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Steamboat Springs — A study published last week pinpoints regional warming as the most probable explanation for tree death rates that have more than doubled in recent decades in the western United States.

The study - led by the U.S. Geological Survey and published last week in the journal Science - also found that the increased mortality rates could indicate that forests throughout the entire West are "vulnerable to sudden, extensive die-back" similar to what has been seen in Colorado and other Rocky Mountain areas in the grip of an unprecedented mountain pine beetle epidemic.

"That may be our biggest concern," said Nate Stephenson of the USGS, a co-leader of the study. "Is the trend we're seeing a prelude to bigger, more abrupt changes to our forests?"

Study co-leader Phil van Mantgem said the study was undertaken after a smaller one that showed a troubling increase in tree deaths in the Sierra Nevada. Van Mantgem said he was surprised to find similar results in pines, firs, hemlocks and other kinds of trees throughout the West, at all elevations and in trees of all sizes.

"It wasn't something we were expecting to see," van Mantgem said. "I was surprised it was that pervasive."

This month, the U.S. Forest Service and Colorado State Forest Service released the results of 2008 aerial surveys showing the cumulative number of acres in Colorado impacted by the current mountain pine beetle epidemic to be 1.9 million - including 310,520 acres in Routt County.

Local and international forest experts have blamed the outbreak on a variety of factors. In addition to global warming, they include modern fire suppression, a precipitous decrease in logging and other factors leading to poor age-class diversity in forests. Through experimental controls, however, the USGS-led team ruled out competing explanations and found that increasing regional temperatures remained correlated with tree deaths.

"Average temperature in the West rose by more than (1 degree) over the last few decades," van Mantgem said. "While this may not sound like much, it has been enough to reduce snowpack, cause earlier snowmelt and lengthen the summer drought."

But van Mantgem said the study does not necessarily discount other theories behind the mountain pine beetle epidemic, in particular.

"I don't think you could look at any one cause," he said. "There's a lot of interacting going on. : I would say warming will be a part of that story. That makes life happier for the beetles."

The study also found that the West's forests are not regenerating as quickly as they are dying. That could lead to a situation similar to the one predicted by Werner Kurz of the Canadian Forest Service. In an article published in the journal Nature, Kurz reported that, by 2020, British Columbia's forests will convert from "carbon sinks" to "carbon sources," meaning they will intensify the warming trends that may have contributed to their death in the first place.