Nerstrand Big Woods
Nerstrand Big Woods
State Park, Minnesota, USA
The phenomenon of landscape disturbance becomes clear in this series about a ravine in the northern portion of Nerstrand Big Woods State Park. I first visited the park on a mild October afternoon in 2009. A pleasant mile-and-a-half hike into the park led to a hillside luminous with yellow maple leaves carpeting the ground. I stood on a promontory across a shallow ravine at the base of the hillside and took the first of many pictures at this location.
When I returned in spring 2010, the ravine seemed a good destination. Nerstrand is known for its spring ephemerals including common Trout lilies and especially the endangered Minnesota dwarf trout lily that emerge early and die-back before the broad leaves of the maples shade the ground. By August that year, after an especially rainy summer, I found the banks of the ravine had been under-cut by run-off; trees clung to the edge of the growing chasm.
On my next visit in May of the following year, the hillside looked bare and scoured, its contours visible under the distinctive texture of the spring lilies’ foliage. The past winter’s snow melt had deepened the gully. Two trees previously at a dynamic angle in my photographic composition had uprooted and tumbled into the ravine.
Over numerous visits, I observed the ravine’s ongoing erosion and noticed several unsuccessful interventions including landscape structures and stone rip-rap to slow the steady wearing-away of the stream banks. Was the ravine in a particularly low point in the surrounding watershed? Did the lack of vegetation encourage run-off? The park is surrounded by agricultural land; where was the water coming from? What is responsible stewardship when faced with these conditions?
According to Shawn Fritcher, Department of Natural Resources (DNR) Specialist who has observed the dynamics in this specific ravine over the past sixteen years, dealing with them has been a constant source of concern, thought, discussion and effort. Many scientists have put their eyes on this ravine, looking for solutions; it poses complex problems.
The park’s landscape was historically an oak dominated forest within a fire-influenced prairie. European settlement and resulting fire suppression initiated the transition of the forest into one dominated by sugar maple, basswood, and similar shade-tolerant species (a process known as mesophication). More recently, the combination of dense canopy and invasive earth worms has decreased forest-floor vegetation diversity and density.
Past land use practices impact the area. Uphill from the ravine, a large opening remains from cutting for wood lots. This area has been replanted with forest canopy species, but the history of clear-cutting and grazing are possible factors that contributed to gully formation.
Hydrologic and geomorphic changes to the river systems surrounding the park also likely compound the erosion. The vast network of drainage systems and loss of wetlands has altered the way that water moves and is stored in the system. Bridges, culverts, and other development can cause severe head-cutting in ravines.
The DNR made several attempts to stabilize the banks that failed within two years. Extensive re-vegetation including cover crops with native plant species was also unsuccessful. It is difficult for plants to grow in such dense shade. The erosion seems impossible to stop. The only viable solution may be to lengthen out and stabilize the ravine – re-grade it to a more sustainable slope – with minimal impact to the surrounding high-quality forest. Intensive human intervention, re-shaping the land, may be the only solution to long-term stability for this native forest.
Grove with conjoined Maple and Red Oak - an example of ecological mutualism. Photographed over three hours on the afternoon of May 15, 2018 (above) amidst a carpet of blooming Trout lilies and on October 19, 2018 during a windy fall afternoon (below).