From Forest to Field, How Fire is Transforming the Amazon
 

Before widespread human settlement began to encroach on the borders of 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.

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  Photograph of Fire in the Amazon Rainforest
 

Slash-and-burn agriculture converts forest to farm land, but that obvious destruction is only the beginning. Intentional fires get out of control and burn through the understory of nearby forests, killing, but not completely burning small trees, vines and shrubs. The dead and dying trees collapse, spilling firewood and kindling to the ground and ripping a great tear in the tent of the forest overhead. Logging has a similar effect. The intense tropical sun, previously deflected by the green canopy, heats the forest floor, pushing fire danger even higher. Smoke hangs over the forest and suppresses rainfall. In this damaged, fragmented landscape, the onset of the natural dry season becomes ominous. The El Niño-driven droughts that typically arrive a couple of times per decade become devastating.

 

A fire sweeps through a portion of the Amazon Rainforest, reducing the vegetation to ash. Once rarely touched by fire, the portions of the Amazon near areas of human development are becoming increasingly susceptible to damaging fires. (Photograph copyright Woods Hole Research Center)

  Satellite Image of Fires in the Mato Grosso Region, Brazil

That is the troubling picture emerging from research by ecologist Daniel Nepstad of the Woods Hole Research Center in Massachusetts and his colleagues in Brazil and the United States, who have been working to identify both the causes and the effects of Amazon fires. Recently, Nepstad has been a part of the Large-scale Biosphere-Atmosphere Experiment in Amazonia. According to Nepstad, the first accidental burn that steals into the forest is the beginning of a long, downward spiral that compromises forest health over an area equal to or greater than the amount that is deforested outright each year. As road-building, slash-and-burn deforestation, and agricultural fires continue to spread across Amazonia, the number of points of possible ignition continues to grow.

The Arrival of Fire

When we think of the Amazon, most of us imagine a vast, dripping jungle where an explosion of plants, vines, and trees rise up out of rich, fertile earth. But the Amazon harbors more than a few surprises, one of which is that large portions of the Amazon experience months of seasonal drought every year. In terms of annual rainfall, says Nepstad, “The east and southeast are right on the edge of what it takes to be a forest, and this has been a paradox for some time: How can this portion of the Amazon continue to be lush and evergreen despite the fact that for 4 to 5 months it is not getting much—if any—rain? The answer, we have found, is that trees can tap soil moisture down as far as 20 meters.”

 

During the height of the dry season, fires are widespread along the margin of the Amazon. Fingers of cleared land typically form a “herringbone” pattern as they extend from roads (right side of image). This image from the Moderate Resolution Imaging Spectroradiometer was acquired on the afternoon of August 31, 2003. Red outlines trace the pixels in which fires were burning when the image was taken. The smoke from these fires is so thick it hides the surface in places. [View the full-size image (3.5 MB JPEG) for a sense of the widespread scale of burning.] (Image courtesy MODIS Land Rapid Response Team at NASA GSFC)

Graph of Rainfall in the Amazon Basin

Historically, even during drought years, there were few, if any, natural fire triggers, since most lightning in the region is accompanied by rain. Scientists think the fire return frequency at any given location in the undisturbed Amazon was on the order of hundreds, possibly even thousands of years. Other than the protection of the dark, humid forest itself, rainforest trees and plants have few defenses or adaptations to fire, and why should they? Until the arrival of fire-wielding humans, the forest had few incentives to evolve them.

These days, human impacts in the Amazon are making it more difficult for the forests to endure dry times. Part of the problem is another of the Amazon’s surprises: the soil is infertile. Most of the richness and productivity we associate with the Amazon is bound up in the living organisms in the ecosystem. Layer upon layer of plants, animals, fungi, and bacteria that are spread throughout the forest endlessly recycle the ecosystem’s nutrients. The infertile soil is not well suited to farming, and when the forest is cleared to make way for farms, farmers face the dilemma of how to enrich the soil. In a part of the country where millions of people earn less than $100 per month, expensive soil additives and fertilizer aren’t options. Instead, farmers clear cut the forest and set it on fire in order to turn the nutrients locked up in the forest biomass into a soil-fertilizing ash.

 

‘Wet’ and ‘Dry’ characterize the seasons in the Amazon. From January through April, the Amazon basin gets about 10 mm (0.4 inches) of rain a day—300 mm (1 foot) a month! In June, July, and August the region averages 2 mm (0.08 inches) a day. In contrast, temperate regions (like Washington, D. C.) typically get 50-75 mm (2-3 inches) a month, year round.

