In 2003 two scientists from Oregon State University, Bill Ripple and Bob Beschta studied trees along the Lamar River and found many tall cottonwoods more than seventy years old - trees that would have been seedlings around the time Yellowstone's wolves were killed off. They also found thousands of tiny cottonwood seedlings that had grown in the few years since the wolves returned. But they found nothing in between.
With wolves absent, fearless and hungry elk stopped the cycle of cottonwood regeneration: seedlings growing into young trees which mature and replace dying old trees. Too few wolves led to too many elk and not enough cottonwoods. In some parts of Yellowstone, cottonwoods faced the possibility of dying off.
Since the return of wolves, the next generation of cottonwoods are surviving - in certain locations - according to another Ripple and Beschta study. They found young cottonwood and willow growing taller each year in sites the scientists labeled as "high risk"- spots where the elk could not see wolves approach or from which the elk had no easy escape route, spots where dining could be deadly. On the other hand, seedlings showed little increase in height at "low-risk" sites - where the elk still felt safe to browse at will. With wolves back in the park, elk were living in what became known as an "ecology of fear" that determined where they browsed.