For the first time in 80 years, Yellowstone National Park is seeing new generations of overstory aspen trees, thanks to the reintroduction of wolves in 1995, according to a study by Oregon State University researchers.
The findings, published in the journal Forest Ecology and Management, mark a significant ecological milestone and coincide with the 30th anniversary of wolves return to the park. Wolves, extirpated from Yellowstone by 1930, were reintroduced in 1995, reshaping predator-prey dynamics and triggering changes that have rippled across the landscape.
Aspen stands in Yellowstones northern range had long been stunted by unchecked browsing from elk, whose populations soared in the absence of predators. The return of wolves — along with bears and cougars, which have also rebounded — has curbed elk numbers and allowed young aspen to grow.
“The reintroduction of large carnivores has initiated a recovery process that had been shut down for decades,” said 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.”
The research team also found that another third of aspen stands surveyed now show patches of tall saplings maturing into new overstory, or canopy, trees, while the remaining stands remain suppressed by herbivory.
“Increasing numbers of bison may be emerging as a new constraint to aspen in some areas,” Painter said.
The studys authors say the recovery provides strong evidence of a trophic cascade — an ecological process in which top predators indirectly benefit plants and other species by controlling herbivore populations. In Yellowstone, this shift has cascading benefits for species like beavers and cavity-nesting birds that rely on aspen habitats.
The study was co-authored by Robert Beschta and William Ripple of OSUs College of Forestry and supported by the Ecosystem Restoration Research Fund of the Oregon State University Foundation.
“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.”
