Fire and Savannization

Fire in the Amazon

Slow-moving fireline on forest floor

Fire is an important agent of transformation in the Amazon landscape, and not only in terms of deforestation (where it also plays a large role). Every year, low intensity fires impact a large percentage of Amazon forest. This is the phenomenon we have called 'cryptic deforestation' (Nepstad 1999). These fires travel slowly (10-20 kilometers an hour) and only reach a few inches in height, but they can be quite destructive because they do affect larger trees and understory vegetation. Once a forest has burned, it becomes more susceptible to further burning because trees killed by fire fall to the ground, the canopy is more open, and there is increased leaf shedding by affected trees, increasing the fuel load . Susceptible forests tend to burn again, and each time the fire become more intense and destructive.

Cryptic deforestation due to forest fires and logging may affect as much forest area as deforestation each year—and even more during periods of prolonged drought. For instance, during El Niño years up to 25,000 km² of Brazilian forests may be affected by fire. Nevertheless, the impacts of forest fires are still poorly understood because satellites cannot detect their scars clearly. This means they have to be studied on the ground.

Accidental fire advances through a transitional forest in Mato Grosso, 2004

For years we have carried out research on the impact of fires and fire frequency in Amazon forests. We have carried out research on the environmental and economic costs of accidental fires and about the impact of drought on the susceptibility of Amazon forests, and have recently begun a field experiment to learn about the impact of fire frequency on the replacement of forests by savanna-like vegetation along the forest-cerrado (Brazilian savanna). This process, known as Savannization, is key to understanding the fragile balance of the savanna/forest boundary as well as the mechanisms that may trigger a runaway expansion of the savanna, and retraction of the edges of the Amazon rainforest.

Feedback Cycles between Forests, Fire and Landuse Activities

Fires cause leaf shedding and open the forest canopy, increasing the risk of future burns

The mains drivers of fire in the Amazon are logging, cattle ranching and soybean farming. Loggers, alter the forest structure as they harvest trees, making the forest more susceptible, and they use fire to clear areas where to establish their camps. These fires often invade the forest already impacted by logging. Farmers and ranchers use fire to clear the forest to plant pastures or crops. The current rapid expansion of the agricultural and logging frontier over the seasonally-dry evergreen forests of Amazonia points to important questions for which answers are not clear: at what rainfall regime do the undisturbed forests of Amazonia become susceptible to fire? and how significant is the impact of human activity on changing the forest's susceptibility to fire?

Fires result in damages, both environmental and economic, that affect everyone: the small-holder, who loses fences and subsistence plantations; the large ranch owner, who loses his investment in pasture reform the logger, who has to go ever further to find intact forests; and society in general, which loses the benefits of the ecological functions which moderate the climatic and the hydrologic cycles exercised by large expanses of forests.

The positive feedback cycle between land-use change, smoke emissions and climate. Smoke released by fires inhibits rainfall. The transformation of forests into pastures may also inhibit rainfall by reducing evapotranspiration and the absorption of solar radiation by vegetation. El Niño episodes provoke droughts in Amazônia, and may increase in the future through global warming.

On an ecological level, virgin forests of the Amazon function as humid barriers throughout the landscape, preventing the spread of fires, whether intentional or accidental, from pastures and agricultural fields. As forests are affected by fire, they may lose this protective function. In that case, it is possible that vast areas of the Amazonian landscape would become subject to periodic fires. This would certainly lead to negative impacts on biodiversity, diminish the amount of biomass stored in the forests, and would reduce the quantity of water liberated from the soil and returned to the atmosphere by vegetation, which is necessary to maintain the hydrologic cycle and climate of the region.

Positive feedback cycle between extensive production systems (e.g. cattle pasture), accidental fire, and damages to intensive production systems (e.g. tree plantations and agroforestry systems). Fires used in the establishment of cattle pasture and farm plots, and in pasture management, often burn beyond their intended boundaries, damaging perennial crops, agroforestry systems, and forest management systems. These losses encourage producers to continue their use of extensive production systems, perpetuating the fire cycle.

There are other potential feedbacks among landuse activities, forests, fire and climate. One potential scenario is that the expansion of the agricultural frontier into Amazonia could stimulate forest conversion to cattle pasture, subsistence farming and conventional, high-impact logging. These activities in turn rely on fire as a management tool and as the frontier encroaches on forest, fires will become more frequent.

Positive feedback cycle between forest understory fire, selective logging, and forest flammability. Both understory fire and logging open the canopy, kill trees, and increase the fuel load on the forest floor, increasing forest vulnerability to fire.

In addition, agricultural management fires that escape their intended boundaries discourage landholders from investing in fire-sensitive forms of production (perennial crops, forest management) and perpetuate reliance upon cattle pasture, subsistence agriculture, and the use of fire as a management tool. Rainfall inhibition caused by forest conversion to pasture or by smoke increases the likelihood of further burning, a trend that is accentuated by higher Amazon temperatures driven by global warming.

Expansion of agricultural frontier into transition forest in northern Mato Grosso

In the long run, herbs, shrubs and resprouting trees will likely replace species-rich forest in eastern, southern, and perhaps central Amazonia through recurrent burning and rainfall inhibition. Despite the Amazon forest’s remarkable resilience in the face of drought and human disturbance a large-scale shift to savanna like vegetation is plausible.

The Tanguro Savannization Experiment

Experimental fire at Tanguro Ranch

"How frequent do fires need to be before a forest is replaced by Savanna?" What is the role of human activities in that transition? These are the main questions driving our current research on savannization. To replicate the forest fires that take place in the Amazon, we have burnt one square kilometer of forest in the transition forest of northern Mato Grosso state, at Fazenda Tanguro in Querencia. The goal of this research is to better understand what is the impact of fire on the transition forests of the edge of the Amazon and the cerrado (savannas of central Brazil). Too many fires in the Amazon could trigger a process of threatens the forest and transforms it into savannas. During the next six years, we plan to burn another two square kilometers, repeating the fires at different intervals.

Aerial view of an experimental fire

The persistence of forest vegetation depends upon the balance between forest resilience and the disturbances that set back or retard forest recovery processes. In the Amazon and elsewhere in the tropics, forest recovery mechanisms - seed/seedling banks, residual tree roots, and tree seed dispersal from neighboring forest patches - are depleted by intensive land uses, such as mechanized agriculture and prolonged pasture management. As a result, non-forest vegetation is being favored along a gradient of increasing land use intensity and fire frequency. Forest fragmentation and its interactions with climatic phenomena and seed/seedling predation are superimposed upon these land-use determinants of forest recovery, leading to unexpected patterns of forest impoverishment.

©Woods Hole Research Center, 2007