Until now, satellite-based methods for measuring deforestation across large areas have only been capable of detecting clear-cut swaths of land, where all the trees are removed to clear space for farming or grazing.
A new satellite imaging method, developed by Gregory Asner of the Carnegie Institution of Washington and colleagues, detects deforestation on a finer scale, allowing researchers to identify areas where trees have been thinned, due mostly to "selective logging." In this type of deforestation, only certain marketable tree species are cut and logs are transported offsite to saw-mills. Little has been known about the extent or impacts of selective logging in Amazonia until now, according to the authors.
To detect and quantify the amount of selective logging in the five major timber production states of the Brazilian Amazon, the researchers used the new Carnegie Landsat Analysis System. This technology allowed them to delve into each pixel of the image produced by a trio of satellites and determine the percentage of forested and deforested land within each pixel. (In contrast, the conventional interpretation of a satellite image would consider each pixel as entirely forested or deforested.)
"This method gives us an incredible map of the ubiquitous but very diffuse types of disturbances that exist in Brazil or in any tropical forest," Asner said.
The researchers found that, from 1999 to 2002, selective logging added 60 to 128 percent more damaged forest area than was reported for deforestation alone in the same study period.
The total volume of harvested trees represents roughly 10 to 15 million metric tons of carbon r emoved from the ecosystem, according to the authors. They estimate that this amount represents a 25 percent increase in the overall flow of carbon from the Amazonian forest to the atmosphere.
Logging causes major ecological disruptions as well. Vines threading through the trees can pull down large amounts of vegetation when a tree falls. The forest also becomes drier and more flammable, as the shady canopy is thinned.
"Logged forests are areas of extraordinary damage," Asner said. "A tree crown can be 25 meters. When you knock down a tree it causes a lot of damage in the understory. It's a debris field down there."