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Bison will once again feast on the grasslands of northern Anoka County this summer as scientists continue to study what role the animals may play in helping save the oak savanna, one of Minnesota's most threatened ecosystems.

The herd will arrive in late May and stay through the summer at Cedar Creek Ecosystem Science Reserve, a University of Minnesota research station that straddles Anoka and Isanti counties. Weekly chances for the public to glimpse the herd begin June 1.

It marks the second straight year for bison to be hauled onto 200 acres near East Bethel for a seasonal stay at one of the largest swaths of oak savanna left in the state. Scientists say the project could uncover a new strategy to maintain the vanishing Midwest ecosystem.

Since the 1960s, researchers at Cedar Creek have studied how to best preserve and restore the oak savanna. The delicate mix of oak trees and prairies once covered huge tracts of the Midwest but has largely been lost to croplands, pasture and development.

Ongoing research has been rooted in prescribed burns at Cedar Creek, one of the best-studied ecosystems in the world. Scientists there have found that fire helps keep woody plants from crowding out prairie grasses and flowers.

But the blazes, fueled by the robust grasses, also have made it tough for oak seedlings to survive. That's why U researchers introduced a herd of bison last year, bringing the animals back to the Anoka County area for the first time in more than a century.

The bison project has largely been funded by the state's Environment and Natural Resources Trust Fund, and scientists say they hope to keep the work going for years to come.

Researchers wonder if bison have been the missing piece that oak savannas need to flourish. The large grazers feed on the very grasses that fuel the fires. Grazing could make those needed burns less intense, giving tiny oaks a better shot.

The bison also weed out some of the seedlings' competition for light and soil nutrients. "The bison last year did a remarkable job of eating down the grass," said Caitlin Potter, Cedar Creek's education and outreach coordinator.

A herd of 32 young bison dined on Cedar Creek's grasses from mid-June to September last summer. This year, the herd will be slightly smaller but will arrive earlier, Potter said, courtesy of NorthStar Bison, a Wisconsin-based ranch that loans the animals for free during summer grazing.

The bison proved to be popular additions to the neighborhood, with the project's viewing gazebo often attracting 50 or so visitors on Saturdays last summer, Potter said.

"It has been far and away one of our most popular sets of events that we hold," Potter said.

Now the trial by fire and next phase of research are about to begin. Ahead of the bison's arrival, crews have started controlled burns on the 200 acres where the bison roamed.

Early observations are promising. In areas where bison were fenced in, the blazes have so far seemed spotty and less intense — good news for young oaks.

"They have a much better chance of surviving these patchy fires," said Forest Isbell, Cedar Creek's associate director who helped spearhead the bison project. "It looks like it played out how we expected."

In coming days, scientists will pay careful attention to the more than 600 oak seedlings planted last year to see how they fared in this spring's fires after surviving the herd's first summer without being trampled or eaten.

The fires also help prepare the prairie buffet. The flames get rid of dead material and provide more light, warming the dark soil more quickly so that flushes of green make a faster appearance than in areas left unburned, Potter said.

That means that by the time the bison arrive next month, they'll find plenty of prime forage under hoof.

Hannah Covington • 612-673-4751