CORVALLIS, Ore. – Yellowstone National Park is celebrating an ecological milestone along with a key anniversary this summer, Oregon State University researchers report.
A paper published today in Forest Ecology and Management documents the first new generation of overstory aspen trees in Yellowstone's northern range in 80 years, three decades after wolves were reintroduced to the nation's oldest national park.
Without predation pressure from wolves, which had been extirpated from the park by 1930, elk populations grew to the point that their browsing was thwarting the growth of young aspen. The ecosystem effects were widespread as aspen stands support a range of species including beavers and cavity-nesting birds.
With wolves back in the mix along with bears and cougars, a nearly extirpated predator whose numbers increased along with wolf reintroduction, elk numbers have been reduced and aspen are once again working toward becoming full-grown trees.
"The reintroduction of large carnivores has initiated a recovery process that had been shut down for decades," said the study's lead author, Luke Painter, who teaches ecology and conservation in the OSU College of Agricultural Sciences. "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."
Another third of the surveyed stands had patches of tall saplings growing into new overstory trees, he added, and the rest remained suppressed by herbivory.
"Increasing numbers of bison may be emerging as a new constraint to aspen in some areas," Painter said.
The fact that stands with many tall saplings have low rates of browsing, whereas other stands continue to be suppressed, indicates aspen recovery is happening because of a trophic cascade and not other factors such as climate or site productivity, he said.
In a trophic cascade, a change at the top of a food web causes ripple effects throughout an ecosystem, altering its structure and balance. In Yellowstone, top predators have reduced herbivory by elk, allowing aspen to begin to recover.
"This is a remarkable case of ecological restoration," Painter said. "Wolf reintroduction is yielding long-term ecological changes contributing to increased biodiversity and habitat diversity."
Collaborating with Painter were Robert Beschta and William Ripple of the OSU College of Forestry. The Ecosystem Restoration Research Fund of the Oregon State University Foundation supported the research.
