Drought, heat killing trees in western N.America

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Trees in the western United States and Canada are dying twice as quickly as they did just 30 years ago, with rising average temperatures almost certainly to blame, researchers reported on Thursday.

These thinner and weaker forests will become more vulnerable to wildfires and may soak up less carbon dioxide, in turn speeding up global warming, they said.

The U.S. and Canadian researchers from a variety of agencies and universities studied trees in old-growth forests for more than 50 years to document the die-off, which they say is beginning to outpace replacement by new trees.

Warmer temperatures may be encouraging pine beetles and other organisms that attack trees, the researchers said. That, along with the stress of prolonged droughts, may be accelerating death rates.

"Average temperature in the West rose by more than 1 degree F (half a degree C) over the last few decades," said Phillip van Mantgem of the U.S. Geological Survey, who helped lead the study.

"While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought."

Writing in the journal Science, the researchers said they found trees of various species, ages and sizes are dying faster at every elevation.

"Wherever we looked, mortality rates are increasing," Nathan Stephenson of the USGS told reporters in a telephone briefing.