Osborne Mill Final Report

Osborne Mill County Park is located along an oxbow in the Huron River. A variety of natural communities typically found along a river floodplain are present here. Southern dry-mesic forest and a prairie remnant are located in the upland areas, while floodplain forest and southern wet meadow are found closer to the river.

Some portions of Osborne Mill (Areas C, D, E) are not connected to the main park property, where there is public access from East Delhi Road. These areas were included in the inventory; however, they are not accessible for the public, as one would have to walk through private property to reach them.

For more information about Osborne Mill or the report, please contact Catherine Marquardt, Naturalist, Washtenaw County Parks and Recreation, (734) 971-6337.

Plant Inventory

The plant inventory was conducted with regular site visits between May and October 2004. A database of plant species was created for five areas in order to reflect the diverse natural communities and the non-contiguous areas of the park. A database for specific areas will also assist in future management decisions based on the presence of plant communities and/or rare species.

Maps which show the five database areas (A-E) as well as the natural communities are available through the following links.

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The plant inventory lists and floristic quality assessments for Areas A-E are available through the following links:

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Site Summary of Natural Communities

Southern dry-mesic forest, in different stages of succession, inhabits most of the upland areas of the park (Area A). Mature oaks (Quercus spp.) and hickories (Carya spp.) are the dominant species of this forest, along with an abundance of black cherry and American elm. There is also a mature stand of Scots pine in Area A which was planted by a previous owner. The quality of the understory is mostly degraded with a dense thicket of invasive shrubs: common buckthorn (Rhamnus cathartica), honeysuckle (Lonicera spp.), and privet (Ligustrum vulgare), but the southern portion of Area A is still relatively open allowing the growth of native understory trees, such as hop-hornbeam (Ostrya virginiana) and witch-hazel (Hamamelis virginiana), and a diversity of native forbs. Shingle oak (Quercus imbricaria) and chinquapin oak (Quercus muehlenbergii), both found in this area, are quite uncommon in Washtenaw County. Twinleaf (Jeffersonia diphylla), a forb of special concern, is also found in Area A. Some plants found here appear to be rare or of high quality, but have most likely escaped from the native garden planted on the adjacent property to the south, such as blue cohosh (Caulophyllum thalictroides) and Virginia bluebells (Mertensia virginica). A species of trillium, which looks very similar to Trillium sessile, a threatened species that can be found along the Huron River, has been identified as Trillium cernuum, a non-native species most likely originating from the adjacent garden. These escapees were not included in the inventory.

Southern dry-mesic forest species are also present on the drier soils of Area E, including Shingle Oak (Quercus imbricaria) and wood-betony (Pedicularis canadensis), both rather unusual plants in Washtenaw County.

Prairie remnants are commonly found on sites along the railroad tracks due to the higher frequency of fire. Prairie species are present in Area A, just south of the tracks, most notably, a common native prairie grass, big bluestem (Andropogon gerardii) along with an abundance of goldenrods (Solidago spp.). In this remnant prairie, these two plants are dominant along with non-native species of old fields.

Floodplain forest species are found along the Huron River where rich alluvial soils have been deposited (Areas B, D, E). The floodplain forest community is relatively small due to the predominance of sandy/gravelly soil deposits from the end moraine through which the Huron River flows. In Area B, dominant trees include black willow (Salix nigra), eastern cottonwood (Populus deltoides), silver maple (Acer saccharinum), and red ash (Fraxinus americana), along with the relatively rare species in Washtenaw County, honeylocust (Gleditsia triacanthos). Red turtlehead ((Chelone obliqua), once thought to be extirpated in Michigan, is found in Area B. Lizard's tail (Saururus cernuus) is growing along the edge of the Huron River in Areas D and E, where the current is not as swift.

Southern wet meadow species are located in two areas adjacent to the river. The habitat for these species is created by groundwater seeps from the upland slopes along with depressional areas and soils which hold standing water for a significant portion of the growing season. The southern wet meadow in Area B borders the railroad tracks and extends to the south in an open area adjacent to the Huron River. Standing water is present throughout most of the growing season in a depressional area, which allows a number of species associated with an emergent marsh to thrive, such as sweet flag (Acorus calamus) and cattails (Typha spp.).

