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Study: Western forests dying at increasing rate

GRANTS PASS, Ore. ” Trees in old growth forests across the West are dying at a small, but increasing rate that scientists conclude is probably caused by longer and hotter summers from a changing climate.

While the death rate is not noticeable to someone walking through the forests, it is doubling every 17 to 29 years, hitting levels of 0.5 percent to 1.7 percent a year, and was seen in trees of all ages, species, and locations, according to a study published in the Friday edition of the journal Science.

“If current trends continue, forests will become sparser over time,” said lead author Phillip J. van Mantgem of the U.S. Geological Survey’s Western Ecological Research Center.



“Eventually this will lead to decreasing tree size,” he said in an interview. “This is important because it indicates future forests might store less carbon than present. Western forests could be a net source of carbon dioxide, further speeding up global warming.”

The rising death rate could also produce a cascading decline in forests that leads to less habitat for fish and wildlife, an increased risk of wildfires, and a vulnerability to sudden forest die-offs, he said.



The likely cause of death for the trees is the increasing average temperature across the West, about 1 degree over the study period, said co-author Nathan L. Stephenson of the U.S. Geological Survey Western Ecological Research Center. That results in greater stress on the trees from lack of water, leaving them vulnerable to disease and insects.

Even if the precipitation remains the same, warmer temperatures mean more rain that runs off than snow that soaks in. Longer summers, typically dry in the West, also mean more moisture in the soil is lost to evaporation.

“So you could conclude that if there is indeed a rising rate of temperature and temperatures continue to increase, very likely mortality rates will continue to rise,” Stephenson said.



These continuing effects of a warming climate should make the nation take a new look at its policies on fighting wildfires, thinning forests, and allowing people to build homes in the woods, said co-author Thomas T. Veblen, a professor of geography at the University of Colorado.

The study examined data between 1955 and 2007 in 76 research plots in British Columbia, Washington, Oregon, California, Idaho, Colorado and Arizona. The average age of the forests examined was about 450 years, with some as old as 1,000 years. Of the 59,736 trees counted, 11,095 died over the study period. They included trees that were young, old, at high, medium and low elevations, in wet and dry climates, and of a variety of species, including hemlock, pine and fir.

The death rate was highest in California’s Sierras, starting at about 0.9 percent in 1980 and rising to about 1.3 percent. It rose fastest in the Northwest, starting at about 0.7 percent in the 1970s and rising to about 1.3 percent. In the Rockies it started at about 0.2 percent in 1955 and rose to about 0.5 percent.

While the rate of trees dying was increasing, the rate of new trees sprouting and surviving was not, so over a long time there would be a net reduction in the numbers, ages and sizes of trees, the study found.

“If it’s a gradual process, we may be fine,” said Mark E. Harmon, professor of forest ecology at Oregon State University. “If it is a real sudden process, it could be problematical.”

“Probably the time for action was yesterday or maybe a decade ago,” he said. “We are losing options as we wait.”

Jerry Franklin, professor of forest ecology at the University of Washington, noted that old growth forests, particularly those in the Northwest, store tremendous amounts of carbon, making them a resource in combatting global warming. But as trees die, they decompose and give off carbon dioxide, contributing to the amount of greenhouse gases. Young forests store very little carbon, and it takes hundreds of years to replace old growth.

“We probably don’t want to get into these older stable forests and mess around, because we are not going to help,” Franklin said. “We could potentially accelerate the process, which we don’t want to do.”

Located in old growth forests, the research plots were only rarely hit by the wildfires and insect infestations that have become more common in recent years, so the death rate did not reflect those factors.

Harmon said the small amount of trees killed by this background mortality could be comparable in volume to the more noticeable losses to wildfires and insect infestations, because it is over such a large area.

The scientists considered such factors as increased air pollution, overcrowding of young trees, a lack of fires that keep forests healthy, forest fragmentation from logging, and large trees falling on small ones as possible causes.

But they rejected each one as the data showed increasing rates of death in both young and old trees, trees in polluted air and clean air, and trees in crowded stands and sparse stands.

Even trees in national parks, where no logging is allowed, died at increasing rates.


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