Warming Doubles Tree Deaths in Western U.S.

Jan. 23, 2009 -- 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 Thursday.

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

Temperatures in western U.S. forests have increased on average more than 0.5 degrees Celsius 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 (3.45 million acres) of lodgepole pines in northwestern Colorado, according to the study led by the U.S. Geological Survey (USGS) study and published in the journal Science.

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

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"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.