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 the Department of Forest Ecosystems and Society at Oregon State University.