The restoration of gray wolves in Yellowstone National Park has helped revive an aspen tree population unique to the region, a new study has found.

Quaking aspen, one of the few deciduous tree species in the northern Rocky Mountain ecosystem, is once again thriving, after suffering severe decline during the 20th century, according to the study, published on Tuesday in Forest Ecology and Management.

“This is a remarkable case of ecological restoration,” lead author, Luke Painter, who teaches ecology and conservation at Oregon State University’s College of Agricultural Sciences, said in a statement.

The decline in aspen growth occurred in tandem with a surge in Rocky Mountain elk, which had lost a key predator following the elimination of wolves from the region by 1930.

Elk and other voracious herbivores, the study authors explained, partake in intensive “browsing” or consumption of high-growing plants like leaves, soft shoots and shrubs.

That browsing menu, however, includes “the palatable young shoots of aspen,” the researchers explained, noting that such activity can both suppress the growth of the trees and inhibit the growth of new saplings and trees.

These circumstances prevented aspens from growing taller and replacing aging trees, even after Yellowstone park officials launched an elk culling initiative, according to the study.

The dearth of aspen trees had wide-ranging knock-on effects across the ecosystem, the researchers added, citing effects on species such as beavers and cavity-nesting birds.

But the situation began improving following the reproduction of gray wolves in 1995-1996, as these animals supplemented the predation efforts of bears, cougars and human hunters outside the park, per the study.

 “The reintroduction of large carnivores has initiated a recovery process that had been shut down for decades,” Painter said.

In 2020-2021, the researchers examined 87 randomly selected, previously sampled aspen “stands,” or groups of genetically identical trees interconnected by a shared root network.

About 43 percent of the stands contained a new cohort of small trees, marking the first documented growth of aspen canopy foliage in northern Yellowstone since the 1940s, according to the study.

“About a third of the 87 aspen stands we examined had large numbers of tall saplings throughout, a remarkable change from the 1990s when surveys found none at all,” Painter observed.

Both saplings and small trees, the authors found, were absent in 1998 but escalated rapidly after 2007.

While the results show a likely restoration of ecological processes that have ensued in widespread growth, the recruitment of new trees has been highly variable, the researchers noted.

Increases and variations in height of young aspen, they concluded, have likely “been driven by a decrease in elk browsing, not climate change or differences in site productivity.”

Reinforcing that conclusion, the researchers added, is the fact that aspens located outside the park, in areas with fewer elk, continued to produce saplings.

Climate-driven trends of warmth and dryness which have wrought havoc on aspens in the Mountain West have not applied as much to northern Yellowstone, where soil moisture has generally remained sufficient during drought periods, per the study.

Meanwhile, with the reintroduction of the wolves, elk changed their foraging behavior by shifting from this “landscape of fear” to less risky places, the scientists explained.

“Wolf reintroduction is yielding long-term ecological changes contributing to increased biodiversity and habitat diversity,” Painter said.

At the same time, however, the authors observed that aspen recovery hasn’t been uniform across northern Yellowstone and the growth is subject to numerous potential threats.

For example, future effects of climate change, as well as the encroachment of coniferous trees, are possible such factors, according to the study.

And while elk presence may have decreased, other browsing herbivores have increased their footprint in the region, the researchers noted.

“Increasing numbers of bison may be emerging as a new constraint to aspen in some areas,” Painter said.