Secret wildfires - 'understory fires' taking momentous toll on Amazon rain forest
Climate conditions, and not deforestation, seen as most important factor for fire risk in region
There's an insidious, not readily visible threat stalking the Amazon rain forests. "Understory" fires that have previously been hidden from view are now seen on NASA satellites. Scientists say that this type of wildfire in the Amazon rainforest is responsible for destroying several times more forest that has been lost through deforestation in recent years.
Understory fires can damage large areas because Amazon trees are not adapted to fire. Long, slow burn gives way to a creeping death that claims anywhere from 10 to 50 percent of the burn area's trees.
"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 says. The study was published April 22 in Philosophical Transactions of the Royal Society B.
Previous understory fire activity, such as the ones that occurred in 2005, 2007 and 2010 was several times greater than the area of deforestation for expansion of agriculture. Morton says the study goes further and assigns blame to climate conditions - in lieu of the long-believed culprit, deforestation - as the most important factor in determining fire risk in the Amazon.
Fires in the Amazon's savanna areas can burn quickly, spreading up to 330 feet per minute. Grasses and shrubs in these ecosystems typically survive low-intensity surface fires.
Understory fires at the frontier and beyond appear "unremarkable when you see them burning," Morton says. Flames reach on average only a few feet high, visible from the air as ribbons of smoke that escape through the canopy and may burn for weeks at a time.
Understory fires can damage large areas because Amazon trees are not adapted to fire. Long, slow burn gives way to a creeping death that claims anywhere from 10 to 50 percent of the burn area's trees. Recovery is also a long and slow, but observable, process.
In order to identify understory fires, Morton and his colleagues used observations from early in the dry season, from June to August. They tracked the timing of fire damage and recovery, which varies depending on the type of forest disturbance.
Areas of deforestation show up in satellite imagery as land that continues to lack signs of recovery for at least two consecutive years. Signs of forest degradation from understory fires, visible in the year after the burn, vanish quickly as the forest replenishes. This pattern of damage and recovery over multiple years provides a fingerprint of understory fire damages in Amazon forests.
Between 1999 and 2010, understory forest fires burned more than 33,000 square miles, or 2.8 percent of the forest. Results also show no correlation between understory fires and deforestation. 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.
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