Amazon Wildlfires Lead Cause of Rainforest Destruction: A Study

NASA scientists have determined via an innovative satellite technique that a certain type of wildfire in the Amazon rainforest is responsible for destroying several times more forest than that lost through deforestation in regards to recent years.

Called “understory fires,” they have long remained hidden because of their location far below the forest treetops; however, using a new method, researchers have developed the first regional estimate of understory fire damage across the southern Amazon.

"Amazon forests are quite vulnerable to fire, given the frequency of ignitions for deforestation and land management at the forest frontier, but we've never known the regional extent or frequency of these understory fires," Doug Morton of NASA's Goddard Space Flight Center and the study's lead author said in a press release.

The study, published in Philosophical Transactions of the Royal Society B, demonstrates that, in years with the most understory fire activity, such as 2005, 2007 and 2010, the area of forest affected was several times greater than the area affected by deforestation for expansion of agriculture.

Moreover, the study goes further, pointing to climate conditions - not deforestation - as the most important factor in determining fire risk in the Amazon at a regional scale.

Fires in the Amazon's savanna areas can burn quickly, spreading up to 330 feet per minute, according to the researchers, with grasses and shrubs in these ecosystems typically surviving low-intensity surface fires.

In contrast, understory fires appear "unremarkable when you see them burning," given that the flames reach on average only a few feet high. Capable of lasting for several weeks, they spread only a few feet per minute.

Despite this seemingly low-key presence, however, understory fires can damage large areas because Amazon trees are not adapted to fire and the long, slow burn can claim anywhere from 10 to 50 percent of the burn area's trees. Furthermore, the recovery that follows is a long, slow process.

In all, the study shows that between 1999 and 2010, understory forest fires burned more than 33,000 square miles (85,500 square kilometers), or 2.8 percent of the entire forest.

Meanwhile, results showed no correlation between understory fires and deforestation; in fact, as the pressure for clearing led to the highest deforestation rates ever seen from 2003 to 2004, adjacent forests had some of the lowest rates of fires.

"You would think that deforestation activity would significantly increase the risk of fires in the adjacent forested area because deforestation fires are massive, towering infernos," Morton said. "You make a bonfire that is a square kilometer in size, throwing ash and live cinders and preheating the adjacent forest. Why didn't we have more understory fires in 2003 and 2004, when deforestation rates were so high?"

As an answer, the researchers point to climate as the reason that fire-driven deforestation didn;t burn more surrounding forests in these years as frequent understory fire activity coincided with low nighttime humidity, as measured by the Atmospheric Infrared Sounder (AIRS) instrument aboard NASA's Aqua satellite.

"You can look within an indigenous reserve where there is no deforestation and see enormous understory fires," Morton said. "The human presence at the deforestation frontier leads to a risk of forest fires when climate conditions are suitable for burning, with or without deforestation activity."

These human sources could include cooking, camping, cigarettes and cars, among other things.

Going forward, this new understanding regarding the scope of understory fires could have implications for estimates of carbon emissions from disturbed forests, according to the scientists.

"We don't yet have a robust estimate of what the net carbon emissions are from understory fires, but widespread damages suggest that they are important source of emissions that we need to consider," Morton said.

Currently, researchers are looking into the climate mechanisms that, given an ignition source from humans, predispose the southern Amazon to burn.

Among the leading suspects is soil moisture, report scientists at the University of California, Irvine, who are using new information derived from satellite-based measurements of the region that indicate that the amount of water in the ground may be a leading cause for flammability.

Heading the search is Yang Chen and his colleagues who, in a recent study, were able to show that water storage estimates from NASA's Gravity Recovery and Climate Experiment, or GRACE, satellites allow monitoring of the evolution of dry conditions during the fire season.

Published in the Journal of Geophysical Research, the study shows how low water storage in the soil leads to a drier near-ground atmosphere, resulting in drier, more flammable vegetation alongside increases in plant litter and fuel availability.

"A severe fire season in the Amazon is often preceded by low water storage in the soil, and this water deficit in the soil can be detected by the satellites several months before the fire season," Chen said.