Global warming and the resulting drought have likely doubled the tree death rate over the past 30 years in old-growth forests in the western United States, according to a study released on Thursday.

Researchers said the accelerated forest loss could trigger an environmental domino effect on the region's wildlife and climate.

Temperatures in western US forests have increased on average more than 0.5 degrees C over the past 30 years, reducing snowfall accumulations, prolonging summer droughts and raising the insect population, including tree-killing bark beetles.

Over the past 10 years, these insects have consumed around 1.4 million hectares of lodgepole pines in northwestern Colorado, according to the study led by the US Geological Survey (USGS) study and published in the journal Science.

Warmer temperatures are also conducive to greater tree disease, the researchers said.

"This regional warming has contributed to widespread hydrologic changes, such as a declining fraction of precipitation falling as snow, declining water snowpack content, earlier spring snowmelt and runoff, and a consequent lengthening of the summer drought," the researchers wrote.

Increasing tree mortality rates mean that western forests could become net sources of carbon dioxide (CO2) to the atmosphere, further speeding up the pace of global warming.

An overabundance of decaying trees on the forest floor is also a source of increased CO2 emissions.

"The increase in tree mortality rates documented in the study is further compelling evidence of ecosystem responses to recent climate warming," said biogeography professor Thomas Veblen of the University of Colorado, a participant in the study.

"The findings are consistent with other well documented, climate-induced ecological changes, including increased wildfire activity since the mid-1980s and bark beetle outbreaks that are occurring at unprecedented levels in western North America forests, including Alaska," Veblen added in a statement.

The study found that the increase in dying trees has been pervasive. Tree death rates have increased across a wide variety of forest types, at all elevations, in trees of all sizes, and in pines, firs, hemlocks and other kinds of trees.

The tree death rate in Canada's British Columbia doubled in only 17 years, 1.5 times faster than in California's old-growth forests, where tree-death doubled in 25 years.

Tree death rates are slower in western US forests that do not border the Pacific Ocean, such as in Colorado and Arizona, the researchers said.

"Tree death rates are like interest on a bank account, the effects compound over time," said Nate Stephenson of the USGS, co-leader of the research team.

"A doubling of death rates eventually could reduce average tree age in a forest by half, thus reducing average tree size."

(China Daily via Agencies January 24, 2009)