The term "fen" is used to describe a class of open, wetland plant communities dominated by sedges, which develop where the ground surface is continuously wet and a layer of dead plant material accumulates to form peat.
Fens are similar to bogs, but the peat layer in fens is not as well developed and fen plants are able to obtain some minerals from groundwater. Fens are also similar to wet meadows, but wet meadows develop where the ground surface regularly dries out over the course of the summer, causing plant material to decay rapidly enough to prevent the formation of the peat soils necessary for a fen or a bog. The differences in the environmental conditions of bogs, fens, and wet meadows are reflected in the plants that grow in them.
Bogs are inhabited by a restricted set of plants that can survive in very acidic and mineral-poor environments, such as sphagnum mosses or Labrador tea. Fens may harbor many of the typical bog plants, but because fens are richer in minerals they provide habitat for other less-restricted wetland plants as well. It is the presence of these mineral-loving plants that distinguishes a fen from a true bog. Wet meadows are rich in nutrients compared to bogs and fens, tend to be richer in grasses and wildflowers, and have fewer mosses than bogs and fens.
Three general types of fens occur in Minnesota: poor fens, rich fens, and calcareous fens. Poor fens develop on acidic peat and are dominated by narrow-leaved sedges such as fen wiregrass sedge (Carex lasiocarpa) or by low-growing, evergreen broad-leaved shrubs such as leatherleaf (Chamaedaphne calyculata) or Labrador tea (Ledum groenlandicum). Poor fens are also characterized by cushiony mats of sphagnum mosses and occasional clumps of stunted tamaracks (Larix laricina) or bog birch (Betula pumila).
Rich fens develop on shallower and less acidic peat than poor fens. Like poor fens, they are dominated by narrow-leaved sedges, but typically support many more broad-leaved wetland plants. If mosses are abundant in rich fens, they are usually not sphagnum mosses.
Calcareous fens develop under very restricted conditions, where cool, mineral-rich groundwater keeps the soil surface continuously moist and allows peat to form, a combination that provides habitat for species typical of rich fens and wet meadows, and also for a distinctive group of rare wetland plants, including sterile sedge (Carex sterilis), small white lady's-slipper (Cypripedium candidum), and marsh arrow-grass (Triglochin palustris). Because they are so uncommon and provide habitat for numerous rare plant species, calcareous fens are given special protection in Minnesota.