U.S. Old-Growth Forests Withering With Warming

Brian Handwerk
for National Geographic News
January 22, 2009
The U.S. West's old-growth forests may look quite a bit thinner in the future.

A new study suggests these forests could feature fewer and smaller trees, and global warming may be driving the change. (Learn about global warming.)

"[Tree] death rates have doubled over the last two decades in old-growth stands across the Western U.S.," said Phillip van Mantgem, a U.S. Geological Survey scientist and co-leader of the research team.

Forests are losing trees faster than new ones are able to grow, he said.

Fading Forests?

Though they contain trees of all ages, the forests van Mantgem and colleagues studied are mature stands established 200 to 500 years ago.

The team spotted the rising death rate trend across a wide range of ages and species in western forest landscapes from regions as diverse as California, Colorado, British Columbia, and Arizona.

Temperatures in the region have risen by about 1 degree Fahrenheit (0.55 degree Celsius) during the past 20 years.

That small but significant warming has helped reduce snowpack and lengthen summer droughts. The added warmth may also be aiding insects and diseases that negatively impact tree health.

The temperature changes may be subtle, but they are contributing to a doubling of tree mortality rates as fast as every 17 years in some areas, the study found. But the rate of tree loss compounds upon itself, so even small fluctuations can make a big impact.

"We may only be talking about an annual tree mortality rate changing from 1 percent a year to 2 percent a year, an extra tree here and there," said Mark Harmon, professor of forest ecology at Oregon State University, in a statement. "But over time a lot of small numbers can add up. The ultimate implications for our forests and environment are huge."

The new research, published in tomorrow's issue of the journal Science, employs data collected over the past 50 years.

There is little knowledge of how tree death rates may or may not have fluctuated before that period.

Consequences Unknown

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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