During the 1997-98 El Niño, rainfall in the Amazon was more than 20 percent below normal. (Graph by Robert Simmon, based on data from the Global Precipitation Climatology Project)

  Photograph of Jungle Understory compared to Ash from Slash and Burn

This slash-and-burn method of agriculture is never more than a short-term solution. Typically within a few years, the initial influx of nutrients from the burned forest is used up.“I think people are very similar whether they are in Texas or Pará, Brazil,” says Nepstad. People are looking to maximize the profit of their land in the short term; they don’t look too far down the line. If you’re poverty stricken, your viewpoint is even shorter.” When the soil fails, farmers move on to a new patch of forest. The old patch may be abandoned or turned into cattle pasture, which must be re-burned frequently to encourage grasses rather than trees or shrubs.

The data used in this study are available in one or more of NASA's Earth Science Data Centers.

 

Nutrients in the Amazon are contained in the multi-layered rainforest itself, not the soil. To make the land suitable for farming, settlers cut and then burn the dense vegetation. The resulting ash fertilizes the soil, but only for a few years. [Photographs Copyright Lucia Enriconi, Miami Museum of Science (upper), Woods Hole Research Center (lower)]

 

Fire Follows Fire

 

The intentional fires have an all-too-frequent tendency to escape control. Fire steals into nearby forest, where it creeps its way through the understory. Understory fires have a radically different effect on Amazon forests than they do on forests in the western U.S. or the boreal forests of the North, which have evolved in the presence of fire. “When you burn a forest out West,” explains Nepstad, “future flammability of that forest goes down because they are big fires that go through and consume everything but the best-protected trees. In the Amazon, the first fires aren’t big enough or hot enough for that.”

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  Photograph of Ground Cover Fire

The first fires are so small and low-key that Nepstad and his colleagues can walk alongside or step over them, but Nepstad describes the fires’ disastrous long-term effects. “Fire goes through and kills small, thin-barked trees, but doesn’t consume them. This creates a lot of dead wood. 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 West, an understory fire in the Amazon causes the flammability to go way up.”

 

Fires in the Amazon start small—low, slow-moving blazes that feed on the thin layer of dead leaves and detritus on the forest floor. With natural fires so rare, occuring perhaps once every 1,000 years, even these small fires kill some trees. Once they fall, holes left in the rainforest’s protective canopy dry out the understory, setting the stage for a new round of more destructive fires. (Photograph copyright Woods Hole Research Center)

  Photograph of Exposed Rainforest

To determine how great a risk fire poses to Amazonian forests, one of Nepstad’s Woods Hole colleagues, Mark Cochrane, undertook a field study of Amazon fires. Cochrane and several other scientists, including Nepstad, observed fires in study plots in the eastern Amazon before and after the severe El Niño-induced drought of 1998. Of the ten half-hectare forest plots (0.5 hectares is 5,000 square meters or 1.2 acres) in the Tailândia region of eastern Amazonia, eight had already been affected by fire at the start of the study, while two were undisturbed. All the team’s observations of fire characteristics, from intensity, to rate of spread, to how high into the canopy the fire reached, painted the same picture: in the Amazon, on the heels of every fire is another, more devastating blaze.

 

Clearings in the Amazon increase the flammability of the forest. Sunlight penetrates into the understory, warming and drying the forest floor, which is normally protected by the dense canopy. Selective logging, permitted on some protected forest land, also produces breaks in the canopy and increases fire risk. (Photograph Copyright Lucia Enriconi, Miami Museum of Science)

  Map of Burn Frequency

While only 23 percent of previously unburned forest in the Tailândia study area burned during the 1997-98 El Niño, 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. The scientists began to realize that these multiple-burn plots of forest were so drastically thinned of trees that they almost appeared to be deforested, and they wondered what effect these accidentally fire-thinned areas might have on the Brazilian government’s annual deforestation estimates.

 

Although virgin forest in the Amazon is resistant to fire, a single low-intensity fire increases the likelyhood and severity of additional fires. Scientists use satellite data to map burned areas and determine the number of times the forest has burned. This map shows a portion of the Tailândia region. Unburned forest is dark green, burned forests are progressively lighter and more yellow. Intentionally cleared areas are beige. (Map adapted from Cochrane, 2000)

 

Hidden Impacts of Fires and Logging

 

Cochrane and his team looked back at satellite images of deforestation in the Paragominas region of Brazil, one of numerous settlements along the “Arc of Deforestation,” on the developing frontier of the southern Amazon. The obvious deforestation that appears in high-resolution satellite images, such as those from NASA’s Landsat satellite missions, is the basis for Brazil’s assessment of human impact on the forest. Between 1993 and 1995, satellite images suggested a dramatic jump in the rate of Amazonian deforestation. The scientists wanted to know whether that jump might have been due not to intentional, slash-and-burn deforestation, but rather to accidental fire during the 1992 El Niño-induced drought.

