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Wetland Plants and Plant Communities of Minnesota and Wisconsin


Alder thickets are a tall, deciduous shrub community similar to shrub-carrs except that speckled alder is dominant. Speckled alder can pioneer exposed peat or alluvial soils because of its tiny seeds and ability to fix nitrogen. Alder thickets are generally found in and north of the vegetation tension zone.

Speckled alder may occur as a monotype, but the alder thicket community can have a diversity of shrubs including high-bush cranberry (Viburnum trilobum), sweet gale (Myrica gale) and common winterberry holly (Ilex verticillata)

The groundlayer may include some of the ferns, sedges, grasses and forbs of sedge meadows and fresh (wet) meadows. The diversity of species in the groundlayer is often dependent on degree of shrub canopy cover, degree of disturbance, and water source (e.g., groundwater versus surface runoff from urban or agricultural lands). Stands with 100 percent shrub canopy cover may have a depauperate groundlayer. The example of an alder thicket shown by the photograph is groundwater fed and minimally disturbed resulting in a rich diversity of species in the groundlayer.

Alder thickets provide high quality habitat for ruffed grouse and American woodcock, as well as white-tailed deer. Rare, threatened and endangered species can be supported by alder thickets. For example, alder thicket communities on the Lake Superior red clay plain of northwestern Wisconsin include state-listed threatened, endangered or special concern species such as sweet coltsfoot (Petasites sagittatus), small yellow water crowfoot (Ranunculus gmelinii var. hookeri) and New England violet (Viola novaeangliae).

An alder thicket

VEGETATION: As shown by the photograph, "thicket" is an accurate description of this community dominated by speckled alder (Alnus incana ssp. rugosa). Non- dominant shrubs include common winterberry holly (Ilex verticillata), common elderberry (Sambucus canadensis), meadowsweet (Spiraea alba) and steeplebush (Spiraea tomentosa). Skunk cabbage (Symplocarpus foetidus), jewelweed (Impatiens capensis) and cinnamon fern (Osmunda cinnamomea) dominate the groundlayer, which also includes Canada bluejoint grass (Calamagrostis canadensis), rattlesnake manna grass (Glyceria canadensis), a sedge (Carex canescens), hummock sedge (Carex stricta), stalk-grain sedge (Carex stipata), sensitive fern (Onoclea sensibilis), crested shield fern (Dryopteris cristata), marsh marigold (Caltha palustris), arrow-leaved tearthumb (Polygonum sagittatum), dewberry (Rubus hispidus), marsh blue violet (Viola cucullata), northern violet (Viola macloskeyi) and sphagnum moss (Sphagnum sp.). Bog bluegrass (Poa paludigena), a species listed as threatened by the State of Wisconsin, occurs this habitat.

SOILS: Dawson peat (Terric Borosaprists), a very poorly-drained soil on floodplains or lake basins with an organic layer between 16 and 51 inches in depth underlain by acidic, sandy material. Dawson soils are typically saturated to the surface and may have as much as 6 inches of standing water after spring snowmelt and heavy rainfall events.

HYDROLOGY: Groundwater discharge (seepages), This alder thicket is immediately adjacent to a small, groundwater fed, perennial stream.

LOCATION: Fort McCoy Military Reservation, Monroe County, Wisconsin.


Speckeled Alder (Alnus incana ssp. rugosa (DuRoi) Clausen)
American Red Raspberry (Rubus strigosus Michx.)
Sensitive Fern (Onoclea sensibilis L.)

Previous Section -- Stalk-grain Sedge (Carex stipata Muhl.)
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