More Wolves Mean More Trees

A study by Oregon State University (OSU) suggests that the reintroduction of wolves into Yellowstone National Park is allowing more cottonwood trees to grow along streams in the park. The number of cottonwoods in the park has been in sharp decline for decades, possibly headed for extinction in Yellowstone.

"In one portion of the elk's winter range along the Lamar River of Yellowstone National Park, we found that there were thousands of small cottonwood seedlings," said Robert Beschta, professor emeritus in the College of Forestry at OSU . "There should also have been hundreds of young trees, but there were none. Long-term elk browsing had been preventing any seedlings from getting taller."

That pattern was common throughout the study area - lots of seedlings in combination with large cottonwood trees generally more than 70 years old, but little or nothing in between. Young cottonwoods, willows, and other streamside woody species are a preferred food for browsing elk during the winter when much of the other forage is buried under snow. But when packs of wolves historically roamed the area, food was not the only consideration for elk, which had to be very careful and apparently avoided browsing in high-risk areas with low visibility or escape barriers.

"I considered a variety of potential reasons that might explain the historical decline of cottonwoods that began in the 1920s and have continued up to the last couple of years," said Beschta. "I looked at climate change, lack of floods, fire suppression, natural stand dynamics, and numbers of elk. But none of those factors really explained the problem. Ultimately, it became clear that wolves were the answer."

With no fear of wolves, elk could graze anywhere they liked and for decades have been able to kill, by browsing, nearly all the young cottonwoods. Other streamside species such as willows and berry-producing shrubs also suffered.

"Before the wolves came back, it was pretty clear that in some areas we were heading towards an outright extinction of cottonwoods," Beschta said. “The removal of wolves for 70 years - and then their return - actually set the stage for a scientific experiment with fairly compelling results."