The big, old trees in long-established stands are particularly good at storing carbon. If they yield to younger, smaller trees, carbon storage would decline. Moreover, the researchers found that in the research plots, the establishment of replacement trees was not keeping pace with mortality, suggesting that old forests could become thinner.
It is even possible, Van Mantgem said, that Western forests could eventually become "net sources of carbon dioxide in the atmosphere -- further speeding up the pace of global warming."
The research team of 11 federal and university scientists reviewed data from undisturbed forest areas in California, the Pacific Northwest and the interior West.
Rising mortality was evident across a spectrum of plots and tree types in all three regions, leading the team to rule out other possible causes of tree deaths such as air pollution or overgrown conditions.
The findings were in sync with other recent studies that have linked rising temperatures to increasing wildfire activity in the West and massive bark beetle outbreaks.
"That may be our biggest concern," Stephenson said. "Is the trend we're seeing a prelude to bigger, more abrupt changes to our forests?"
Veblen argued that "society needs to discuss policies that will help adapt to the changes that are well underway."
For example, he said it may be better to deal with the growing wildfire risk by limiting development in fire-prone areas than by stepping up firefighting or forest-thinning efforts.
Hugh Safford, a U.S. Forest Service regional ecologist in California not involved in the study, said the paper's linkage of tree death and warming seemed sound.
But he added that the picture was much gloomier in many of the West's forests, which are overgrown as a result of decades of fire suppression and are experiencing much higher mortality rates than those documented in the study.
If death rates are climbing in undisturbed old forest, Safford said, "that's extremely bad news" for areas where tree density is increasing.
"The ante is going up constantly, and when you add a highly dense stand and increasing fire and insect beetle issues, it's alarming."
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bettina.boxall@latimes.com
