Enough Amazon rainforest to cover Connecticut was razed last year, according to satellite images. But a new survey, reported in tomorrow's issue of Nature, suggests the loss may be even more staggering.
The destruction of Amazon rainforest peaked in the late 1980s but still remains at roughly 17,000 square kilometers per year. To measure that pace, researchers have relied on the Landsat satellite's Thematic Mapper to identify vegetation. But although clear-cutting is plain to see, partial logging is often obscured by remaining trees and green vegetation. "There's some ambiguity when you look at those images," says Daniel Nepstad, a forest ecologist at the Woods Hole Research Center in Massachusetts.
To determine just how well satellites capture the big picture of forest damage, Nepstad and colleagues at Brazil's Amazon Institute for Environmental Research in Belém convinced more than 1300 sawmill owners in the Amazon to estimate how much wood they harvested in 1996 and 1997. The researchers confirmed the accuracy of these reports by comparing them to airplane surveys, which can detect partial logging. Nepstad and his colleagues estimate that between 10,000 and 15,000 square kilometers are logged to some degree each year without appearing deforested on the Landsat maps. Partial logging can reduce shade on the forest floor by 15% to 50%, drying it out and creating a fire hazard.
Clearly, satellite photos can't pick up every bit of forest damage, says Diogenes Alves of the National Institute for Space Research in São Paulo. But estimates from Nepstad's interviews may not be entirely accurate, because the amount of logging may vary widely across the Amazon, he says. Nepstad says more sophisticated analyses of light are being developed to improve Landsat's ability to distinguish a healthy section of forest from a logged one.