View of Wicken Fen showing vegetation typical of a fen in the foreground and carr vegetation featuring trees and bushes in the background
A fen is one of the main types of wetlands, the others being grassy marshes, forested swamps, and peaty bogs. Along with bogs, fens are a kind of mire. Fens are minerotrophicpeatlands, usually fed by mineral-rich surface water or groundwater. They are characterized by their distinct water chemistry, which is pH neutral or alkaline, with relatively high dissolved mineral levels but few other plant nutrients. Continuous input of groundwater into fens maintains a stable water table throughout the course of a year. The stable water table helps maintain multiple defining characteristics of fens, namely the neutral pH, high base (Mg, Fe, Ca) saturation, and low nutrient availability. They are usually dominated by grasses and sedges, and typically have brown mosses. Fens frequently have a high diversity of other plant species including carnivorous plants such as Pinguicula. They may also occur along large lakes and rivers where seasonal changes in water level maintain wet soils with few woody plants. The distribution of individual species of fen plants is often closely connected to water regimes and nutrient concentrations.
Fens are distinguished from bogs, which are acidic, low in minerals, and usually dominated by sedges and shrubs, along with abundant mosses in the genus Sphagnum. Bogs also tend to exist on dome-shaped landmasses where they receive almost all of their usually-abundant moisture from rainfall, whereas fens appear on slopes, flats, or depressions and are fed by surface and underground water in addition to rain.
Fens have been damaged in the past by land drainage, and also by peat cutting. Some are now being carefully restored with modern management methods. The principal challenges are to restore natural water flow regimes, to maintain the quality of water, and to prevent invasion by woody plants.
Carr is the northern European equivalent of the wooded swamp of the southeastern United States, also known in the United Kingdom as wet woodland. It is a fens overgrown with generally small trees of species such as willow (Salix spp.) or alder (Alnus spp.). In general, fens may change in composition as peat accumulates. A list of species found in a fen can therefore cover a range of species from those remaining from the earlier stage in the successional development to the pioneers of the succeeding stage.
Where streams of base-rich water run through bog, these are often lined by strips of fen, separating "islands" of rain-fed bog.
Temporary flooding by beavers can have negative effects on fens.
Use of term in literature
Shakespeare used the term "fen-sucked" to describe the fog (literally: rising from marshes) in King Lear, when Lear says "Infect her beauty, You fen-sucked fogs drawn by the powerful sun, To fall and blister."
^William Shakespeare (2008). "King Lear, Act II, Scene IV, line 162". Penguin Books. Retrieved 2015. You nimble lightnings, dart your blinding flames, Into her scornful eyes! Infect her beauty, You fen-sucked fogs drawn by the powerful sun, To fall and blister.
Charlton, D. L.; S. Hilts (1989). "Quantitative evaluation of fen ecosystems on the Bruce Peninsula". In M. J. Bardecki; N. Patterson (eds.). Ontario Wetlands: Inertia or Momentum. Toronto, ON: Federation of Ontario Naturalists. pp. 339-354. Proceedings of Conference, Ryerson Polytechnical Institute, Toronto, Oct 21-22, 1988.
Godwin, Kevin S., James P. Shallenberger, Donald J. Leopold & Barbara L. Bedford (2002). "Linking landscape properties to local hydrogeologic gradients and plant species occurrence in New York fens: a hydrogeologic setting (HGS) framework". Wetlands. 22 (4): 722-737. doi:10.1672/0277-5212(2002)022[0722:LLPTLH]2.0.CO;2.CS1 maint: uses authors parameter (link)
Sheail, J.; T. C. E. Wells (1983). "The Fenlands of Huntingdonshire, England: a case study in catastrophic change". In A. J. P. Gore (ed.). Mires: Swamp, Bog, Fen and Moor - Regional Studies. Ecosystems of the World. 4B. Amsterdam, the Netherlands: Elsevier. pp. 375-393. ISBN9780444420046.
Wheeler, B. D.; K. E. Giller (1982). "Species richness of herbaceous fen vegetation in Broadland, Norfolk in relation to the quantity of above-ground plant material". Journal of Ecology. 70 (i): 179-200. JSTOR2259872.