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. Credit: NASA's Earth Observatory Enlarge

( —Using an innovative satellite technique, NASA scientists have determined that a previously unmapped type of wildfire in the Amazon rainforest is responsible for destroying several times more forest than has been lost through deforestation in recent years.

In the southern , fires below the forest treetops, or "understory fires," have been hidden from view from NASA satellites that detect actively burning fires. The new method has now led to the first regional estimate of understory fire damages 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," said Doug Morton of NASA's Goddard Space Flight Center in Greenbelt, Md., and the study's lead author. The study was published April 22 in Philosophical Transactions of the Royal Society B.

In years with the highest understory fire activity, such as 2005, 2007 and 2010, the area of forest affected by understory fires was several times greater than the area of deforestation for expansion of agriculture, according to Morton.

The study goes further and fingers climate conditions – not deforestation – as the most important factor in determining fire risk in the Amazon at a regional scale.

Uncovering the Story Behind Understory Fires

Fires in the Amazon's savanna areas can burn quickly, spreading up to 330 feet (100 meters) per minute. Grasses and shrubs in these ecosystems typically survive low-intensity surface fires.

In contrast, understory fires at the frontier and beyond appear "unremarkable when you see them burning," Morton said. Flames reach on average only a few feet high, visible from the air as ribbons of smoke that escape through the canopy. They may burn for weeks at a time, spreading only a few feet (0.5 meters) per minute.

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