THE rainforests of the Amazon are being destroyed twice as quickly as previously thought, with companies exploiting less easily detectable logging techniques, satellite images reveal.

An imaging method developed by scientists in the United States has shown for the first time the damaging effects of “selective logging”, in which trees are thinned out, but the forest is not cleared completely. The images indicate that an area of more than 5,000 square miles — about two-thirds of the size of Wales — is disturbed in this way every year. Until now, satellite-based methods for measuring deforestation across large areas have been capable only of detecting clear-cut swaths of land where all the trees are removed.

The findings, which are published today in the journal Science, are based on analysis of the rainforests from three different satellites using a computer programme that allows every pixel of an image to be studied in detail.

By delving into the pixels, the researchers were able to determine a more exact percentage of deforested land whereas, before, analysts could consider each pixel only as entirely forested or deforested.

The technique, developed by Gregory Asner of the Carnegie Institution of Washington, and colleagues, identified areas in the five main timber production states of the Brazilian Amazon 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 sawmills.

Little was known previously about the extent or impact of selective logging in Amazon rainforests, the authors said, adding that the technique could be employed for other endangered ecosystems. In recent years, as logging has come under greater scrutiny, some operators have taken to stealthily extracting specific types of tree one by one, with the forest canopy covering their tracks.

Dr Asner said that the new image-analysis technology would have far-reaching impact on such fragile environments. “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,” he said.

The researchers found that, from 1999 to 2002, selective logging added 60 to 128 per cent more damaged forest area than was reported for deforestation alone in the same study period. They said that the total volume of harvested trees would have removed as much as 15 million metric tons of carbon from the ecosystem, representing a 25 per cent increase in the overall flow of carbon from the Amazonian forest to the atmosphere.

Logging is responsible for other serious ecological disruptions as well, such as the destruction of vegetation and vines that are pulled down when a tree falls. The forest also becomes drier and more flammable, as the shady canopy is thinned. The uncontrolled opening of logging tracks is also responsible for the destruction of much forest wildlife.

“Logged forests are areas of extraordinary damage,” Dr Asner said. “A tree crown can be 25 metres [wide]. When you knock down a tree it causes a lot of damage in the understory. It’s a debris field down there.

“Selective logging negatively impacts on many plants and animals and increases erosion and fires.”

Dr Asner said that the researchers’ ultimate goal was to provide the satellite results to government officials in Brazil, since much of the logging was illegal but difficult to enforce because it was usually clandestine.

Although Brazil’s Space Research Institute has used satellites to measure deforestation for more than 20 years, little has been known about the extent of selective logging in the region because the techniques could not penetrate the upper layers of forest leaves.

Natalino Silva, from the Brazilian Agricultural Research Corporation, said the Carnegie Landsat Analysis System (CLAS) had added much to knowledge of human activity and its impact on the area.

“With the new Carnegie system, we can now see what is happening from the top of the forest all the way to the soil; we have a whole new picture of the Amazon region and selective logging,” he said.

The researchers scanned millions of square miles of rainforest and selected on-ground surveys over the five main states, which account for 90 per cent of all deforestation in the area. The annual extent of selective logging was found to be between 4,685 square miles and 7,973 square miles.

Several protected national reserves, parks and indigenous lands were found to have been logged illegally.

The research is published two days after Greenpeace activists dumped a tonne of timber outside the offices of a government department in Central London in protest at illegal logging. The conservation group claims that illegally logged timber from Papua New Guinea has been made into plywood in China and ended up on British building sites.

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