The wet meadow in Area C is situated upon a flat, river terrace with seepages draining from the hillside into the river. Standing water fluctuates throughout the season. The presence of a small population of shrubby cinquefoil (Potentilla fruticosa) and bog goldenrod (Solidago uliginosa) indicate that a fen may have existed on the western edge of this area where the upland forest meets the wet meadow. Historically, fens were more common in a transition area such as this where groundwater seeps from a hillside of sand and gravel deposited by a moraine. But changes in hydrology, a lack of fire, and/or the deposition of woody brush from adjacent property owners most likely altered the ecosystem here. Shrubs now dominate the edge of the wet meadow and will most likely advance if not set back by fire or cutting.

Dominant plants of both wet meadows include sedges (mostly Carex stricta), joe-pye weed (Eupatorium maculatum), and marsh fern (Thelypteris palustris). Other characteristic species of wet meadow present include soft-stem bulrush (Schoenoplectus tabernaemontani), marsh bellflower (Campanula aparinoides), swamp thistle (Cirsium muticum), southern blue flag (Iris virginica), great water dock (Rumex orbiculatus), and wild mint (Mentha arvensis).

Bird Inventory

The bird survey was conducted most intensively during May and June during the spring migration and nesting seasons. The Huron River is an excellent corridor for migratory birds in general, but the lack of development on the north side of the oxbow also provides an undisturbed area for nesting. The list below includes 47 species observed between April and October 2004.

Frog and Toad Inventory

The park was visited in the evening once per month during April, May, and June to listen for frog and toad calls. A total of 3 species were recorded during the inventory. Green frogs were observed and their calls recorded. This species is adapted to live along the river, and this park has a healthy population as determined by a high density of calls and by the frequency of frogs observed leaping into the water. Wood frogs were observed in the woodland areas throughout the summer, along with American toads.

A unique habitat for amphibians is created by the headrace from the former dam (Area E). Certain species would not inhabit the banks of the Huron River due to the current, but the headrace on the western side of the park provides an alternative habitat with standing water in the spring. Depending on the amount of seasonal precipitation, pools of water may remain standing into the fall. No species which prefers this type of habitat was observed during the inventory, but this site should be monitored further in the future to record changes.

Management Recommendations

Osborne Mill has some areas with high quality native plant communities that should be preserved, along with areas so dense with invasive species that restoration would be a timely and costly endeavor.

While the former farm fields of Area A are highly degraded with dense thickets of buckthorn and honeysuckle, there is a healthy portion of the southern dry-mesic forest in the southern portion of this area which has mature trees, an open understory and a nice diversity of native forbs. Threats to this habitat include the advancing presence of invasive species: garlic mustard (Alliaria officinalis), bitter cress (Cardamine impatiens), honeysuckle (Lonicera spp.), common buckthorn (Rhamnus cathartica), autumn-olive (Elaeagnus umbellatus), and common privet (Ligustrum vulgare). In addition there is a patch of the garden escapee, lily-of-the valley (Convallaria majalis) in the middle of the forest. This area should be monitored and encroaching populations of non-natives pushed back to keep this natural community in tact by weeding garlic mustard, as well as cutting and painting woody invasives with herbicide. The population of lily-of-the valley could be eliminated with an application of herbicide. A prescribed burn would also deter the young invasives and encourage the growth of native species.

Burning the prairie remnant (Area A) along the railroad tracks would encourage the growth of native grasses and forbs and discourage the growth of non-native annuals and woody plants, but this may not be as high of a priority as other areas due to the lower FQI.

Area E would benefit from a similar management plan as described for the forest in Area A, but is perhaps a higher priority due to the higher diversity of plants and because of the clearing of the understory by an unknown party. Since both the native and non-native understory trees and shrubs have been cut, the non-natives are recovering much more quickly and will outpace the growth of native plants. The native species will not be able to compete unless the young non-native woody species are burned and/or cut and painted with herbicide. In addition, garlic mustard seeds are spreading quickly from the dense population near the railroad track, and the disturbed ground along with increased light could allow this seed bank to spread rapidly. This area should be burned and invasives cut back over the next several years so that the native understory can recover. In addition to being a high priority due to the diversity, this area is also very appealing to visitors due to the view of the river, including rapids and the diversity of mature trees.

Purple loosestrife (Lythrum salicaria) and dame's rocket (Hesperis matronalis) are present throughout the floodplain forest and southern wet meadow (Areas B, C, and E). The populations are not yet dense, and could be controlled with herbicide application. Since these areas are located along the Huron River, they are a special challenge to manage as seeds are deposited annually by spring flooding. Purple loosestrife should be controlled before these populations outcompete the native species.

The southern wet meadow contains diverse plant communities, which benefit from periodic fire. Fire would burn many years of duff or dead plant material and therefore, allow for better germination of the native seed bank. A prescribed burn would also set back the encroachment of native and non-native shrubs.