About
Pembina Trail SNA is notable in its own right, but also for being part of a landscape-scale tallgrass prairie initiative in Polk County that includes Glacial Ridge National Wildlife Refuge and over half a dozen nearby Wildlife Management Areas. The SNA takes its name from its proximity to Pembina Trail (also sometimes called the Woods Trail), one of the routes used in the mid-1800s to haul trade goods by ox-cart between the northwest prairies of the Red River Valley and St. Paul, Minnesota.
This SNA is composed of three units: from north to south these are the Crookston Prairie, Pembina Trail Preserve SNA (owned by The Nature Conservancy and largest of the three, at 1660 acres) and Foxboro Prairie.
The site ranges from near-level to gently undulating, with a prominent Lake Agassiz beach ridge running along the eastern side of the SNA, expressed in the local topography as a 10-15' rise in elevation. As explained by Dr. Harvey Thorliefson, Director of the Minnesota Geological Survey, this wave-deposited, sand and gravel ridge marks the historic shoreline of the lake during its Tintah phase, and was formed an estimated 11,500 to 13,000 calendar years before present. This ancient beach ridge not only offers insight into Minnesota's geologic past, it adds to the biodiversity of the landscape by supporting sand-gravel prairie.
Most of the SNA has been mapped by the Minnesota Biological Survey as Agassiz Interbeach Prairie Complex, defined as "a mosaic of upland prairie, wetland prairie, and sedge meadow communities in areas of shallow swales and rises on calcareous soils formed in glacial Lake Agassiz lacustrine deposits." Relatively smaller areas of wet-mesic aspen and aspen-oak woodland may be found as well, along with small fen communities in areas of groundwater discharge high in calcium and magnesium bicarbonates.
Despite a land-use history that includes haying, cultivation and grazing in parts of the SNA, a remarkable diversity of native species persists here. Rare species include Hall's sedge, hair-like beak rush, Dakota skipper butterfly, small white lady's-slipper, northern gentian, and grassland birds such as Upland Sandpiper, Yellow Rail, greater prairie chicken, marbled godwit and Wilson's Phalarope. This is one of few places in the state where the federally-threatened western prairie fringed orchid still perfumes the summer nights.
The Nature Conservancy (TNC) has long had a presence in this region, not only acquiring native prairie remnants for preserves but also bringing together public and private partners to restore the intervening lands to prairie. These efforts culminated in a 24,158-acre land acquisition by TNC in the year 2000, which became the heart of the Glacial Ridge National Wildlife Refuge.
Brian Winter has worked in various capacities for TNC in this region for more than 30 years, and is now the organization's Program Director for Stewardship in Minnesota, North Dakota and South Dakota. "Pembina Trail SNA stands out in my mind for its native prairie and rare species," says Winter. "It's been both the blueprint and seed source for the massive Glacial Ridge Project, which is mostly reconstructed prairie." He adds, "For me, personally, I'll always remember Pembina as the place where I saw three of Minnesota's native grouse in one day: Ruffed Grouse, Sharp-tail Grouse and Greater Prairie Chickens. It was a real treat to be able to do that."
So often we read about the tallgrass prairie as something of the past: a broken, fragmented landscape. Here, we see the beginnings of a new story, one that gives hope that we may retain at least some of the prairie's rich diversity and ability to function. Among the good signs observed by TNC is that hydrology is returning to more natural conditions in parts of the Glacial Ridge landscape, with increased retention of water in the surficial soils. With each passing season, the native prairie at Pembina Trail SNA finds itself in better and better company.
"Lake Agassiz was an immense ice-dammed lake that filled the Red River Valley during the retreat of the continental ice sheet at the end of the most recent Ice Age. The history of the lake was complicated, but overall its level fell stepwise, as the outlet channel to the Minnesota River Valley was eroded, and as lower outlets across the Iron Range, or leading to the Great Lakes and/or Arctic Ocean were opened as the ice retreated.
Each level of the lake left behind shoreline features that are now in many cases mined for sand and gravel. The principal shorelines that drained south to the Minnesota River Valley are named after towns on a rail line in western Minnesota between Morris and Wahpeton – from highest to lowest, these are the Herman, Norcross, Tintah, and Campbell. Much research has addressed the age of these shorelines, with conflicting estimates for the Tintah's age ranging from about 11,500 to 13,000 calendar years."