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Graph of Deforestation in the Amazon  

In 1994-95, the annual rate of deforestation in the Amazon appeared to almost double over the rates of the previous two years. Cochrane and Nepstad believe this apparent jump was actually due to accidental fire following the 1992-93 El El Niño-related drought. [Graph by Robert Simmon, based on data from the Brazilian Instituto Nacional de Pesquias Espaciais (National Institute of Space Research)]

 

They pored over satellite images from the Landsat satellite captured between 1993 and 1995 to see what happened to all the area that was “deforested” in 1992. They decided that cleared areas with nice, straight lines that had sprung up next to pre-existing forest edges were probably intentional clearings for pasture or farmland. But Cochrane and his colleagues also discovered that much of the area labeled as deforested in 1992 was far from forest edges, was irregular in shape, and was already beginning to regrow. These areas, it seemed clear, were probably not the result of intentional deforestation. Instead, the scientists concluded, these tracts of forest had been “impoverished” by accidental fire. Cochrane estimated that accidental fires inflated the estimates of intentional deforestation during that period by 129 percent; in other words, more than half of what was called deforested during that time period was due to accidental fire.

This distinction between accidental and intentional is important, explains Nepstad, because “it suggests that much of the impoverishment of the Amazon may have nothing to do with people’s plans for the land.” It gives governments and conservation groups a target for better land management that doesn’t immediately conflict with local land use needs. If the land was only burned accidentally, then on the surface at least, protecting it should not be as difficult as convincing a local land owner to give up his cattle pasture in the interest of forest conservation.

The impacts of fires during the 1997-98 El Niño were dramatic and easily visible in satellite imagery, leading to an overestimate of deforestation for that time period. But unless an area has been repeatedly thinned through multiple burns, it may not be easily detected in satellite imagery. This is especially true of the small, first-time burns whose low-intensity presence sets the stage for future devastation. By failing to include these not-so-obvious burns, we may be seriously underestimating human impacts on the forest. In addition, we may be underestimating how greatly fire in the Amazon may be adding to global greenhouse gases. Nepstad says that during the last El Niño in 1998, the real amount of carbon emissions from fires in the Amazon may have been 200-300 million tons—nearly twice what scientists first thought.

  Comparison of normal and fire-induced deforestation patterns

Cochrane estimated that more than half of what was counted as deforested in 1994-95 was actually cleared by accidental fires. In satellite observations, such as the two example regions shown here, intentionally cleared forest areas had sharp, geometric edges (olive green shapes in top image) and usually extended from existing pasture (light yellow). Areas cleared by accidental fire (red, bottom image) had more ragged edges and were often far from developed land. The burned areas began to regrow shortly after the El NiNiño—another indication they were accidentally cleared.(Map adapted from Cochrane, 2000)

  Landsat Image of Degraded Forest
 

The hidden impact of accidental fire combines with another threat to forests: logging. Unless an area is completely deforested by loggers, it’s not likely to be labeled as such in Brazil’s annual tally. Partially logged or burned areas are visible in satellite imagery, but they begin to regrow so quickly that they are often overlooked in the annual estimate. Nepstad decided to try to determine how much undocumented, impoverished forest might exist throughout the Amazon.

He and his colleagues combed though timber mill records and interviewed mill operators about how much wood they harvested between 1996 and 1997, and then they back-calculated the area of forest needed to provide that much wood. They went out into the field to verify that the records and their estimates were correct. Based on their research, Nepstad says that logging impoverished 10,000-15,000 square kilometers of undisturbed forest per year in 1996 and 1997, an amount that is at least half of the amount that was totally deforested during the same time.

 

Selective logging and understory fires don’t completely destroy the rainforest, but instead degrade the forest. Satellite data, like the image above (composed of near-infrared, red, and green wavelengths of light) does not clearly differentiate untouched from degraded forest. Forested regions (dark red and textured areas) are easily distinguishable from cleared regions (light red or gray areas). The appearance of burned forest and logged forest is similar to primary forest. (Image by Robert Simmon, based on Landsat-5 Thematic Mapper data provided by the UMD Global Land Cover Facility)

  Maps of Degraded Forest
 

Writing in a letter to the scientific journal Nature, Nepstad presents a stark picture of human impact on the Amazon: “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.”

