Two giant storms slammed the Western Hemisphere in 2005. Everyone knows about Hurricane Katrina, which struck the U.S. Gulf Coast in late August. But in January of that year, a powerful line of thunderstorms roared through the Amazon basin in Brazil. That storm, researchers report, may have felled hundreds of millions of trees. Just as striking, they've found evidence that such windstorms aren't unique.
Trees have a long life span, but in any given patch of forest they're dying all the time—from drought, insects, disease, old age, and wind. "Blow downs," is what scientists call windstorm casualties, says forest ecologist Jeffrey Chambers. Chambers, along with atmospheric scientist Robinson Negrón-Juárez, both of Tulane University in New Orleans, Louisiana, and colleagues had begun studying the effects of the 2005 Amazon storm around Manaus, Brazil, when Katrina hit their home city. After about a year's hiatus, during which they studied the storm's wind effects on Gulf Coast forests, they resumed their research with colleagues at Brazil's National Institute for Amazonian Research in Manaus.
Using satellite images of about 34,000 square kilometers of the region from 2004 and comparing them with post-storm images, the researchers discovered that the wind had cut an enormous swath through the rainforest, running in a northeasterly direction across the Amazon basin. In all, the storm affected possibly 70% of the basin. After analyzing the satellite images, the team chose 30 random plots at five sites around Manaus for field studies. The trees in some of the plots had been devastated by the storm, while others were untouched. Over the next 4 years, Chambers says, he and colleagues derived a "body count" of trees in the plots that were knocked down by the wind.
The team reports in a paper accepted for publication in Geophysical Research Letters that the storm knocked down an estimated 500 million trees across the entire Amazon basin. For the Manaus region alone the storm accounted for about 30% of deforestation in 2005. Some research has suggested that drought was the primary cause of natural tree death in the Amazon rainforest in 2005, Chambers says. But much of the Manaus area was unaffected by drought in that year, he says, so the findings show that the windstorm clearly took an enormous toll.
"These large windstorms are proving to be important to our understanding of how the Amazon works, in terms of carbon storage, biodiversity, and other factors," says global ecologist Gregory Asner of the Carnegie Institution for Science in Stanford, California. If the storms become more intense or frequent, possibly because of climate change, Asner adds, "We can expect big changes to the structure and composition of ecosystems resting in their paths."
*This article has been corrected. It originally stated that the storm was responsible for 30% of the deforestation in the entire Amazon region, when that figure was meant to reflect only the Manaus region.