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Presence of wolves allows aspen recovery in Yellowstone

t have grown from tiny shoots in the past decade to heights of more than seven feet a key point in their long-term survival, placing their crowns above the height easily browsed by elk and other animals. Tree growth in some stands has been particularly apparent just in the past 4-5 years.

The long-term decline, to the point of localized extinctions, of aspen and cottonwood trees in Yellowstone National Park dates to the extirpation of the last known wolf packs in the 1920s. Prior to the re-introduction of wolves, scientists found there were many small sprouting shoots of these important tree species, and numbers of large trees 70 years old or more but practically nothing in between. High populations of grazing ungulates, primarily elk, had grazed on the small tree shoots at leisure and with little fear of attack.

But the ecological damage, researchers say, went far beyond just trees. The loss of trees and shrubs opened the door to significant stream erosion. Beaver dams declined. Food webs broke down, and the chain of effects rippled through birds, insects, fish and other plant and animal species.

Aspen, a beautiful hardwood tree with golden fall color, a key to ecosystem biodiversity and a hallmark feature of mountain areas across the West, has been the focus of concern. Unlike willows, aspen are more easily killed or suppressed by browsing and have been the slowest to show any recovery. In some areas of the West, up to 90 percent of the aspen have disappeared.

When I first looked at these degraded ecosystems in the mid-1990s in Yellowstone, I had doubts we would ever be able to bring the aspen back, said Robert Beschta, a professor emeritus of forestry at OSU and co-author on the study. There were so many elk, and the stream ecosystems were in such poor shape. The level of recovery were seeing is very encouraging.

The OSU researchers say they believe there are two forces at work both the lower populations of elk
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Contact: William Ripple
bill.ripple@oregonstate.edu
541-737-3056
Oregon State University
26-Jul-2007


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