Fire has been used as an agricultural management tool for millennia. It helps to combat pests, weeds and disease, clears debris, and recycles nutrients in the soil. But over the last decade, destructive fires have been taking their toll on the Amazon. These fires lead to forest degradation, release stored carbon into the atmosphere, impair air quality, and damage property.

While previous studies have found that drought and proximity to roads increase the frequency of fires, a team of researchers from Columbia University and the Center for International Forestry Research have found a new cause. Their study, published in the Proceedings of the National Academy of Sciences, found that decreases in rural populations is an additional factor in the increased incidence of fires.

Factors that contribute to fires are poorly understood, and climate variability has only made the picture murkier. Climate is changing patterns of drought and humidity. Severe droughts hit Amazonia in 2005 and 2010, and agriculture-related fires became a major problem in their aftermath. In addition to drought, increased fire risk in the Peruvian Amazon is likely due to a several factors that interact with drought severity, such as increased flammability of forests due to timber extraction, and the repeated burning for extension of roadways.

In addition to physical factors, economic factors also influence increased fire risk. Policies that stimulate agricultural development and road construction provide farmers with economic incentives and access to develop the land, and they have led to increased fire activity elsewhere in the Amazon.

The researchers analyzed climate, remote sensing, province-level census data, and farmer surveys to determine the contributions of climate, land use patterns, and socioeconomic factors (namely rural migration) on fire activity. Farmer surveys were collected in 2010 for 37 communities in a smaller focus area, and looked at things like population density, land use, location of residence of land owners, and degree of implementation of fire control methods. These surveys also provided data on the frequency and extent of burn scars, bare patches of ground where some of the vegetative cover has been burned off by either controlled or uncontrolled fires.

This smaller regional model revealed that the occurrence and location of fires was associated with drought severity, proximity to roads and rivers, and the extent of pastures and agricultural crops. In areas where agricultural crops covered more than 20 percent of the land area, fire risk more than doubled from wet to dry years.

The association of fires with economic activity might lead you to expect that having more people in rural areas would increase fire risk. But, contrary to that, the researchers discovered that declining rural populations at the provincial scale were associated with greater fire frequency and larger burn scars. The decline in rural populations and expansion of road infrastructure, when combined with an increasingly unpredictable climate, signal greater damage from fire in the future.

Although several initiatives have been implemented to avoid or minimize the risks associated with agricultural fires, these policies won’t be effective unless they address the factors that contribute to the fires. The authors note that the implementation of early warning systems could reduce the risk to humans and ecosystems, and policies that promote low-fire land use in areas where land owners are often absent, could also reduce the damage.

PNAS, 2012. DOI: 10.1073/pnas.1215567110 (About DOIs).