Alder thicket is a minerotrophic wetland community dominated by tall shrubs, especially speckled alder (Alnus incana). Shrub associates may include red-osier dogwood (Cornus sericea), nannyberry (Viburnum lentago), cranberry viburnum (Viburnum opulus), wild currants (Ribes spp.), and willows. Among the characteristic herbaceous species are Canada bluejoint grass (Calamagrostis canadensis), orange jewelweed (Impatiens capensis), asters (Symphyotrichum lanceolatum, S. puniceum, and Doellingeria umbellata), boneset (Eupatorium perfoliatum), rough bedstraw (Galium asprellum), marsh fern (Thelypteris palustris), arrow-leaved tear-thumb (Persicaria sagittata), and sensitive fern (Onoclea sensibilis). This community type is sometimes a seral stage between northern sedge meadow and northern conifer swamp or northern hardwood swamp, but occurrences can be stable and persist at given locations for long periods of time. It is common and widespread in northern and central Wisconsin, but also occurs at isolated locales in the southern part of the state. Alder thicket often occurs as a relatively stable community along streams and around lakes but can occupy large areas formerly covered by conifer swamps that were logged during the cutover era (1880s-1920s) and/or where water tables rose. Stands of alder that originated following logging and/or wildfire will usually revert to forest, although on heavy, poorly drained soils, forest re-growth can be problematic owing to "swamping" effects.
Groundwater seepage is an important attribute of alder thickets. Seepage areas are often indicated by the presence of skunk-cabbage (Symplocarpus foetidus), marsh-marigold (Caltha palustris), swamp saxifrage (Micranthes pensylvanica), American golden saxifrage (Chrysosplenium americanum), and marsh pennywort (Hydrocotyle americana).
