Researchers found a 152-fold increase in sapling and young-tree density with wolves’ return

A wolf in Yellowstone. Photo / Supplied

Three decades after wolves were reintroduced in Yellowstone National Park, aspen stands are recovering, a new analysis suggests.

Published in Forest Ecology and Management, the study looks at what happened after 1995, when wolves were reintroduced to the park.

During most of the 20th century, there were no apex predators in the park aside from bears and cougars, and elk increased to record numbers.

In previous years, the elk had kept young aspen to 1m or less in height, preventing the trees from growing to their natural height of up to 12m.

Now that wolves are back and bears and cougars prey on elk in the park, the researchers write, elk numbers are down and aspen stands are growing new trees again, with young trees more than 5cm in diameter at breast height found for the first time in more than 80 years.

“The reintroduction of large carnivores has initiated a recovery process that had been shut down for decades,” the study’s lead author, Luke Painter, who teaches ecology and conservation in the Oregon State University College of Agricultural Sciences, said in a news release.

“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.”

All told, the researchers found a 152-fold increase in sapling and young-tree density, with 38% of aspen stands still suppressed by grazing elk and bison and 32% showing only patchy growth.

Aspens grow in groups called stands, reproducing mostly asexually and thriving through collective roots.

Debate has long raged as to whether Yellowstone could be used as an example of a trophic cascade, referring to downstream effects, often catastrophic, of an absent apex predator.

Without predation, grazing species like elk increase until limited by starvation; they also can eat too much vegetation, causing other problems down the food chain.

Now, the park contains “historically and ecologically significant” amounts of growing aspen saplings, the researchers write - key to the ecosystem’s future as it continues to recover.

Aspens support habitat and species diversity, they add.