Bison have returned to East Bethel for the second year of an experiment in restoring oak savanna ecosystems.
The 17 bison recently released into the University of Minnesota’s Cedar Creek Ecosystem Science Reserve are part of an experiment to see if the large, grazing animals can help young oak trees survive prescribed burns.
On Saturday, May 25, the small herd of male bison was released from an enclosed trailer into a fenced-in portion of the reserve, where they quickly gathered into a tight knot and trotted over the hills and out of sight of onlookers.
“It’s part of long-term research that’s been going on since the 1960s trying to understand the dynamics of the threatened oak savanna ecosystem and how we can restore and enhance them,” Education and Outreach Coordinator Caitlin Potter said.
The oak savanna ecosystem is a transitional ecosystem between forests and prairies, according to the National Park Service. Savannas are characterized by a scattering of trees in prairie grasses.
However, the savanna can only be maintained with regular fires that keep trees from transforming the land into a forest, according to the National Park Service. That is the purpose of regular prescribed burns used in ecological management.
“If you don’t burn, the trees take over and you end up with a forest,” Potter said. “If you burn too often or you chop down trees to put in houses or run cattle or something you end up with a prairie.”
But those prescribed burns have proven too much for young oak trees, which are needed to replace older trees when they die.
“What we found out is that no matter what frequency of fire you put on the landscape, the young oaks get charcoaled,” Potter said. “The grass grows so high, so fast here that the fires are really hot and even the fire-resistant trees can’t survive.”
That’s where the bison come in. The idea is the bison will graze on plants targeted by prescribed burns and reduce fuel for fires – in turn making it easier for the trees to survive.
So far the experiment has had positive results. In locations where bison have grazed the fires burn about 356 degrees Fahrenheit, cooler than where bison did not graze.
The grazing has also led to an increase of grass growth by 50-100%, according to Cedar Creek representatives, but it’s expected the increased growth rate will decrease with time.
One question scientists want to answer is whether the grasses are growing faster overall, or if they’re simply redirecting resources from their roots to their leaves. One way the reserve will try to answer that question is by measuring root growth by taking core samples of the root systems, which can extend 10 to 20 feet under the surface.
In the meantime, scientists will be watching the young oaks planted most recently. So far the bison have left the trees alone, with around 98 percent surviving, Chad Zirbel a post-doctoral researcher working on the project said.
Now they have to wait to see how many of the trees survived the prescribed burns as the season warms up and the trees put out leaves, Zirbel said.
“In the next couple weeks we’ll know the answer to that question because, unfortunately, it’s been such a cool spring we don’t know for sure,” Zirbel said.
The oak savanna, which covered approximately 10 percent of the state before European colonization, is considered important because of the biodiversity it can support, according to the National Park Service.
Today approximately one-hundredth of a percent of the land originally covered by oak savanna remains, according to the United States Department of Agriculture.
Transitional ecosystems like oak savanna can harbor species from both ecosystems it borders as well as house species unique to that ecosystem, according to the park service.
Maintaining high levels of biodiversity is important for creating a stable and predictable environment, Eric Seabloom a University of Minnesota professor of ecology evolution and behavior said.
He compared ecosystems to stock investments, using the portfolio effect, which averages multiple single components into a single, more stable average.
“The more plants you put out the more likely it is that you are going to have one that’s going to do well in that year,” Seabloom said.
The oak savanna used to cover wide swaths of land in the U.S., but is now one of the rarest ecosystems in North America, Potter said.
“So there’s a lot of plants and animals that are specialized for that ecosystem,” Potter said.
One such species is the red-headed woodpecker. The colorful bird has dropped sharply in population and is one of Audubon Minnesota’s target species, which the organization considers in urgent need of conservation action, according to audubon.org.