Satellite images reveal Amazon forest shrinking faster

Brazil's Amazon rain forest - one of the most biologically productive regions on the planet - is disappearing twice as fast as scientists previously estimated.

That is the stark conclusion ecologist Gregory Asner and his colleagues reached after developing a new way to analyze satellite images to track logging there.

The team traces the additional loss to selective logging, which some environmental groups say is ocurring illegally. The technique removes trees piecemeal from a forest, rather than carving large swaths. This has made it easier to hide. This project is the first time satellites have been used to track selective logging. [Editor's note: The original version identified selective logging as illegal. Not all groups agree that the practice always occurs illegally.]

For the region, this activity increases the forest's vulnerability to wildfires and undermines its biological productivity. Selective logging in the region releases nearly 100 million tons of additional carbon dioxide into the atmosphere each year.

Ecologists and Brazilian officials long have known that selective logging occurs, says Dr. Asner, a researcher at the Carnegie Institution's Department of Global Ecology, based at Stanford University in Palo Alto, Calif. But gauging its extent by looking at changes in forest cover has been difficult. Sawmill surveys can yield results that either are incomplete or unreliable. Previous methods for analyzing Landsat satellite images couldn't render such tiny details.

Asner and his colleagues suspected their more tightly focused view would bring bad news. But the extent of the damage still surprised them. As the team huddled around a supercomputer terminal watching the first numbers emerge, "They were more than double what I expected," Asner recalls. "It's exciting science, but sobering."

For Daniel Nepstad, an ecologist at the Woods Hole Research Center an environmental policy and research organization in Woods Hole, Mass., the study "puts to rest a long-standing debate about how extensive selective logging is in the Amazon."

The results come from new supercomputer software the team developed. Over several years, the research group - which included scientists from Brazil - gradually squeezed more detail out of satellite images. Initially the team could detect changes to a patch of forest roughly 14 miles on a side. That shrank to a patch 98 feet on a side - small enough to spot the holes selective logging leave in the forest canopy. Asner's group fed Landsat images from 1999 to 2002 into the computer, which hunted for the telltale holes.

The results boosted the estimates of deforestation during the period from 60 to 123 percent, depending on which of the five logging-intensive Brazilian states they examined. The group verified the results with field surveys. Their results appear in Friday's edition of the journal Science.

Over the long term, Asner adds, he plans to use the technique to look at other tropical rain forests, such as those in Peru and Bolivia.

Deforestation can radically alter the environmental "services" the forests provide - from scrubbing the atmosphere of CO2 and harboring useful plants and animals to reducing erosion.

For example, in a related Science research paper, Columbia University ecologist Daniel Bunker and colleagues found that above-ground carbon storage varied widely, depending on which tree species vanish from within a patch of tropical rain forest and what triggered their loss.

In a 123-acre tropical-forest research site in Panama, they experimented with different mixes of species. The results show that carbon storage is strongly influenced by the types of trees present and the ways in which they are lost. Selective logging of prized hardwoods removes a small number of species from a forest. But it substantially reduces the forest's above-ground carbon storage because the lost wood is dense.

The bottom line, Dr. Bunker says, is that preserving species diversity may be the best way to ensure humans continue to reap the services healthy ecosystems provide.