For the most part, logging and forest clearing in the Amazon are carried out by different groups for different purposes.
Selective tree harvesters have an interest in maintaining the forests. But ranchers and farmers carry out most of the Amazon's deforestation in their efforts to clear new areas for agricultural use.
The new study highlights how the two practices are linked by road building.
Within about three miles (five kilometers) of main roads, both logged and unlogged areas are highly vulnerable to deforestation, the research showed.
In a broader band, from 3.1 to 15.5 miles (5 to 25 kilometers) from main roads, logged areas are two to four times more likely than unlogged areas to be cleared within four years.
"Logging often opens up a Pandora's box of spontaneous forest colonization by slash-and-burn farmers, ranchers, hunters, miners, and land speculators," the Smithsonian's Laurance said.
Because of its destructive consequences, selective logging as it's currently practiced appears not to be effective in keeping rain forests intact, the scientists conclude.
"Selective logging has been heralded by the timber industry and conservation groups as a sustainable alternative to clear-cutting," Asner said.
"But if the recent past is any indication, logging has not been treated as a sustainable land use."
The researchers say sustainability depends on the type of logging being practiced.
One encouraging finding was that nearly one-fourth of logged areas showed only minimal damage.
Study co-author Michael Keller says these were likely regions harvested by small-scale operators specializing in particular tree species.
The report also backed other recent studies in showing that reserve areas appear to be highly effective in limiting both logging and deforestation.
(Read related news: "Indigenous Lands Help Protect Amazon Forests, Study Finds" (February 28, 2006).]
The new study also showcases a powerful new method for measuring logging impacts and assessing the condition of tropical forests.
Scientists have long used satellite imagery to monitor deforestation, but damage caused by small-scale logging has been much harder to track from space.
Asner's group spent years developing the first remote monitoring system sensitive enough to detect small disturbances in the forest canopythe overhead cover of leaves and branchesthat signal selective logging.
The more refined system can not only identify where logging is occurring but also measure its impact on forest habitat.
In a written commentary that accompanies the study, Lisa Curran of the School of Forestry and Environmental Studies at Yale University calls the new remote sensing technique a "major breakthrough."
"The broad application [of the new remote sensing method] could potentially revolutionize monitoring of logging operations and improve land management," Curran writes.
The development comes at a critical time for Brazil, which recently enacted sweeping new legislation for Amazon forest management, including much tighter control of logging.
Brazil's National Space Research Institute is developing its own system for large-scale forest monitoring, using methods similar to those of Asner's group.
"I'm very hopeful," Asner said. "The new Brazilian logging regulations promise to radically change what we've observed in the recent past."
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES