Wolves Bring Aspen Trees Back

Before the park's wolf reintroduction, loss of aspen trees in Yellowstone National Park was traced to elk grazing . Now wolves help control the elk population.
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Why Have Yellowstone Aspen Trees Been Making a Comeback?

Biologists had a question: what was happening to the aspen in Yellowstone National Park? Although the quaking aspen tree is the most widely distributed tree in North America, its abundance has been shrinking over the past century. This loss has been especially apparent in our nation's first park, where stands of the white-barked tree, with its trembling leaves and brilliant fall foliage, have decreased dramatically over the last century.

This disappearance is puzzling, especially since aspen grows rapidly and can quickly resprout from its extensive underground root systems or from seeds after fires, floods and other disturbances.

In 1997, William Ripple, Oregon State University forest ecologist, and OSU graduate student Eric Larsen, set out to unravel the mystery.

"We had several theories on the cause of the decline," Ripple said in an interview from his Corvallis, Ore., office. "It could be caused by the lack of fire, climate change, or other environmental factors. We weren't sure."

To answer the question, the researchers studied tree rings, tree size classes, and aerial photographs of the park. "We took core samples of the aspen to count the rings," Ripple said. "From that data, we developed an age structure for the aspen."

The results were surprising. Ripple discovered the aspen quit regenerating in the 1920s. For 70 years, young aspen hadn't fared well, with few surviving to become mature trees.

About the same time that the aspen quit regenerating, wolves were eliminated from the park. Ripple said the timing begged the question: Is there a connection?

Connection Between Wolves, Elk, and Aspen

"We developed the hypothesis that there was some link among wolves, elk and aspen," Ripple said.

The primary culprit for the loss of young aspen was elk feasting on the sprouts. The elk browsed in the willow bottoms and other open country, leisurely gobbling up tender young trees, shrubs and grasses.

With wolves absent, the elk grazed anywhere they liked and for decades have been able to kill, by browsing, nearly all the young aspen. Other streamside species such as willows and berry-producing shrubs also suffered. That in turn began to play havoc with an entire streamside ecosystem and associated wildlife, including birds, insects and fish.

Wolves were reintroduced to Yellowstone in the winter of 1995-1996. In 2018, the park had about 450 wolves, too many to track them all with radio collars. They are no longer classified as an endangered species, but are now "threatened," and, once a dispute between the Fish and Wildlife Service and the state of Wyoming is resolved, they may soon be "delisted" altogether.

Before the wolves,  Yellowstone had the largest single elk population in the world. Weighing as much as 700 pounds apiece, they had no serious rivals. Grizzly bears, Yellowstone's top predators, are capable of bringing down an adult elk, but they mainly prey on calves. Coyotes, though numerous, were much too small to attack elk.

The fact that wolves have returned and done well has resulted in a boon for the park's aspen.

Amber Travsky is a freelance writer from Laramie, and is the author of Mountain Biking Wyoming and Mountain Biking Jackson Hole. She is an outdoor writer for the Wyoming Tribune-Eagle newspaper in Cheyenne and the Laramie Boomerang. In addition, she is a wildlife biologist and owns her own environmental consulting firm.

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