Drought, heat killing trees in western N.America

Thu Jan 22, 2009 8:45pm EST

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.

'CARBON SOAKS'

Forests are usually called "carbon soaks" because plants take in carbon dioxide and release oxygen, removing carbon from the atmosphere. But when trees die or burn, this carbon is released back into the atmosphere.

So a dying forest adds to the carbon that in turn helps warm the planet's surface.

The findings fit in with other studies and with changes that have become obvious -- such as the 3.5 million acres (1.4 million hectares) of pine forest that has been destroyed by mountain pine bark beetles in northwestern Colorado.

Thomas Veblen of the University of Colorado said new regulations may be needed to help the forests survive.

"We need to consider developing land-use policies that reduce the vulnerability of people and resources to wildfires," Veblen said.

"Activities include reducing residential development in or near wildland areas that are naturally fire-prone and where we expect fire risk to increase with continued warming."

Mark Harmon, a professor of forest ecology at Oregon State University, said the overall mortality rates are low but they add up.

"We may only be talking about an annual tree mortality rate changing from 1 percent a year to 2 percent a year, an extra tree here and there," Harmon said in a statement.

"Forest fires or major insect epidemics that kill a lot of trees all at once tend to get most of the headlines. What we're studying here are changes that are much slower and difficult to identify, but in the long run extremely important."

(Reporting by Maggie Fox; Editing by Will Dunham and John O'Callaghan)

http://www.reuters.com/article/environmentNews/idUSTRE50L5UQ20090123