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American Association for the Advancement of Science

Logging, a bigger threat to the rainforest than we knew

The amount of land in the Amazon rainforest that's being damaged by human activities is twice as large as we had previously thought, suggests a new study. That's because researchers didn't have a good idea of how much logging was occurring there, until now.

The Amazon rainforest is the largest single, continuous area of tropical forest on Earth. It contains a large portion of the world's species, and the trees act as the "lungs of the planet," converting carbon dioxide to oxygen through photosynthesis.

Being able to measure the actual amount of damage occurring to the rainforest is important so that efforts to protect the rainforest can be based on solid information about how big the problem is.

Most of the damage to the forest that scientists have measured until now has been from clear-cutting, which is when people chop down or burn large areas of vegetation in order to clear the land for farming or grazing.

These large, treeless chunks of land can be seen from space. They appear in photographs taken by special cameras aboard satellites, allowing researchers to estimate how much of the rainforest has been clear-cut.

When loggers go into the rainforest, however, they only cut down certain large trees, thinning the forest out but not clearing it away altogether. This type of change is more difficult to spot in a satellite image.

Gregory Asner of the Carnegie Institution of Washington and his colleagues have developed a new way to analyze satellite images. Using computers to measure several different kinds of information in each dot or "pixel" that makes up the image, the researchers were able to detect areas that had been thinned by logging.

The researchers studied images of the five major timber-producing states in the Brazilian Amazon. When they added these areas to the clear-cut areas, the amount of damaged forest approximately doubled.

Asner's team estimated that the amount of trees removed from the forest by logging translates into a 25 percent increase in the overall flow of carbon from the Amazonian forest to the atmosphere, since the trees aren't available to convert carbon dioxide into oxygen.

Logging causes important local disruptions as well. Vines threading through the trees can pull down large amounts of vegetation when a tree falls. When the leafy canopy at the top of the forest is thinned, the forest also becomes drier and more prone to burning.

"Logged forests are areas of extraordinary damage," Asner said. "A tree crown can be 25 meters across. When you knock down a tree it causes a lot of damage in the understory. It's a debris field down there."