The true extent of rainforest damage in the Amazon is more than twice as great as present estimates suggest, researchers say.

Daniel Nepstad: "The degradation will continue"

The team says field surveys of logging and burning show far more deforestation than satellite monitoring has revealed.

The researchers are based at several Brazilian and US institutions, including the Woods Hole Research Center, Massachusetts.

Their work is reported in the current issue of Nature magazine.

The researchers interviewed 1,393 wood mill operators, representing more than half the mills in 75 Amazonian logging centres.

As well, they interviewed 202 landlords, whose properties covered 9,200 sq km.

They found that logging crews annually cause severe damage to between 10,000 and 15,000 sq km of forest that are not included in current deforestation estimates.

Insidious damage

They also discovered that fires burning on the surface consume large areas of forest which again are not recorded.

The researchers say the failure so far to register the much greater loss rate they have discovered is because the loggers reduce tree cover, but do not eliminate it.

By contrast, ranchers and farmers deforest land in preparation for pasture and crops by clear-cutting it, and by burning whole areas.

The more the forest burns, the more vulnerable to fire it becomes

And where logging and fires have caused damage, they say, the vegetation will grow back fast enough to dupe a satellite.

The only reliable way to find out what is happening is by field surveys.

Logging and surface fires seldom kill all the trees. But they help to make them more vulnerable.

Logging increases the flammability of the forest by reducing leaf canopy coverage by up to 50%.

This lets the sunlight strike through to the forest floor, where it dries out the organic debris created by the logging.

Worsened by drought

And fires leave the surviving trees more susceptible to future blazes.

The researchers say the area of surface fires may be much larger than usual during severe droughts.

An unpublished Brazilian Government report says15,000 sq km of standing forest may have burned in the northern state of Roraima alone during the 1997-98 El Nino drought.

More deforestation means more carbon emissions

These so far unreported forms of forest loss, the researchers say, imply a need to look again at climate change calculations.

They write: "Forest impoverishment through logging and surface fire causes a significant release of carbon to the atmosphere that is not included in existing estimates of the Amazonian carbon balance."

Carbon dioxide is the principal gas caused by human activity that is implicated in global warming.

The team says: "Logging and fire can virtually eliminate previously undisturbed forest in regions with seasonal drought and high concentrations of wood mills."

One area in eastern Amazonia, they say, was classified as 62% forested according to conventional deforestation mapping techniques.

Satellites not enough

But they found that only about a tenth of the area classified as forest actually supported undisturbed forest.

The researchers say: "Satellite-based deforestation monitoring is an essential tool in studies of human effects on tropical forests, because it documents the most extreme form of land use, over large areas, and at low cost.