SATELLITE analysis of the Brazilian rainforest has revealed that logging is causing more than twice as much damage as previously thought. Many ecologists believe that it may be contributing to the region鈥檚 worst drought in 40 years.
Previous satellite analyses could not tell normal forest from areas that had been thinned by selective logging, in which only valuable timber such as mahogany is removed. Only areas that had been completely cleared were discernible from orbit, and most estimates of selective logging came from questionable reports of local sawmill activities.
Now researchers from the Carnegie Institution of Washington in Stanford, California, have overcome this blind spot by combining data from three satellites. They started with the sensitive hyperspectral sensors on a NASA satellite called EO-1. These sensors can pick up small differences in the frequency of light reflected by various materials on Earth鈥檚 surface: for instance, vegetation reflects light in the near infrared, while soil and ground clutter reflect more red light.
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鈥淲e saw this Swiss-cheesing of the forest canopy, and the gaps are enough to let sunlight through and moisture out鈥�
This information was used to fine-tune the satellite Landsat 7 to enable it to distinguish forest canopy from soil and ground clutter. The sensors of Landsat 7 have lower resolution than those of EO-1 but cover more ground, so researchers could use it to gather data for five Brazilian states that together account for nearly 90 per cent of all deforestation. Readings from a third satellite, Terra, were used to filter out the effects of smoke, water vapour and other atmospheric conditions.
The combination of the three satellites鈥� data revealed something shocking. 鈥淲e saw this Swiss-cheesing of the forest canopy,鈥� says team leader Greg Asner. Between 1999 and 2002, selective logging cleared between 12,075 and 19,823 square kilometres 鈥� up to an extra 120 per cent on top of the areas that were already known to have been deforested.
The gaps in the canopy caused by selective logging are drying up the rainforest. 鈥淓ven though these can sometimes be just small gaps, they鈥檙e enough to let sunlight through and moisture out,鈥� says Asner. Normally, only 1 or 2 per cent of the sunlight that shines on the forest reaches ground level. But selective logging is allowing sunlight to heat the moist forest-floor clutter, evaporating away moisture that would otherwise be retained by the ecosystem, says Dan Nepstadt, a senior scientist at Woods Hole Research Center in Massachusetts.
The dry debris has turned the forest floor into a tinderbox and kindled an unprecedented number of wildfires this year. Brazilian officials have declared 16 cities within the country鈥檚 Amazonas region as disaster areas. Similar situations may be developing in countries such as Indonesia, Peru and Bolivia, where selective logging is suspected of playing a large part in damaging local ecology, Asner says.
