Forests on the wane

Early last decade, the world’s tree coverage dropped by more than 3 percent

Some people can’t see the forest for the trees, but many can’t see them because they’re truly disappearing: About 3 percent of forests standing in 2000 were gone by 2005, a new analysis of satellite images reveals.

WHEN A TREE FALLS Satellite images indicate that about 3 percent of the world’s forests in 2000 were gone by 2005 (percent deforestation shown on map). Forests vanished largely because of massive wildfires and human-induced deforestation for logging and agriculture. Hansen et al./PNAS

Forests vanish for a variety of reasons, from agriculture and logging to natural phenomena such as wildfires, storms and insect infestations, says study coauthor Matthew Hansen, a geographer at South Dakota State University in Brookings.

For purposes of forest accounting, researchers classify an area as forest if trees cover more than 25 percent of the landscape. In 2000, forests blanketed almost 33 million square kilometers of land worldwide. But by 2005 more than 1 million square kilometers of those forests had disappeared, Hansen and his colleagues report online April 26 and in an upcoming Proceedings of the National Academy of Sciences.

On a percentage basis, forests are vanishing most quickly in the boreal region encircling the Arctic. More than 350,000 square kilometers — about 4 percent of the forests in that ecosystem — fell victim. Fires caused about 60 percent of those losses, says Hansen. Logging accounted for a substantial fraction of the remaining deforestation, which also included losses due to storms and beetle infestations (SN: 5/10/08, p. 9).

More than 58 percent of forest loss worldwide occurred in just four nations — Brazil, Canada, the Russian Federation and the United States. Together, these sprawling nations account for about 45 percent of the world’s forests, Hansen notes.

The new study didn’t measure the spread of forests between 2000 and 2005, so recovery from previous natural disasters such as wildfires or storms wasn’t included. Also, Hansen notes, future analyses by the group will use techniques better able to distinguish natural deforestation from that caused by humans.

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