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Study: Hidden wildfires taking big toll on Amazon rainforest

June 8, 2013
Courtesy of NASA Jet Propulsion 
Laboratory/Kathryn Hansen
and World Science staff

A pre­vi­ously un­mapped type of Am­a­zon rain­for­est wild­fire is de­stroy­ing sev­er­al times more for­est than defor­esta­t­ion wipes out, NASA sci­en­tists say. 

In the south­ern rain­for­est, fires be­low the tree­tops, or “un­der­story fires,” have been in­vis­i­ble to NASA satel­lites, but a new meth­od has led to the first re­gion­al es­ti­mate of their dam­age.

Re­search­ers 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. (Im­age cred­it: NA­SA's Earth Ob­serv­a­to­ry)


“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 most 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, Mor­ton said. The study fin­gers cli­mate con­di­tions, not defor­esta­t­ion, as the most im­por­tant fac­tor in de­ter­min­ing fire risk.

Under­story fires look “un­re­mark­able when you see them,” 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, he added—much slower than in the Am­a­zon sa­van­na fires, which can spread up to 330 feet (100 me­ters) per min­ute.

But un­der­story fires can dam­age large ar­e­as be­cause un­like some of the sa­van­nah plants, Am­a­zon trees are ill-adapted for fire. The long, slow burn gives way to a creep­ing death that claims 10 to 50 pe­rcent of the burn ar­e­a’s trees, Mor­ton ex­plained. Re­cov­ery is long and slow.

Mor­ton and col­leagues used ob­serva­t­ions from early in the dry sea­son, from June to Au­gust, col­lect­ed by the Mod­er­ate Res­o­lu­tion Im­ag­ing Spec­tro­ra­dio­meter in­stru­ment on NASA’s Ter­ra sat­el­lite. They tracked the tim­ing of fire dam­age and re­cov­ery.

An un­der­story fire in the south­ern Am­a­zon (Im­age cred­it: NA­SA/­Doug Mor­ton)


They found 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 for­est. 

And even though the fires tend to occur near defor­esta­t­ion acti­vity, no link was seen to the ac­tual rate of de­for­es­ta­t­ion. In­stead, fre­quent un­der­story fire ac­ti­vity co­in­cided with low night­time hu­mid­ity, as meas­ured by the At­mos­pher­ic In­fra­red Sound­er in­stru­ment aboard NASA’s Aq­ua sat­el­lite.

“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,” Mor­ton said. Ig­ni­tion could come from cook­ing, camp­ing, cigarettes, cars, ag­ri­cul­tur­al waste burn­ing or other hu­man sources, he added. The find­ings could have im­plica­t­ions for es­ti­mates of car­bon emis­sions from dis­turbed for­ests, the sci­en­tists said.


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A previously unmapped type of Amazon rainforest wildfire is destroying several times more forest than deforestation wipes out, NASA scientists say. In the southern rainforest, fires below the treetops, or “understory fires,” have been invisible to NASA satellites, but a new method has led to the first regional estimate of their damage. “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 most 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, Morton said. The study fingers climate conditions—not deforestation—as the most important factor in determining fire risk in the Amazon regionally. Understory fires look “unremarkable when you see them,” 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, he added—much slower than in the Amazon savanna fires, which can spread up to 330 feet (100 meters) per minute. But understory fires can damage large areas because unlike some of the savannah plants, Amazon trees are ill-adapted for fire. The long, slow burn gives way to a creeping death that claims anywhere from 10 to 50 percent of the burn area’s trees, Morton explained. Recovery is long and slow. Morton and colleagues used observations from early in the dry season, from June to August, collected by the Moderate Resolution Imaging Spectroradiometer, instrument on NASA’s Terra satellite. They tracked the timing of fire damage and recovery, which varies depending on the type of forest disturbance. They found 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 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. The researchers point to climate as the reason that fire-driven deforestation didn’t burn more surrounding forests in these years. Frequent understory fire activity coincides with low nighttime humidity, as measured by the Atmospheric Infrared Sounder instrument aboard NASA’s Aqua satellite. “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,” Morton said. Ignition could come from cooking, camping, cigarettes, cars, agricultural waste burning or any number of human sources, he added. The findings could have implications for estimates of carbon emissions from disturbed forests, the scientists said.