Field Notes
Field Notes

Battling buckthorn to restore natural resilience

Tuesday, May 16, 2017
Posted by
Greg Seitz

Anemone quinquefolia (Wood Anemone)

Wood Anemone (Anemone quinquefolia)

I was walking through the tangled woods of the Research Station’s Tanglewood Nature Preserve when I heard cheeping from above. I looked up, but never saw the source of the urgent cries. My eyes were instead drawn to a Red-shouldered Hawk zipping through the treetops. It was gone in a moment and the woods were silent again.

Red-shouldered Hawks are birds of tall, mature forests. They like open woods where they can easily hunt creatures on the forest floor. Unfortunately, the hawk’s numbers are dwindling in the Midwest, just like those woodlands it prefers. In 1848, U.S. government surveyors noted lots of “scattering timber” on dry, sandy soil in upland areas near the Research Station.

The soil was carpeted by grass, flowers, and sedges — with roots woven thicker than any fabric. Fires periodically burned through, beating back woody brush. The sunny environment was ideal for the plants that germinated and died back every year.

This living layer held the soil firm against the forces of water. Today, things are much different.

A century of change

When European immigrants arrived in the nineteenth century, they changed the ecosystem by driving away the Dakota people, who used to set many of the fires. But, the settlers’ hungry cattle grazed among the trees, keeping the ground free of most shrubbery — while also eating the native plants and preventing them from reproducing.

Then most of the cattle disappeared from the pastures as beef production was industrialized, and the prairies were plowed for crops. The forests were largely neglected. The plants the cows didn’t eat were weedy, and too weak to resist new competition.

That is why today, much of the wooded land in the region is choked with dense, spiky thickets of invasive buckthorn. It was brought by the Europeans and went feral. It’s nearly impossible to walk through.

But, buckthorn’s impacts are more serious than simply making the woods impassable, because what happens on the land, happens to the water.

Downstream impacts

What has happened on the land is buckthorn has created thick stands of bushes that leaf out earlier and stay green later than anything else, shading the ground through the whole growing season.

Buckthorn also changes the soil chemistry, poisoning native plants and making the soil susceptible to invasive earthworms that eat dead leaves and other forest floor detritus (there are no native earthworms in Minnesota).

The soil ends up nearly naked, the native plants long gone.

Buckthorn and bare ground

“Buckthorn’s leaves can intercept and slow down rain, but once it gets down to the soil surface, if there’s not much green vegetation, it won’t soak in. If there’s no ground layer there, it will cause damage.” says Paul Bockenstedt, an experienced ecologist and project manager at consulting firm Stantec, which is working with the Research Station to restore parts of its property.

When it rains, the soil under buckthorn can easily wash away, forming gullies and ravines, and sending more soil, leaf matter, and other substances flowing downhill. Most if it eventually ends up in a waterbody somewhere, which is often already overwhelmed by sediment and organic nutrients from farm fields, lawns, and other sources.

While the impact of agriculture on water quality has become better known in recent decades — and the Research Station has studied it intensively — buckthorn’s ongoing takeover of the region’s forests and what it means for lakes and rivers has so far received less attention.

In the St. Croix watershed, 58 percent of the land is covered with forest (a 34 percent decline since Europeans arrived in the region). Yet forests are only responsible for 8 percent of the phosphorus that feeds algae and plagues the lower St. Croix River.

"Our forests are our best defense against water quality degradation in the St. Croix,” says Monica Zachay, water quality steward with the St. Croix River Association. “We simply can’t lose any more and expect to meet water quality standards.”

Restoring resiliency

This winter, the Research Station and Stantec commenced a multi-year project to clear buckthorn and other invasive plants from about 70 acres of the property. The project is supported by Minnesota’s Outdoor Heritage Fund, which distributes one-third of the sales tax approved by voters in 2008 with the Clean Water, Land and Legacy Constitutional Amendment.

At the same time, researchers from the University of Minnesota are conducting studies in the cleared areas to determine the effectiveness of several strategies to slow buckthorn’s return.

The restoration project began with a terrible grinding sound. Starting in early January, a tracked skidsteer with a masticating drum mounted on its front tore through the brush, shredding any and all wood, and spitting it out as chips. It made short work of vast swaths of invasive shrubs.

Other workers used brush saws to clear places the skidsteer couldn’t get. They daubed the stumps with herbicide to prevent them from growing back. Afterwards, the cleared areas were seeded with the types of plants that grew here historically (most of the seeds originated within ten miles).

It took several weeks of labor by young people with energy and muscles, guided by the company’s decades of experience. The work was done in winter when the ground was frozen to minimize damage to the soil. What was left after this winter’s work was a wide-open forest covered in wood chips.

Their efforts made the woods walkable, but the project is not nearly finished. The goal was not just to cut down the buckthorn, but to restore the land’s natural resistance to the imbalance of invasive species.

“I view restoration projects like this as trying to put landscapes on wellness programs,” says Bockenstedt. “Think of the invasive species as a patient having an infection. Our goal is to treat the cause not the symptoms, and build up the landscape’s ability to be more resistant to re-invasion by buckthorn and invasive species in the future.”

In the past 20 years, Stantec’s restoration staff have seen how that helps improve the success of the seeds that they distribute afterwards.

At the end of April, they burned a large prairie on the station’s Tanglewood property, suppressing nonnative grasses like brome and bluegrass, returning nutrients to the soil to feed the seeds they spread afterwards. In the years ahead, fires are planned in woodland areas too, as well as herbicide treatments, to prevent buckthorn re-growth and give native species an edge.

Fixing the sunlight fight

Almost as soon as the brush had been knocked down this winter, white posts with blue flags popped up in the woods. They marked plots where forestry researchers will manage and measure several seeding strategies.

The “Cover It Up” project is seeking to better understand how to use native plants to stop buckthorn from growing back after clearing. It’s led by Drs. Mike Schuster and Peter Wragg of the University of Minnesota’s Invasive Terrestrial Plants and Pests Center, collaborating with Drs. Peter Reich and Lee Frelich. They are working on several recently-cleared lands around the Twin Cities area, including eight plots at the Research Station.

This spring, the researchers planted the plots with specific kinds of vegetation that once grew in these forests, but which were largely driven out during European settlement and buckthorn invasion. Species include Bottlebrush Grass, Brown-eyed Susan, Pennsylvania Sedge, and Elderberry bushes.

The idea is to see if heavy seeding can limit buckthorn by re-establishing the native plant community. In a couple years the project should provide valuable new data about what species have the best chance of winning the competition with buckthorn, by taking up light, space, and other resources.

Buckthorn is notoriously shade tolerant, which is why it has overtaken so much forest understory, but these experiments are intended to see if native plants can block enough light to slow down buckthorn regrowth.

Eventually perhaps, more Red-shouldered Hawks will fly through these forests, because they’ll be able to see their prey on the ground. And people will be able to explore the woods on something similar to its original carpet of vegetation. And when water falls from the sky, it will soak in, stay put, and keep more harmful runoff out of the St. Croix and other waters.

In the months ahead, we’ll continue sharing the story of our land restoration, the buckthorn research — and how it’s all connected to water.

Red-shouldered Hawk (Buteo lineatus)

Red-shouldered Hawk (Buteo lineatus)

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