Brazilian Amazon being cut down twice as fast
'Selective logging' could harm forest's ability to suck up greenhouse gases.
Loggers are cutting down trees in the Amazon rainforest at twice the rate of previous estimates, according to a new analysis of satellite images of the region. Earlier attempts to gauge the scale of deforestation were not sensitive enough to spot the occurrence of selective logging - the cutting down of individual trees without clearing the surrounding forest.
Selective logging is also rife on land set aside for conservation, report the researchers, led by Gregory Asner of the Carnegie Institute of Washington in Stanford, California. Across the area surveyed, some 1,200 square kilometres of supposedly protected lands showed the effects of timber-cutting, they report in the journal Science1.
Selective logging has been difficult to monitor because, whereas bare patches of deforested land can be easily spotted by satellite, the hallmarks of cutting down some trees but leaving others are harder to spot.
But it wasn't the satellites that weren't up to the task - it was the way scientists have analysed the pictures. Asner and his colleagues used improved pattern-recognition software to spot subtle signs of tree removal in images captured by the Landsat satellite over the years. The method spots the small patches of dead vegetation left behind when loggers take a tree.
Although selective logging can be kinder on ecosystems than clearing, as it leaves the soil and undergrowth more intact, it isn't all good for the forest.
Removing trees decreases the forest's ability to suck up carbon from the atmosphere, and whether those trees are taken singly or in a large swath doesn't matter. In fact, selective loggers tend to harvest the trees with the highest wood density, which pack more carbon than less dense trees.
If left unchecked, selective logging could alter the rainforest ecosystem, potentially upsetting the balance of different plant species that live there, says Asner's colleague Michael Keller of the University of New Hampshire, Durham. "Another effect is a change in microclimate that makes the forest more susceptible to fire," Keller adds.
Coping with change
The overall effects of selective logging on the rainforest and its ability to store carbon are not yet clear, the researchers say. And they could change as the climate alters, they add.
In another study published this week in Science2, a team led by Daniel Bunker of Columbia University in New York evaluated the consequences of factors ranging from logging to climate change on Amazonian carbon storage. They note that climate change could result in less rain over this part of the world, increasing the number of drought-tolerant species, which store carbon more densely in their tissues.
That might be good news for carbon storage, and so might lessen the impact of selective logging. But there are other factors to consider too, says Bunker. "While there may be a handful of species that are best for storing carbon, these species are not likely to be able to maximize flood control, water quality, recovery from disturbance of any of the myriad other services that ecosystems provide," he says. "Any effort to manage a forest must consider all the services that we require from that ecosystem."
The best strategy for helping forests to cope with future climate change is to preserve as many different species as possible, Bunker argues. "Having lots of species gives an ecosystem more ways to respond to environmental change, and that may be essential in years to come," he says. So the fact that selective loggers tend to weed out certain species of trees is not good news, he adds.
- Asner, G. P., et al. Science, 310. 480 - 482 (2005).
- Bunker D. E., et al. Science, doi:10.1126/science.1117682 (2005).
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