Some past approaches to calculating the impacts of forest fires have grossly overestimated the number of live trees that burn up and the amount of carbon dioxide (CO2) released into the atmosphere as a result, in a recent study at Oregon State University, US.
The research was done on the Metolius River Watershed in the central Oregon Cascade Range, where about one-third - or 100,000 acres - of the area burned in four large fires in 2002-03.
Although some previous studies assumed that 30 percent of the mass of living trees was consumed during forest fires, this study found that only 1-3 percent was consumed.
Some estimates done around that time suggested that the B and B Complex fire in 2003, just one of the four Metolius fires, released 600 percent more carbon emissions than all other energy and fossil fuel use that year in the state of Oregon.
But, this study concluded that the four fires combined produced only about 2.5 percent of annual statewide carbon emissions.
Even in 2002, the most extreme fire year in recent history, the researchers estimate that all fires across Oregon emitted only about 22 percent of industrial and fossil fuel emissions in the state - and that number is much lower for most years, about 3 percent on average for the 10 years from 1992 to 2001.
The OSU researchers said there are some serious misconceptions about how much of a forest actually burns during fires, a great range of variability, and much less carbon released than previously suggested.
Some past analyses of carbon release have been based on studies of Canadian forests that are quite different than many U.S. forests, according to the researchers.
"A new appreciation needs to be made of what we're calling 'pyrodiversity,' or wide variation in fire effects and responses," said Garrett Meigs, a research assistant in OSU's Department of Forest Ecosystems and Society. "And more studies should account for the full gradient of fire effects," he added.
The past estimates of fire severity and the amounts of carbon release have often been high and probably overestimated in many cases, according to Beverly Law, a professor of forest ecosystems and society at OSU.
"Most of the immediate carbon emissions are not even from the trees but rather the brush, leaf litter and debris on the forest floor, and even below ground," Law said.
"In the past we often did not assess the effects of fire on trees or carbon dynamics very accurately," Law added.