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 we’re seeing is very encouraging.”
The OSU researchers say they believe there are two forces at work – both the lower populations of elk, and their changed behavior due to fear of wolves – but it’s difficult to determine exactly which force is the most significant.
Of note, they say, is that elk populations now are actually higher than they were in the mid-1960s, when aspen trees were still in significant decline. The major change from that period of time is the presence of wolves. The effect of behavioral changes “may be equal to or even greater than” lower elk population levels in allowing tree survival, the researchers said in their report.
“In riparian zones, where wolves can most easily sneak up on elk, and gullies or other features make it more difficult for elk to escape, we’ve seen the most aspen recovery,” Ripple said. “We did not document nearly as much recovery in upland areas, at least so far, where elk apparently feel safer. But even there, aspen are growing better in areas with logs or debris that would make it more difficult for elk to move quickly.”
This element of fear, the OSU scientists said, is a concept that is now getting more attention in ecology – it factors in not just the numbers or species of animals, but also their behavior and the reasons for that behavior. Predators such as wolves or cougars, OSU researchers have shown, have the ability to strike fear into their prey and significantly change their behavior as a result.
The recovery of aspen, the researchers said, appears to have no link to climate or local terrain, since unbrowsed aspen in upland sites are growing just about as much as those in riparian zones. More details on this research and the findings can be found on the web at www.cof.orst.edu/cascades
“The issue of aspen decline in the American West is huge, and their recovery will depend on local conditions and issues in many areas,” Ripple said. “In northern Yellowstone, we finally have some good news to report. It’s just a start, but it’s a pretty good start.” |