Drought may be becoming a more frequent occurrence in the Amazon, with the fires themselves adding insult to injury. The smoke released by biomass burning interferes with the Amazon’s great self-watering cycle, in which evapotranspiration from the forest feeds some of the clouds that add to the regional rainfall. More smoke equals less rain. Add to that the feeling of some experts that climate change may bring more frequent El Niño events, which amplify the seasonal droughts, and parts of the Amazon may be facing a much drier future.

To help Brazil cope with droughts and fire, Nepstad and his colleagues use rainfall, soil moisture data, and satellite observations of the total solar radiation (which drives evaporation from the soil as well as evapotranspiration from vegetation) to model how much soil moisture is available to the forest during times of drought. They turn this information into Fire Risk maps that the Brazilian government can use for planning. In 2001, available soil water dropped to 50 percent of maximum over half the Amazon basin and to only 25 percent of maximum in about one third of the forests. When the available water drops below the 25 percent level, the forests’ leaf area is reduced by about the same amount. The thin canopy and dry leaves on the ground cause fire danger to skyrocket. One trigger can ignite thousands of square kilometers of forest.

 

Standard deforestation analysis (above left) categorizes land as either forested (deep green) or cleared (beige). When Nepstad refined satellite observations using interviews with landholders, it became clear that much of the forested area around Paragominas was not undisturbed, but instead had been degraded by logging and accidental fire (above right). These images were derived from the Landsat scene above and show a wider view of the region around the town of Paragominas. Degraded forest can be easily destroyed by accidental fire and does not support the diversity of species of undisturbed forest. (Map adapted by Robert Simmon from Nepstad et al., 1999)

 

People and Fires

 

Controlling those triggers, says Nepstad, isn’t easy in a social climate where fire has come to be viewed as a ‘chronic emergency.’ “Every few years,” he explains, “there will be a really bad fire season, where smoke closes airlines and parks and preserves are threatened. Then there is a knee-jerk reaction among the media and some in the government to treat it as a sudden emergency that you can only combat by throwing firefighting resources at it, rather than seeing the fires with a long-term perspective, which would cause you to see that poor frontier management is causing it.”

The ever-present threat of fires traps local people into superficial uses of the land that don’t require much investment. These superficial uses are also called “extensive land use” because they encourage people to make minimal use of a large extent of land, rather than putting a smaller area of forest to more intensive use. “Every farmer or landowner you talk to has a story,” says Nepstad, “about how his neighbor’s cattle-pasture fire got out of control and burned into his property. He’ll say, ‘Yeah, I used to have some orange trees,’ or something, ‘but they got burned up by my neighbor’s pasture fire.’“

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  Photograph of Pasture Cut out of the Amazon Rainforest

Those stories draw attention to how difficult it is to escape the cycle of accidental fire. Logging causes fire; drought causes fire; fire causes fire. With the increasing threat of fire, landholders are reluctant to use their land for anything that might be wiped out by fire or to spend their money on fire prevention measures on their own property when they are just as threatened by their neighbor’s land management decisions. Nobody wants to try to maintain a plot of sustainably harvested timber land if it is just going to get burned up when the neighbor’s pasture fire gets out of control.

 

Frequent fires along the developing margin of the Amazon discourage settlers from improving their land. As a result, landholders graze cattle or abandon their property when the land becomes infertile. These types of land use increase the frequency of fire, further discouraging land management practices that better preserve the forest. (Photograph courtesy Compton Tucker, NASA GSFC)

  Map of Fire Susceptibility

Instead, land owners anticipate failure, and when it happens, they just move deeper into the forest. In the wake of slash-and-burn operations both big and small, the Amazon is being transformed from forest to savanna, and thousands of species of plants, animals, fungi, and bacteria are likely being lost to extinction—living organisms that in addition to their own innate value have also been a source of everything from food to furniture to pesticides to medicines. Looking into the future of his study area around 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.”

