In Yellowstone drama, angry bison threaten a key Western tree

The record numbers of bison in Yellowstone National Park threaten the recovery of the park’s iconic quaking aspen, which scientists had hoped was on the mend, a study has found.

The findings, published Tuesday in Ecology and Evolution, reveal a complex drama between plants, predators and prey in the oldest U.S. park — and one of the earliest and most intensive sites of U.S. federal intervention in a complex ecological landscape.

At the center of that story is the surprising — and now interrupted — recovery of the aspen.

The aspen is a keystone Western tree species known for its vivid fall foliage and its smooth white bark, which is puckered by black scars where the trees have dropped branches they no longer need.

That white bark is a botanical wonder because, unlike the bark of most trees, it can carry out photosynthesis, allowing the aspen to make its own food even in winter, long after those brightly colored leaves have turned brown and fallen. 

That special quality has helped aspen achieve its current wide geographic range. 

But unfortunately for the tree, that bark — and the tender trunks of young aspen — are also attractive to the park’s resurgent northern bison herd.

Researchers found that the enormous, shaggy herbivores were rubbing the bark off of young aspen saplings, killing them. 

Perhaps more frustrating for landscape managers was the bison’s habit of simply knocking young aspen trees over.

In the nearly 90 aspen stands researchers surveyed, they found that almost 20 percent of saplings had been broken by bison.

The researchers, from Oregon State University (OSU), concluded that bison were now hindering the recovery of the tree species, which had previously been driven into decline by another large herbivore — Yellowstone’s ubiquitous elk. 

For the Yellowstone elk, the 20th century was a golden age. It was a period when humans had suppressed or eradicated their main predators — wolves, bears and cougars.

This allowed the elk herd to grow to unusual size, which was particularly bad news for aspen, a species that reproduces largely through sending up identical “clones” from a giant shared root system.

While saplings are shooting up through the soil, they are vulnerable — and, to elk, delicious.

In one region, the aspen canopy in 2015 had fallen to 1 percent of its extent in 1954, another OSU study found.

Previous work by the lead author of that study, Luke Painter, built on findings that the reintroduction of wolves to Yellowstone National Park in 1995 reshaped the ecosystem.

The growing numbers of wolves both cut down the population of elk in the park by nearly two-thirds — and changed their behavior in key ways.

With the cooperative carnivores patrolling the forests, the elk were no longer able to camp out above the new aspen growth, eating all the new, tender saplings.

Instead, the surviving herds began to spend the winter outside the park, where wolves were suppressed — allowing saplings time to grow to maturity.

“Some young aspen began growing into saplings — young trees taller than 2 meters — which was an indication they were no longer being consumed by elk and were likely to grow into mature trees,” Painter said.

With fewer elk, the aspen could recover, Painter concluded in that earlier paper.

“Restoring aspen in northern Yellowstone has been a goal of the National Park Service for decades,” he wrote.

“Now they’ve begun to achieve that passively, by having the animals do it for them. It’s a restoration success story.”

Except, the Ecology and Evolution study found, it wasn’t. 

Because as elk declined, bigger and more formidable herbivores filled the space they had left behind: the American bison, which federal managers had helped save from local extinction through one of the nation’s earliest conservation breeding programs in the 1920s.

With wolves targeting the elk, bison populations in Yellowstone have quadrupled since the 1990s — climbing to nearly 4,000. (While Yellowstone wolf packs target bison too — generally going after females or calves — they generally prefer easier prey. In this video, bison cooperate to save a calf from wolves.)

Like bison everywhere, the resurgent herd in the park reshaped the landscape around them. As they trundled along creek sides, they compressed the banks, changing the way streams flow.

They trampled vegetation, creating new paths; rubbed their horns on trees; and punctured the soil with their hooves, aiding the growth of new trees.

And, as the Ecology and Evolution findings showed, bison bulls simply broke the recovering aspen saplings, in what researchers conjectured was a macho show of aggression toward their rivals.

In a heartbreaking detail for aspen lovers, these shaggy showboaters tended to vent their anger on saplings that — with a height of more than 12 feet — could have survived the depredations of elk and other herbivores.

Concentration of bison on the Yellowstone landscape might have been relieved in other cases, as bison, left to their own devices, wander out of the park and into the surrounding countryside.

But since the late 1990s, a deal between the federal government and Montana — pushed by the state’s powerful ranching industry — has required Yellowstone officials to kill, capture or scare back into the park any bison that roam free.

This has bottled the bison up in the park, concentrating their ecological effects, Painter noted.

While the recovery of the bison was a great conservation success, “one important conservation goal is affecting another important conservation goal,” he added.