June 08, 2013
The research, published April 22 in the journal Philosophical Transactions of the Royal Society B, used satellite data to show that in some years, understory fires burn a far larger extent of forest than the area deforested for agriculture and cattle pasture. Yet the study found no correlation between understory fires to deforestation.
"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," said lead author Doug Morton of NASA's Goddard Space Flight Center in a statement. "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?"
The answer lay in humidity data from the Atmospheric Infrared Sounder (AIRS) instrument aboard NASA's Aqua satellite. Understory fire frequency coincides with low nighttime humidity, which allows the low-intensity surface fires to continue burning. In other words, climate conditions seem to be the most important factor in the area affected by understory fires.
Therefore areas with low deforestation may experience high rates of burning, according to the Morton.
"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."
Researchers for the first time mapped the extent and frequency of understory fires across a study area (green) spanning 1.2 million square miles (3 million square kilometers) in the southern Amazon forest. Fires were widespread across the forest frontier during the study period from 1999-2010. Recurrent fires, however, are concentrated in areas favored by the confluence of climate conditions suitable for burning and ignition sources from humans. Caption photo credit: NASA's Earth Observatory
While small fires may seem unimportant if they aren't correlated with deforestation, other research has shown that these fires can have a significant impact on forests by reducing their resilience.
"When a forest burns for the first time flame heights rarely exceed 30-40 cm, and the fire moves slowly through the leaf litter," Jos Barlow, an Amazon researcher unaffiliated with the Philosophical Transactions of the Royal Society B study, told mongabay.com. "Although these fires appear relatively innocuous, they are actually very destructive as most rainforest trees have a low tolerance of heat, and their slow speed means that flames stay in contact with trees for long periods of time. As a result, even low-intensity fires kill up to 40 percent of trees. They are also the first step in positive feedback cycle, where the dead trees act to open up the canopy and add fuel to the forest floor, making the forest more flammable and increasing the severity of any additional fire."
Once-burned forests are twice as likely to be deforested as unburned forests, largely because subsequent fires burn with increased velocity and intensity and cause higher tree mortality.
The new findings thus have significant implications for forecasting the future of Amazonian forests. With climate models projecting drier conditions across large swathes of the Southern Amazon, the incidence and extent of small surface fires may increase, exacerbating the effects of climate change-induced drying. The result could be increased carbon emissions from burned forests.
"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.
Emissions from fires in dry years in the Amazon Basin can sometimes rival emissions from some of the world's largest countries. For example, a 2011 study published in the journal Environmental Research Letters estimated that the epic 2010 drought in the Amazon triggered the release of nearly 500 million tons of carbon (1.8 billion tons of carbon dioxide), or more than India's annual greenhouse gas emissions.
- D. C. Morton, Y. Le Page, R. DeFries, G. J. Collatz and G. C. Hurtt. Understorey fire frequency and the fate of burned forests in southern Amazonia. Philosophical Transactions of the Royal Society B June 2013 vol. 368 no. 1619 20120163. Published 22 April 2013 doi: 10.1098/rstb.2012.0163
- Christopher Potter, Steven Klooster, Cyrus Hiatt, Vanessa Genovese and Juan Carlos Castilla-Rubio. Changes in the carbon cycle of Amazon ecosystems during the 2010 drought. Environ. Res. Lett. 6 (July-September 2011) 034024 doi:10.1088/1748-9326/6/3/034024