A four-year, comprehensive survey of the Amazon Basin in Brazil reveals that selective logging--the practice of cutting down just one or two tree species in an area--creates an additional 60 to 123 percent more damage than deforestation alone. Combining field surveys with data gathered from a satellite-imaging system that has a resolution as fine as one tree, scientists at the Stanford, Calif., research department of the Carnegie Institution in Washington, D.C., determined that not only have traditional analysis methods missed more than 50 percent of the damage caused by timber harvest, but that selective logging results in 25 percent more greenhouse gases entering the atmosphere than previously thought.

"This was totally surprising to us and alarming to our colleagues, especially those interested in conservation, climate change and the ability of governments like Brazil to enforce environmental laws," says assistant professor Gregory Asner, lead author on the study, which was published today in the journal Science.

Conventional analysis of satellite data can only detect clear-cut and burned areas, and has been unable to distinguish selective logging hidden by the rainforest's dense canopy. It took Asner and his team eight years to develop a technique that can detect and quantify selective logging by analyzing each pixel in the image for spectral data--which contains information about the extent of vegetation in the forest canopy, the amount of debris on the forest floor and how much bare soil is exposed--while also taking into consideration climate conditions.

Using their computer method, called the Carnegie Landsat Analysis System, the team analyzed observations from three NASA satellites--Landsat 7, Terra and Earth Observing 1--which accumulated data on five Brazilian states between 1999 and 2002.They were able to distinguish areas 30 meters by 30 meters and determined that selective logging ranged from 12,000 square kilometers to nearly 20,000 square kilometers in some states.

To confirm their results, the researchers compared their data with field studies that measured the canopy damage of 11,000 individual global-positioning-satellite-plotted locations where selective logging had occurred. They found that their satellite analysis system was within 5 percent accuracy and concluded that previous analysis had missed 50 percent of damage.

In short, they determined that for every tree removed, 30 more will become severely damaged.That's because selective logging is inherently destructive, the team says. When a tree is cut down, vines growing between it and other trees will pull down neighbors. The space that opens up becomes dry and susceptible to burning. Additionally, tractors and skidders used to remove the hardwoods destroy the forest floor and promote additional logging.

What's more, felled trees, the decomposing debris left behind on the forest floor and the large amounts of sawdust produced at sawmills release carbon dioxide gas into the atmosphere. Traditional deforestation unleashes 400 million tons of carbon every year, and Asner and his colleagues estimate selective logging produces an additional 100 million tons.

Asner is making his research available to the Brazilian government that is now charged with enforcing logging laws across an area even more afflicted that previously thought.