The biggest killers of Amazon trees might not be the massive walls of flame set by humans for deforestation, but stealthy understory fires that eat away at the trees like a cancer beneath the canopy.

Understory fires in the Amazon aren’t as dramatic as the infernos set by people clearing the forest for agriculture. However, NASA satellite images found that inconspicuous understory fires ravage several times more area than massive land-clearing blazes.

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As their name implies, understory fires burn through the vegetation of the lower levels of the Amazon forest. The fires creep slowly through the vegetation, moving only a few feet per minute, but killing 10 to 50 percent of the trees in the area.

These undercover infernos appear as only wisps of smoke rising from the forest canopy, which make them hard to detect. However Doug Morton of NASA led a team that developed a way of tracking understory fire frequency using data from the Moderate Resolution Imaging Spectroradiometer, or MODIS, instrument on NASA’s Terra satellite. Their research appeared in the Philosophical Transactions of the Royal Society B.

Between 1999 and 2010, understory fires devoured 33,000 square miles (85,500 square kilometers), or 2.8 percent of the forest, according to the study.

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The culprits behind the blazes seem to have been a combination of dry conditions and the presence of humans to spark a blaze with untended cooking fires, discarded cigarettes or other mundane flames. However, the researchers didn’t find a correlation between massive land-clearing infernos and the understory 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 in a press release. “You can look within an indigenous reserve where there is no deforestation and see enormous understory fires. 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.”

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Sea surface temperatures can help forecast a bad burning season for the Amazon. Warmer surface temperatures often correlate to more fires.

This year, could be worse that average for the Brazilian Amazon, according to a forecast made by University of California, Irvine and NASA scientists.


Sunrise over the Amazon River Basin (Galen Rowell/Corbis)

Map of fire frequency in the Amazon understory (NASA’s Earth Observatory)