There is little knowledge of how tree death rates may or may not have fluctuated before that period.
If future forests are filled with younger, smaller trees, it could have unknown consequences for the many species that make the forests their homes.
Higher mortality rates would also mean more dead wood on the ground, which would increase the risk of fire.
It's even possible that tree loss could turn some forests into carbon sources, rather than carbon-absorbing "sinks," further feeding the warming cycle. Carbon dioxide is a greenhouse gas associated with global warming (interactive: how greenhouse gases heat the planet).
Perhaps most disturbing possibility is that old-growth forests could become more vulnerable to massive "die back" events like droughts or infestations.
Increased tree mortality rates "may be indicators of climate-induced stress that, in turn, may increase trees' susceptibility to much more abrupt causes of tree mortality such as bark beetle outbreaks," Which are occurring at unprecedented levels across western North America, said co-author Thomas Veblen, from the University of Colorado, during a teleconference Wednesday.
(Related: "Pine Beetles Turn Forests From Carbon Sinks to Sources" [April 24, 2008].)
Pollution, Fire Suppression Not to Blame
The study examined other plausible causes for the trend but found no strong correlations other than warming temperatures.
One possibility was that decades of fire suppression had made forests denser than they used to be, so that increased competition for resources raised the death rate. But the team found no evidence to support the theory.
Another potential culprit, air pollution, was also ruled out. For example, death rates in Washington's relatively pristine Olympic National Park were no better than those in the heavily ozone-polluted southern Sierra Nevada.
Study co-leader van Mantgem stressed that the broad background mechanism at work across the West appears to be warming.
Chadwick Oliver, of the Yale School of Forestry and Environmental Studies, said that forests have adapted to many dramatic climate shifts over the eons.
"I don't want to imply that we're not contributing to warming—we may very well be—but these types of climate variations occur and they always have," he said. "This is the way forests behave."
Even so, Oliver doesn't advocate sitting on the sidelines.
"Over millions of years many species have gone extinct as a result [of dramatic climate change], but that doesn't necessarily mean it's something that we want to happen [again]. We may be losing some species that we'd like to keep for our children and grandchildren."
Study co-author Jerry Franklin, of the University of Washington, said the research stressed how managers and policymakers must adapt to the shifting environment.
"So much conservation has been focused on going back, or keeping things as they are," he said. "We've got to have a much more adaptive kind of strategy."
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