Aspen trees are getting a boost from wolves at Yellowstone National Park, new research suggests.

For the first time in 80 years, baby aspens are growing tall and healthy in the northern part of the park, scientists report in the journal Forest Ecology and Management. They’ve linked the trees’ resurgence with the reintroduction of gray wolves (Canis lupus), which are keeping Yellowstone’s plant-eating elk in check.

Gray wolves were once abundant in Yellowstone National Park. But by the 1930s, the apex predators had been extirpated from the area, largely due to hunting, government eradication programs and habitat loss.

Without the wolves, the park’s elk population ballooned, reaching around 17,000 individuals in winter 1995. The huge numbers of elk wreaked havoc on Yellowstone’s quaking aspen trees (Populus tremuloides) by chowing down on saplings before they had a chance to get established. When ecologists conducted surveys in the 1990s, they couldn’t find a single aspen sapling in the park.

“[The aspens] would grow new sprouts, but then the sprouts couldn’t get any larger [because of the elk],” says lead author Luke Painter, an ecologist at Oregon State University, to Oregon on the Record’s Michael Dunne. “The stands basically had older trees … and those were dying out, and then there wasn’t any new growth underneath, of young aspens, to replace those older trees.”

Then, in 1995, biologists reintroduced gray wolves to the park. The carnivores quickly made meals out of Yellowstone’s abundant elk, helping drive their population down to the roughly 2,000 individuals that spend winter in the park now. With fewer elk grazing, aspen saplings have been able to regain a foothold, the study authors suggest.

Scientists had previously surveyed 87 aspen stands in northern Yellowstone in 2012. When the new study’s team returned in 2020 and 2021, 43 percent of the sample sites had new, young trees with trunks that were at least two inches in diameter at chest height—something researchers hadn’t seen since the 1940s.

They also documented a more than 152-fold increase in the density of aspen saplings between 1998 and 2021.

“It doesn’t mean that they’re not going to get killed or die from something, but it’s a pretty good indication that we’re getting some new trees,” Painter says to Live Science’s Chris Simms. “As they get bigger, they get more resilient.”