Not for a century have such thunderous hoofs struck the tree-studded grasslands of northern Anoka County.

Under clear June skies, a herd of young bison barreled from trailers and tore through fields stippled with oaks and wildflowers while scientists nearby hugged the fence of the grassy enclosure. Some snapped photos. Others seemed too gobsmacked to move. At least one wiped away tears.

A shared question hung in the air: Could these burly newcomers become the heroes of a landscape in peril?

“Look at them running!” cried Caitlin Potter, a University of Minnesota scientist. “This is where they belong.”

Bison are being reintroduced on 200 acres near East Bethel, part of a broader project to see what role the storied animals may play in helping save the oak savanna, one of Minnesota’s most threatened ecosystems.

A delicate blend of prairie and scattered oak trees, oak savanna once covered about 9 percent of Minnesota, according to the state Department of Natural Resources. Less than 1 percent remains, largely lost to croplands and pasture.

Since the 1960s, scientists have studied how to preserve and maintain oak savanna at Cedar Creek Ecosystem Science Reserve, a 5,500-acre U research station straddling northern Anoka County and southern Isanti County.

Fire, they say, is crucial, but it comes at a price. Without prescribed burning, woody plants and trees shade out prairie grasses and flowers. But fires also may have made it tough for oak seedlings to survive, spurring the slow demise of savanna as it gives way to grassland.

That’s where the bison come in. Scientists think that with the large grazers feeding on the grasses, oak seedlings may face less competition for light and soil nutrients. Grasses also are the primary fuel for fires, so burns may be less intense on grazed fields, giving tiny oaks a fighting chance.

Hopes are high that the bison project could uncover a new strategy to restore this vanishing Midwest ecosystem.

“It’s bringing back a missing piece of the puzzle,” said Forest Isbell, an assistant professor of ecology at the U who helped spearhead the project. “It’s about time.”

Fire and forage

Millions of bison once dominated the Great Plains, but systematic slaughter by European settlers and loss of habitat pushed them to the brink of extinction a century ago. Ranching and conservation efforts have helped their numbers rebound; still, Cedar Creek researchers say, bison likely have been absent from the Anoka area for more than 150 years.

A month before bison charged into Anoka County, Isbell sat in his quiet St. Paul office at the U and scrutinized a photo on his laptop screen. It captured a striking savanna landscape at Cedar Creek, with shapely, mature oak trees embedded in lush prairies. But what he saw was an ecosystem in trouble.

“Notice, no young oak trees,” said Isbell, who’s also the associate director at Cedar Creek. “This looks nice, but it’s fleeting in its current form.”

The oak seedlings, he said, are getting caught in a fire cycle and can’t seem to get past the small shrubby stage, if they survive the blazes at all.

Through decades of prescribed burns at Cedar Creek — one of the longest ongoing fire experiments in the world — researchers here have found that fire alone can’t restore the savanna. So the plan is to study how the savanna responds to bison grazing alongside various fire frequencies, layering new findings with Cedar Creek’s deep cache of existing research. Some plots will be fenced off from the bison.

Scientists say they’re ready to be surprised. While bison prefer grazing on grasses, it’s unclear how much the animals may trample or eat the oak seedlings, too.

“Whatever it is, it is — whether the bison come out to be the hero or goats,” said David Tilman, Cedar Creek’s director who’s also involved in the project.

Scientists know that the herds that once defined the American West are a keystone species for restoring and preserving grassland, along with prescribed burning. At Konza Prairie Biological Station in Kansas, researchers have found that bison, by eating and knocking down grasses, promote plant diversity and make fires there less intense.

“A healthy grassland is one that’s grazed,” said John Briggs, professor of biology at Kansas State University and Konza’s director.

Yet not much is known about the role of bison in rescuing oak savanna, an ecosystem they once roamed. Konza scientists are keeping an eye on Cedar Creek, eager to compare findings.

“I think it’s just going to be a bonanza and a gold mine for researchers,” Briggs said.

‘A great fit’

Scientists at Cedar Creek got nearly $390,000 in funding for the bison project from Minnesota’s Environment and Natural Resources Trust Fund. Among other things, the money paid for the fence to keep the bison within the 200-acre enclosure.

The herd is on loan for free for the summer from NorthStar Bison, a Wisconsin-based ranch that also provides bison to Belwin Conservancy in Afton for prairie restoration work.

At Cedar Creek, researchers say the bison will be feasting in one of the largest swaths of oak savanna left in the state.

“Our benefit is to have grass for our bison, and their benefit is they get to research bison on their grass,” said Marielle Hewitt of NorthStar. “It seemed like a great fit.”

The ranchers will return in August and use a portable corral system to remove the bison until next summer. Scientists hope to continue the research for years to come.

On Wednesday morning, trailers carrying young bulls from NorthStar Bison rumbled into Cedar Creek. Scientists clustered around the enclosure opening in the noonday sun as a herd of swarthy 2-year-olds pounded into the savanna, tails whipping. The bulls kicked up an earthy smell that signaled good forage.

Isbell stood smiling near the truck, gripped, he said, by their power. Nearby, Tilman compared their entrance to a “football team taking the field.” Then he swung his eyes back to the enclosure: “Look at that! Bison grazing!”

And so they were. One by one, hulking heads lowered into the grasses and tugged at the earth. Dinner.