 

Fires are widespread along the south and east edges of the Amazon, and flammable forests are located deep in the heart of the region. Fire threatens to transform the dense rainforest into scrub and grassland. This map shows the state of the forest and occurence of fires in 1998. (Map adapted from Nepstad et al., 2001)

 

Amazonia’s Future

 

Nepstad hopes his research will help Brazil prepare. Along with colleagues from Brazil, Nepstad formed a research and education institute in Brazil called IPAM (which in Portuguese stands for Instituto de Pesquisa Ambiental da Amazonia, or, in English, the Amazon Institute of Environmental Research). IPAM provides scientific expertise to governments and conservation groups throughout the country who are trying to develop good land management policies for the Amazon but lack the information they need to create them.

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.”

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  Landsat Image of Deforestation along a Road
 

Nepstad and his colleagues at IPAM don’t, for example, try to fight every single road that is planned for the Amazon frontier, even though their analysis of satellite imagery collected over the already developed frontier suggests that Brazil’s current road paving plans will likely stimulate an additional 120,000-270,000 square kilometers of deforestation and forest impoverishment. Instead, he hopes the research will help the government see the wisdom in investing more in roads that will link existing frontier communities.

 

Development in the Amazon usually extends from roads cut through the forest. This true-color Landsat Enhanced Thematic Mapper plus (ETM+) image of a portion of the Brazilian state of Pará was acquired on July 9, 2001. (Image by Robert Simmon, based on Landsat data provided by the UMD Global Land Cover Facility.)

  Map of Planned Road Paving

Nepstad thinks the best chance for the sustainable use of the forest will come from policies geared toward encouraging landowners to make more intensive use of the already developed land along the frontier rather than allowing the frontier to expand farther into the forest interior. Nepstad sees an Amazon that is a network of parks and preserves interspersed with permanent, well-managed agricultural areas, and the harvest of sustainable timber and other forest products.

 

Paved roads accelerate forest degradation, enabling damage up to 50 km from the path of the road. This map shows areas currently influenced by paved roads (red) and areas vulnerable to planned road paving (yellow). The planned paving projects will isolate areas of virgin forest from one another, and allow development in previously inaccessible regions of the Amazon. (Map adapted from Nepstad et al., 2001)

 

He’s optimistic, but nevertheless seems saddened by some of the destruction he’s seen in the 20 or so years he has been visiting the Amazon. He described seeing an intense canopy fire consume all of the few remaining trees left in an already burned patch of forest. “Once I was about 50 meters from a crown fire moving through a patch of logged forest, and I couldn’t get any closer—it was so hot. That for me symbolizes what we are trying to prevent—a patch of forest with only a few trees remaining, and then even that goes up in flames and there is nothing left.”

  • References:
  • Cochrane , M.A. Forest fire, deforestation and landcover change in the Brazilian Amazon. 2000. Volume 1: Proceedings from, The Joint Fire Science Conference and Workshop, June 15-17, 1999, “Crossing the Millennium: Integrating Spatial Technologies and Ecological Principals for a New Age in Fire Management” , L.F. Neuenschwander, K.C. Ryan, G.E. Gollberg, and J.D. Greer (eds.), University of Idaho and the International Association of Wildland Fire, Moscow, Idaho. 170-176.
  • Cochrane, M.A., A. Alencar, M.D. Schulze, C.M. Souza, Jr, D.C. Nepstad, P. Lefebvre, and E.A. Davidson. 1999. Positive feedbacks in the fire dynamic of closed canopy tropical forests. Science 284: (5421): 1832-5.
  • Nepstad, D., A. Veríssimo, A. Alencar, C. Nobres, E. Lima, P. Lefebvre, P. Schlesinger, C. Potter, P. Moutinho, E. Mendoza, M. Cochrane, and V. Brooks. 1999. Large-scale impoverishment of Amazonian forests by logging and fire. Nature 398: 505-508.
  • Nepstad, D., D. McGrath, A. Alencar, A.C. Barros, G. Carvalho, M. Santilli, and M. del C. Vera Diaz. 2002. Frontier governance in Amazonia. Science 295: 629-631.
  • Nepstad, D., G. Carvalho, A.C. Barros, A. Alencar, J.P. Capobianco, J. Bishop, P. Moutinho, P. Lefebvre, and U. Lopes da Silva, Jr. 2001. Road paving, fire regime feedbacks, and the future of Amazon forests. Forest Ecology and Management 5524: 1-13.
 

Photograph of Children and SmokeThe entire Amazon ecosystem cannot be protected from all human development. Nepstad’s vision of the Amazon includes people, but people who are making their living from the forest in a sustainable way. He hopes his research will help planners make sustainable choices for the Amazon and its residents’ future. (Photograph courtesy United States Forest Service)