printable versionSmall-scale logging often leads to major clear-cutting in the Amazon rainforest
Greg Asner
Selective logging of individual trees in the Brazilian Amazon is often followed by large-scale clear-cutting of the rainforest, according to a new study led by scientists from Stanford and the Carnegie Institution's Department of Global Ecology. The study, published in the July 31 online edition of the Proceedings of the National Academy of Sciences (PNAS), is the first to quantify the relationship between small-scale logging and complete deforestation, or clear-cutting, according to the authors.
"The link between selective logging and clear-cutting is a one-two punch," said Carnegie scientist Greg Asner, lead author of the study and assistant professor, by courtesy, of geological and environmental sciences at Stanford. "Once a forest is selectively logged, it is likely on the path to destruction."
The Amazon Basin contains the largest contiguous rainforest on Earth—a vast region nearly as big the continental United States that includes Brazil and seven other South American countries. In a typical year, approximately 5,800 square miles of Amazon forest—an area roughly the size of Connecticut—are burned or clear-cut to make way for cattle ranching, farming and other development, according to satellite estimates. Unregulated selective logging of mahogany and other hardwoods also is widespread in the region.
In the PNAS study, Asner and his colleagues discovered that 16 percent of selectively logged forests in Brazil were clear-cut within one year, and 32 percent were cleared within four years. Virtually all of the damage occurred within 15 miles of major roadways, they found.
Satellite surveysFor the study, the scientists used advanced satellite remote-sensing techniques developed in the Asner lab to calculate how much forest had been selectively logged, and then integrated those data with official Brazilian deforestation maps. The remote-sensing techniques measure canopy foliage at a spatial resolution of 98 feet by 98 feet. Foliage cover is important because it regulates such processes as the rate of photosynthesis, water balance, plant and animal population dynamics, and most critically, the probability of drought and fire. Through advanced computational methods, the scientists can determine the level of canopy damage and how long it takes to grow back.
The research team identified areas that had been selectively logged, then tracked those gaps in the harvested forests, which covered 17,760 square miles across four Brazilian states. "We surveyed an area that is about three times the size of Texas from 1999 to 2004," Asner noted.
According to the five-year surveys, the likelihood that a selectively logged area would be clear-cut was highly dependent on its proximity to significant roadways. Most selective logging was concentrated within 3 miles of major roads, and logged areas located 3 to 15.5 miles from roads were two to four times more likely to be cleared than intact forests—an indication that selective logging blazes the trail for deforestation, according to the authors.
Asner and his colleagues were surprised to discover such a tight relationship between these two land-use activities, because they involve different financial interests—selective logging is done for timber, while clear-cutting is a technique primarily used by farmers and ranchers.
Overall, the researchers concluded that selective-logging operations in Brazil were highly damaging, although federally regulated preserves were much less disturbed than unprotected forests. These results come on the heels of recent Brazilian legislation designed to regulate logging to improve sustainability and the announcement by Brazilian officials to develop a remote-sensing system to monitor logging from space.
"The new Brazilian timber-concession laws for federally protected lands could bring more control over both the high levels of forest damage caused by current logging operations and the loss of selectively logged forest to full deforestation," Asner said.
Other co-authors of the PNAS study are Eben N. Broadbent, Paulo J. C. Oliveira and David E. Knapp of the Carnegie Department of Global Ecology at Stanford; Michael Keller of the U.S. Forest Service and the University of New Hampshire; and José N. M. Silva of the Brazilian Ministry of Agriculture. This research was funded by NASA, and high-resolution digital deforestation maps were provided the Brazilian National Institute for Space Research (INPE).
