about climate change projections because with higher temperatures and lower rainfall, there is greater stress on both these [burned] forests and primary forests,” she explained.

—Rishika Pardikar (@rishpardikar), Science Writer