BILLINGS, Mont. -- The return of gray wolves has dramatically altered the landscape in portions of Yellowstone National Park, as new trees take root in areas where the predators have curbed the size of foraging elk herds, according to scientists in a new study.

Stands of aspen, willow and cottonwood are expanding in areas where for decades dense elk populations prevented new growth, said study author William Ripple from Oregon State University.

While other factors might play a role, from a changing climate to wildfires, more than a decade of research has confirmed earlier assertions that the return of Yellowstone's elk-hungry wolves has spurred new plant growth, he said.

The findings from Ripple and co-author Robert Beschta will be published in the scientific journal Biological Conservation. The study already has been released online.

Wolves are "apex predators, on top of the food web," Ripple said. "They're more than just charismatic animals that are nice to have around. We're finding that their function in nature is very important."

Wolves have spin-off benefits too, the researchers said: As trees grow taller, the stands provide more habitat for yellow warblers and other songbirds and more food for beavers, which in turn construct ponds that attract fish, reptiles and amphibians.

The phenomenon has been described as a "landscape of fear" in which a predator's pursuit of prey has a cascading effect across the ecosystem.

Wolves were reintroduced to Yellowstone from Canada in 1995 and 1996 after being killed off early last century. About 100 now roam the park, thinning elk herd numbers as they feast on the big game animals year-round.

Some scientists dispute the claim that wolves have sparked a restoration among Yellowstone's aspen.

In a 2010 study, U.S. Geological Survey scientist Matthew Kauffman agreed that foraging elk were the leading cause of the trees' decline in the park during more than a century. But Kauffman said the decline has continued since wolves returned, even in areas considered risky to elk because they are frequented by the predators.

That's because elk alter their behavior only slightly to avoid wolves, concluded Kauffman, who also heads the Wyoming Cooperative Fish and Wildlife Research Unit.