Tree deaths soar in Western U.S.

— -- Tree deaths, spurred by global warming, have more than doubled in older forests across Western states, federal scientists reported Thursday.

Droughts and pests brought on by warmer temperatures have killed firs, hemlocks, pines and other large trees in particular over the past 30 years without allowing replacements to sprout, the study published in the journal Science finds.

"Very likely the mortality rate will continue to rise," says lead author Phillip van Mantgem of the U.S. Geologic Survey.

In 2007, the United Nations' Intergovernmental Panel on Climate Change found it "very likely" that average temperatures have increased more than 1 degree Fahrenheit over the past century and probably will rise 3 to 7 degrees in this one. In the American West, temperature increases have led to longer summers, drought and the survival of tree-killing beetles at higher elevations. These beetles are widespread in outbreaks reaching to Alaska.

A research team analyzed unmanaged, old-growth forest records at 76 sites across Western states from 1955 to 2006. Tree death rates increased at 87% of the sites. Pacific Northwest forests were particularly hard-hit, with death rates doubling in the past 17 years. Forest fires played no role, the study found; rates were similar across fire-prone and fire-resistant locales.

"Climate change is not just affecting the ice cover of the Arctic Ocean — it's closer to home," says climate scientist Raymond Bradley of the University of Massachusetts in Amherst, who was not part of the study. "Climate changes in mountain regions of the world are occurring at a much faster pace than has generally been recognized."

Though some people blame inadequate thinning of older trees by state forest managers, the study makes a "convincing case" that drought and pests are responsible, says entomologist Kenneth Raffa of the University of Wisconsin in Madison.

Because the data do not include recent bark beetle outbreaks in the Rockies, it may understate tree death rates, says study co-author Thomas Veblen of the University of Colorado in Boulder.