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Rising Temps Killing Trees In Western US, Canada

January 23, 2009

Researchers from a variety of agencies and universities in the United States and Canada reported Thursday that trees throughout western North America are dying at twice the rate of 30 years ago. They say rising temperatures are the likely cause.

These weaker and thinner forests will become more vulnerable to wildfires and may absorb less carbon dioxide, accelerating global warming, they say.

The researchers studied trees in old-growth forests for more than five decades in order to document their rate of dying, which they said is beginning to outpace the rate of replacement by new trees.

Pine beetles and other organisms that attack trees, along with the stress of prolonged droughts, may be accelerating the death rates, according to researchers.

“Average temperature in the West rose by more than 1 degree F (half a degree C) over the last few decades,” Phillip van Mantgem of the U.S. Geological Survey, one of the study’s leaders, told Reuters.

“While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought.”

The researchers said they discovered that trees of a wide variety of species, sizes and ages are dying faster at all elevations.

“Wherever we looked, mortality rates are increasing,” said Nathan Stephenson of the U.S. Geological Survey during a telephone briefing with reporters.

Forests are typically referred to as “carbon soaks” because plants take in carbon dioxide and release oxygen, thereby reducing levels of atmospheric carbon. However, when trees burn or die, this carbon is then released back into the atmosphere, such that a dying forest adds to the carbon, which in turn accelerates atmospheric levels of carbon.

The findings support other studies and reinforce observed changes in the environment, such as the 3.5 million acres of pine forest that has been destroyed by mountain pine bark beetles in Colorado.

University of Colorado’s Thomas Veblen told Reuters that new regulations may be required to help the forests survive.

“We need to consider developing land-use policies that reduce the vulnerability of people and resources to wildfires,” he said.

“Activities include reducing residential development in or near wildland areas that are naturally fire-prone and where we expect fire risk to increase with continued warming.”

Oregon State University forest ecology professor Mark Harmon said the overall mortality rates are low, but significant.

“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 Harmon in a statement.

“Forest fires or major insect epidemics that kill a lot of trees all at once tend to get most of the headlines. What we’re studying here are changes that are much slower and difficult to identify, but in the long run extremely important.”

The findings were published in the journal Science.

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