Once burnt…

 作者:康惑     |      日期:2019-03-08 02:20:01
By Fred Pearce THE cycle of fires in the Amazon threatens to transform vast tracts of rainforest into scrub or grassland, say researchers in Massachusetts and Brazil. Mark Cochrane of the Woods Hole Research Center in Massachusetts and his colleagues studied the dynamics of forest fires by looking at experimental plots and satellite images of rainforest. The researchers, who published their findings in Science last week (vol 284, p 1832), warn that the volume of dead and dry timber is accumulating rapidly, turning large areas into a tinderbox. “Tens of thousands of square kilometres of forest may be difficult or impossible to save,” says Cochrane. Fires create a feedback which makes a forest more likely to succumb to future blazes. The first fire to sweep through a region leaves many trees charred though intact, but any dead trees left standing provide fuel for the next fire. “The first fire leaves more combustible fuel than it consumes,” says Cochrane. As a result, later fires engulf areas more quickly and are more intense. Cochrane’s research shows that these second fires, whether started by people or triggered by lightning, often generate ten times more heat than the first—enough to consume any remaining large trees. Conventional satellite images of deforestation often miss areas at risk. Cochrane has re-examined satellite images of the Amazon rainforest from recent years, using detailed spectral analysis of each pixel to determine how much of a forested area is actually dead wood and leaves. In many areas, around half of the forest has recently burnt and is primed for lasting destruction in the event of a second fire. The team’s analysis also indicates that fires now return to areas within 7 to 14 years, rather than following a cycle lasting centuries or longer. People may be to blame for more frequent fires, as fires are used to clear pasture. “These findings are typical for much of the Amazon and other rainforests such as Indonesia,” says Cochrane,