Text Size

From Forest to Field: How Fire is Transforming the Amazon

Current estimates of Amazon deforestation may capture less than half of the area degraded by logging and accidental fire. If the current trends continue, the entire Amazon frontier could be transformed into grass or scrubland.

Before widespread human settlement began to encroach on South America's Amazon forests, there was no such thing as an Amazon fire season. Now, fire may pose the biggest threat to the survival of the Amazon ecosystem.

Ecologist Daniel Nepstad of the Woods Hole Research Center in Massachusetts and his colleagues in Brazil and the United States have been working to identify the causes and the effects of Amazon fires. A long-time researcher in the field of tropical forest ecology, Nepstad has recently been a part of the Large-scale Biosphere-Atmosphere Experiment in Amazonia (LBA), an international research effort led by Brazil. Through its LBA-ECO program, NASA funds scientists who participate in LBA.

Photograph of burning field and smoke in the Amazon
  Image Left: Once rarely touched by fire, portions of the Amazon near areas of human development are becoming increasingly susceptible to damaging fires. Even small, mild fires will increase the frequency and intensity of future fires. (Photograph courtesy United States Forest Service)

The obvious slash-and-burn deforestation that is used to clear land for farming and cattle grazing is only one of the problems facing the forests of the Amazon. Accidental fire and logging may be damaging as much forest area as the amount that is totally deforested each year.

Fire Follows Fire
Until the arrival of fire-wielding humans, rainforest trees and plants were under little threat from fire. Although large portions of the Amazon do experience months of seasonal drought each year, "the forest survives because the trees can tap soil moisture down as far as 20 meters," explains Nepstad. Lightning, the only natural fire trigger, was almost always followed by rain.

Now, however, fire is a common land management tool in the Amazon, and fires that people set on purpose to clear pasture or farmland often get out of control and burn through the understory of nearby undisturbed forests.

"Fire goes through and kills small, thin-barked trees, but doesn't consume them," says Nepstad. "When the dead trees fall, they create openings in the forest canopy. Any time you punch a bunch of holes in the canopy and let the sunlight in--either through understory fires and the killing of trees, or logging, or drought--you increase fire risk. Unlike the forests out in the western U.S., an understory fire in the Amazon causes the flammability to go way up."

Nepstad participated in a field study of fire in ten forest plots in the eastern Amazon. The study was led by Mark Cochrane, one of Nepstad's colleagues at Woods Hole. The scientists found that while only 23 percent of previously unburned forest burned during the 1997-98 El Niño (which produced a severe drought and fire season), the percentage jumped to more than 39 percent for areas that had already burned once before, 48 percent for areas that had burned twice already, and to a staggering 69 percent for those areas that had burned three times before the study began.

Landsat image of forest degredation in the Amazon
  Image Left: Scientists use satellite data, like this Landsat image (composed of near-infrared, red, and green wavelengths of light) to differentiate between degraded and untouched forest. Dark red areas are forested, while bright pale red areas have been cleared. Computer classification systems also detect areas that have been burned and logged. (Image by Robert Simmon, based on Landsat-5 Thematic Mapper data provided by the UMD Global Land Cover Facility )

Trying to figure out what effect these accidental fires could be having on annual deforestation estimates, they studied Landsat satellite images collected between 1993 and 1995 over the Paragominas region on the southern Amazon frontier. The images revealed that more than half of the land originally classified as deforested immediately following the 1992 El Niñ,o drought was growing back. The areas had probably been severely burned by accidental fires and were starting to re-grow.

Hidden Impacts of Fire
On the other hand, unless the impact of the burning is dramatic enough to mimic deforestation, it's not easily detected in the satellite imagery that forms the basis of Brazil's annual deforestation estimates. Small, first-time burns in particular often go unnoticed, as do selectively logged areas, even though both increase forest flammability. Nepstad calls these degraded areas of forest "impoverished."

Writing in a letter to the scientific journal Nature, Nepstad said, "Overall, we find that present estimates of annual deforestation for Brazilian Amazonia capture less than half of the forest area that is impoverished each year, and even less during years of extreme drought."

In the wake of fires, the Amazon is being transformed. Looking into the future of his study area around the Amazon frontier in Paragominas and Tailândia, Brazil, Cochrane wrote in 1999:

Left unchecked, the current fire regime will result in an inexorable transition of the entire area to either scrub or grassland. Effects on the regional climate, biodiversity, and economy are likely to be severe. These fire-induced changes will take several years to occur, but are likely to be irreversible under the current climate conditions.

How many years are "several"? Nepstad hesitates. "Let's see how prepared Brazil is for the next El Niño--and how quickly the next El Niño comes back."

It's hard not to think that the Amazon is doomed, but Nepstad still has hope. "I really do feel optimistic,"he says, "but I admit I have lowered my expectations. When I first started research in the Amazon, my colleagues and I used to talk about preserving the whole thing as some sort of 'last great sanctuary.' But to do that we would be asking the Brazilian government and people to do something that no other civilization has managed to do. Instead, we think we have to focus on an Amazon with people in it, but people who are making their living in a sustainable way from the forest."

This article was excerpted from From Forest to Field: How Fire is Transforming the Amazon on NASA's Earth Observatory

Rebecca Lindsey
NASA's Earth Observatory