* Forests may be vulnerable to more extreme weather
* Effect of storms on forests still relatively unknown
RIO DE JANEIRO, July 13 (Reuters) - A powerful storm destroyed about half a billion trees in the Amazon in 2005, according to a study on Tuesday that shows how the world's forests may be vulnerable to more violent weather caused by climate change.
Researchers at Tulane University in New Orleans used satellite data, on-site observations and computer models to calculate that between 441 million and 663 million trees were killed by the storm that swept through the region in January 2005.
The destruction was equivalent to about 30 percent of the total deforestation caused by humans in the region around the city of Manaus that year, the study found.
"In terms of deforestation in the Amazon they're not comparable. They are completely different processes," study co-author Jeff Chambers, who has been studying the Amazon for nearly 20 years, told Reuters. "That being said, it was a huge storm."
Chambers said the results of the study showed a widespread drought in the Amazon that year, which had been blamed for the tree loss, was not the main culprit.
The trees killed by the storm would have released carbon into the atmosphere equivalent to more than a fifth of the amount that is created each year as the world's largest forest grows, the study found.
The destruction of the world's forests is believed to contribute up to 20 percent of the carbon emissions that cause global warming. The biggest drivers of destruction in the Amazon are cattle ranchers and small farmers who clear trees for pasture.
The Tulane researchers said as more intense storms are likely to be one consequence of global warming, it is increasingly important to find out the effect of powerful winds on the world's forests.
"It's really important that we start establishing some baselines here and understanding how frequently these storms occur," Chambers said.
"What fraction of trees in the Amazon every year are being killed by wind? We don't even know that." (Reporting by Stuart Grudgings; editing by Chris Wilson)