The majestic old-growth forests of western North America, greening patches of the landscape from Arizona to British Columbia, may be far more vulnerable to subtle climate change than scientists previously believed. A study published today in the journal Science reveals that these western forests are dying at faster rates as regional average temperatures climb more rapidly than the global average.

"Tree death rates have more than doubled," says study co-author Phillip van Mantgem, a research ecologist with the U.S. Geological Survey (USGS).

He and his team analyzed data (collected from 1955 to 2007) on about 58,000 trees, including firs, pines, hemlocks and others, in 76 old-growth forest plots covering six western states and a Canadian province: Arizona, Colorado, California, Idaho, Washington, Oregon and British Columbia. Their findings: 11,000 trees had perished during the observation period, even though no logging, development or other major activities occurred in the study zones.

The researchers pinpointed the rise in regional temperatures as the likely culprit in their demise after ruling out other possible suspects, such as air pollution and forest management practices.

They note that the average regional temperature, though a mere one degree Fahrenheit (0.6 degree Celsius) warmer, translated into less snow, longer dry seasons, and increased soil evaporation, which stress out trees, making them more vulnerable to destructive insects and disease. Meanwhile, bugs and pathogens, which thrive in hotter temperatures, grow stronger, making them an even bigger threat to the fading forests, according to Kenneth Raffa, a professor of forest entomology at the University of Wisconsin in Madison.

"You sort of have a double whammy there," says David Breshears, a professor of natural resources at the University of Arizona's Institute for Environment and Society in Tucson.

Exacerbating the problem: not enough new trees are sprouting to replace the dead and dying old ones.

"It's like a human population," says study co-author Nathan Stephenson, a USGS research ecologist. It shrinks, he says, when the mortality rate outpaces the birth rate. "If you saw that going on in your hometown," he adds, "you'd become concerned."

This pattern could eventually lead to sparser forests in which trees are younger and about half the size of what they are now.

Today's towering old-growth forests help to mitigate climate change by absorbing carbon dioxide (a greenhouse gas that speeds global warming). But if the trends observed in this study continue, van Mantgem says, these key carbon-sinks could actually become a net source of carbon in the atmosphere, a phenomenon that has already been observed in tropical climates.

"If anything, it's a warning bell," van Mantgem says about the study's findings. "A lot of people like to think of these majestic old-growth forests as unchanging, but this showed us that they do in fact respond rather quickly to the environment."