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Aspen stand
A withering stand of aspen in Yellowstone National Park bears testimony to the decline of these trees around much of the Rocky Mountains.

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Wolves Believed to Affect Aspen Population

It's long been know that eliminating a predator such as wolves from an area can affect populations of many other animals in the area. Now, a study by Oregon State University suggests that the elimination of wolves from Yellowstone and other areas of the Rocky Mountains is responsible for a decline in aspen groves of as much as 50 to 90 per cent.

"This hypothesis is not yet proven, and we're working closely with National Park Service biologists in more than 115 permanent research plots to test the theory," says Eric Larsen of the Department of Geosciences at Oregon State University. "What is clear is that the wolves disappeared during the same era that the successful development of mature aspen stands ground to a halt."

Using historic documents, aerial photographs, and ring dating techniques, it was determined that Yellowstone Park aspen successfully recruited tree-sized aspen into their overstory until about 1928, but have been unable to do so since then. Various theories have been proposed to explain the lack of aspen overstory recruitment, including the effects of fire suppression and a trend towards a warmer and drier climate. A key factor on which virtually all scientists agree is that elk browsing has had a major effect on suppressing the growth of young aspen in Yellowstone.

"During winter, elk browse off the aspen suckers, preventing them from growing to a full tree height," Larsen said. "But elk have been in this ecosystem for centuries, so the question becomes why are the aspen declining only now?"

One answer is that due to their protected status, elk populations may now be unusually high. However, there may be additional factors other than just the total number of elk present.

"The difference between the effect of elk on aspen now, compared to periods prior to 1900, may be a reflection of both their population levels and their behavior," says William Ripple, professor of forestry and director of the Oregon State University Environmental Remote Sensing Applications Laboratory. "Foraging behavior of elk may be influenced by the risk of predation on them."

Wolves are a natural elk predator. Wolf packs not only lower the overall elk population, but may also change elk behavior by their very presence. Elk avoid areas frequented by wolves, which can include aspen thickets, and protect themselves by staying in open areas. By influencing both the total number and foraging behavior of elk, the wolf packs may historically have prevented extensive elk browsing in some of Yellowstone's aspen stands.

The ecological link between wolves, elk and aspen is being tested with continued research in Yellowstone Park. The study is comparing aspen growth and survival rates both inside and outside the territories of Yellowstone's wolf packs.

The researchers will acquire data on the amount of elk use of those areas and its effect on aspen growth. This project and others are part of a larger "Aspen Project" at OSU, focusing on the condition of aspen throughout the western United States and Canada.