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Old-Growth Forests Dying Off in U.S. West

Tree deaths have doubled, and global warming may be the cause, experts say

THURSDAY, Jan. 22 (HealthDay News) -- Trees in old-growth forests in the Western United States are dying at twice the rate they were a few decades ago, and experts suspect regional warming is to blame.

The report, led by the U.S. Geological Survey (USGS), found that the increase in tree deaths has included trees in a variety of forests, elevations and sizes. Species have included pine, fir, hemlock and other coniferous trees. In addition, the rate of new tree growth has not changed, according to the report in the Jan. 23 issue of Science.

"If current trends continue, forests will become sparser over time, and average tree ages will decrease by half," study co-author Phillip van Mantgem, a USGS research ecologist, said during a teleconference Wednesday.

In the future, forests will store less carbon than they do now, van Mantgem said. "It introduces the possibility that Western forests could become net sources of carbon dioxide, further speeding up the pace of global warming," he explained.

In addition, fewer trees could result in a loss of habitat for animals that depend on old-growth forests, van Mantgem said, and there might also be an increased risk of forest fires, with increasing drought and more fallen trees.

To determine the causes of increased tree death, the researchers considered problems in the forest themselves, such as overcrowding. "Every way we cut the data and examined it, it looks like internal dynamics are not a significant source of the increase in mortality rates," Nathan Stephenson, a USGS research ecologist and co-author of the study, said during the teleconference.

The researchers also looked at external causes, such as air pollution. However, they concluded that these were unlikely causes of the troubling trend, Stephenson said.

"What we were left with was temperature," he said. "Increasing temperature was correlated with the increase in mortality rates."

Rising temperatures in the Western United States have changed weather patterns, Stephenson said. Summers are getting longer, increasing drought conditions. "It is possible that trees are under more drought stress," he said.

Moreover, warmer temperatures favor an increase in insects and other organisms that feed on trees, he said.

"Projections for the future are for continued warming, and even an accelerated rate of warming," Stephenson said. "It's very likely that mortality rates will continue to rise."

Thomas Veblen, a professor of geography at the University of Colorado at Boulder and another co-author of the study, noted during the teleconference that the findings are consistent with other ecological changes brought on by global warming.

"These include increased wildfire activity across the Western U.S., as well as bark beetle outbreaks that are occurring at unprecedented levels across Western North America," Veblen said.

He noted that these changes in climate necessitate a reevaluation of policies on how forests are managed, including new ways of dealing with wildfires and limiting development.

One critic of the concept of global warming, Steven Milloy, publisher of, does not think that tree deaths have an effect on climate change.

"If they are trying to add on to climate alarmism, their paper is way short of that," Milloy said. "To say tree death is going to contribute to global warming is extremely debatable. It's all kind of silly to me."

However, two papers in the Jan. 22 issue of Nature describe other changes occurring as the earth's temperature rises.

Seasons now arrive two days earlier than they used to, one study from scientists at the University of California, Berkeley, and Harvard University concluded. Not only have average worldwide temperatures been rising for the last 50 years, according to the report, but the hottest day of the year has shifted to almost two days earlier.

And the other study found that temperatures in Antarctica have increased about half a degree in the past 50 years. The warming of Antarctica is related to changes in atmospheric circulation and declines in sea ice in the pacific region of the southern polar ocean, the University of Washington researchers concluded.

More information

For more information on climate change, visit the U.S. Environmental Protection Agency .

SOURCES: Jan. 21, 2009, teleconference with: Phillip van Mantgem, Ph.D., U.S. Geological Survey (USGS) research ecologist, Arcata, Calif.; Nathan Stephenson, Ph.D., USGS research ecologist, Three Rivers, Calif.; and Thomas Veblen, Ph.D., professor, geography, University of Colorado at Boulder; Steven Milloy, publisher,; Jan. 23, 2009, Science; Jan. 22, 2009, Nature

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