CORVALLIS, Ore. - A new study suggests that the decline of aspen groves in Yellowstone National Park during much of the past century may be at least partly due to the absence of wolves.
The loss of native aspen groves in Yellowstone and other areas of the Rocky Mountains is reaching crisis proportions, experts say, having declined as much as 50-90 percent in certain areas.
Now, scientists have outlined in more detail the magnitude of the aspen decline in Yellowstone National Park, and developed a new theory for the tree's decline within the park. It links those declines to the loss of wolves, a key predator species, and their interactions with elk and bear populations.
The study was done by Eric Larsen, with the Department of Geosciences at Oregon State University, and William Ripple, professor of forestry and director of the OSU Environmental Remote Sensing Applications Laboratory. It was just published in the journal Biological Conservation.
"This hypothesis is not yet proven, and we're working closely now with National Park Service biologists in more than 115 permanent research plots to test the theory," Larsen said. "What is clear is that the wolves disappeared during the same era that the successful development of mature aspen stands ground to a halt."
The loss of aspen, scientists say, is an ecological crisis that's poorly appreciated by much of the public.
Aspen not only adds scenic beauty to the landscape with their rich golden fall color, but they are often the only significant hardwood present in these conifer-dominated ecosystems. Groves of aspens, which are biologically rich with herbs, shrubs, insects, birds and berries, offer a diversity of plant and animal life often exceeded only in riparian zones in the mountain West.
Using historic documents, aerial photographs, and dendrochronological, or tree ring dating techniques, Ripple and Larsen determined that Yellowstone Park aspen successfully recruited tree-sized aspen into their overstory from about 1751 to 1928, but have been unable to do so since. Various theories have been proposed to explain the lack of aspen overstory recruitment in Yellowstone since the 1920s, 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," Ripple said "Foraging behavior of elk may be influenced by the risk of predation on them."
Wolves are a natural elk predator, the OSU researchers say. 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.
Seen as a threat to local herds of elk and bison, the wolves in and near Yellowstone Park were eliminated by 1926. By counting the annual growth rings on a sample of Yellowstone Parks' aspen trees, the OSU research has determined that the 1920s were also the last decade in which aspen overstory trees were able to regenerate.
"When the wolves were eliminated the aspen overstory began to decline, and young trees were unable to join the mature overstory," Larsen said. "Prior to the elimination of the wolf, we documented successful aspen regeneration for a period of at least 170 years."
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. More information can be obtained on the Internet.
"This is one of the most important tree species in these ecosystems," Larsen said. "And in the fall it provides much of the natural beauty in the Yellowstone forests. We have to determine what is happening to these trees and what we can do to prevent their decline."