Calcareous fens occur mostly in southern Wisconsin on sites that are fed by carbonate-enriched groundwater. Fens occur in several landscape settings, including the bases of morainal slopes, on sloping deposits of glacial outwash, in the headwaters areas of spring runs and small streams, and on the shores of alkaline drainage lakes. Most fens are small, covering no more than a few acres, and are often associated and can intergrade with more abundant and widespread wetland communities such as southern sedge meadow, wet-mesic prairie, shrub-carr, emergent marsh, and southern tamarack swamp. An accumulation of peat due to groundwater upwelling can raise the fen surface to a height of several meters above the adjoining lands.The diverse fen flora is distinctive, containing many calciphiles of restricted distribution. Common or representative plants include sedges (Carex sterilis and C. lanuginosa), marsh fern (Thelypteris palustris), shrubby cinquefoil (Dasiphora fruticosa), shrubby St. John's-wort (Hypericum kalmianum), Ohio goldenrod (Solidago ohioensis), grass-of-Parnassus (Parnassia glauca), twig-rush (Cladium mariscoides), brook lobelia (Lobelia kalmia), boneset (Eupatorium perfoliatum), swamp thistle (Cirsium muticum), and asters (Symphyotrichum spp.). Many fens have a significant number of prairie or sedge meadow components, and some contain plants often associated with bogs, such as tamarack (Larix laricina), bog birch (Betula pumila) and pitcher-plant (Sarracenia purpurea).
