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Researchers have managed to count the uncountable in Brazil's Amazon rainforest. In a study published today, they show using satellite imagery how much of the forest is 'selectively logged' for mahogany and other trees.

The team says that deforestation figures for the Amazon should be doubled to take account of this form of logging, which involves felling only those trees that are valuable in the timber trade.

"Mahogany is the one everybody knows about," explains lead researcher Gregory Asner of Stanford University, United States. "But in the Amazon, there are at least 35 marketable hardwood species, and the damage that occurs from taking out just a few trees at a time is enormous."

According to Asner: "On average, for every tree removed, up to 30 more can be severely damaged by the timber harvesting operation itself. That's because when trees are cut down, the vines that connect them pull down the neighbouring trees."

Large areas of deforestation are easily
spotted in satellite pictures
Photo Credit: Digital Globe / Google Earth

Compared with deforestation, where large areas are burnt or cut down to make way for agriculture, the environmental impact of selective logging "has been mostly invisible until now", he says.

Efforts to monitor selective logging have so far used imprecise methods. These include surveys of sawmills that do not reveal where the logs originated, or labour-intensive — and potentially dangerous — fieldwork to count felled trees.

None of these methods is effective for large-scale assessments.

In research published in this week's Science, Asner and colleagues used ultra-high-resolution satellite data to measure the amount of selective logging in the five states where about 90 per cent of the deforestation in Brazil's Amazon has occurred.

"With this new technology, we are able to detect openings in the forest canopy [left by] just one or two individual trees," says Asner.

His team found that from 1999 to 2002, between 1.2 and 2 million hectares of forest were selectively logged each year. The upper limit corresponds roughly to the combined area of Jamaica and Puerto Rico.

The selective logging that Asner's team recorded totals 60–123 per cent more forest damage than was reported for deforestation alone in the same period.

The findings have implications for climate change, as the researchers say the additional logging caused a 25 per cent increase in carbon released to the atmosphere, compared to the amount arising from deforestation alone.

Generally, the researchers found that protected areas were not logged. However, there were notable exceptions in Mato Grosso and Pará, the two states where selective logging was most intense.

Reference: Science 310, 480 (2005)