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Illustrated diagram of a fen.jpg
Avaste Fen, Estonia
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 minerotrophic peatlands,[1] usually fed by mineral-rich surface water or groundwater.[2] 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.[3] 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.[4] Fens frequently have a high diversity of other plant species including carnivorous plants such as Pinguicula.[5][6] They may also occur along large lakes and rivers where seasonal changes in water level maintain wet soils with few woody plants.[7] The distribution of individual species of fen plants is often closely connected to water regimes and nutrient concentrations.[8][9]

Fens have a characteristic set of plant species, which sometimes provide the best indicators of environmental conditions. For example, fen indicator species in the State of New York include the flora Carex flava, Cladium mariscoides, Potentilla fruticosa, Pogonia ophioglossoides and Parnassia glauca.[10]

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.[4] 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.[11] Some are now being carefully restored with modern management methods.[12] The principal challenges are to restore natural water flow regimes, to maintain the quality of water, and to prevent invasion by woody plants.

Distribution and extent

Fens are distributed around the world, but are most frequently found at the mid-high latitudes of the Northern Hemisphere.[13] They are found throughout the temperate zone and boreal regions, but are also present in tundra and in specific environmental conditions in other regions around the world.[14][15] In the United States, fens are most common in the Midwest and Northeast, but can be found across the country.[16] In Canada, fens are most frequent in the lowlands near the Hudson Bay and James Bay, but can also be found across the country.[15] Fens are also spread across the northern latitudes of Eurasia, including the British Isles and Japan, but east-central Europe is especially rich in fens.[15][16] Further south, fens are much rarer, but do exist under specific conditions. In Africa, fens have been found in the Okavango Delta in Botswana and the highland slopes in Lesotho.[15] Fens can also be found at the colder latitudes of the Southern Hemisphere. They are found in New Zealand and southern Chile and Argentina, but the extent is much less than that of the northern latitudes.[15][13] Locally, fens are most often found at the intersection of terrestrial and aquatic ecosystems, such as the headwaters of streams and rivers.[15][17]

It is estimated that there are approximately 1.1 million square kilometers of fens worldwide, but quantifying the extent of fens is difficult.[13] Because wetland definitions vary regionally, not all countries define fens the same way.[15] In addition, wetland data isn't always available or of high quality.[15] Fens are also difficult to rigidly delineate and measure, as they are located between terrestrial and aquatic ecosystems.[15]


Rigidly defining types of wetlands, including fens, is difficult for a number of reasons. First, wetlands are diverse and varied ecosystems that are not easily defined by inflexible definitions. They are often described as a transition between terrestrial and aquatic ecosystems with characteristics of both.[17] This makes it difficult to delineate the exact extent of a wetland. Second, terms used to describe wetland types vary greatly by region.[14] The term bayou, for example, describes a type of wetland, but its use is generally limited to the southern United States.[18] Third, different languages use different terms to describe types of wetlands. For instance, in Russian, there is no equivalent word for the term swamp as it is typically used in North America.[17] The result is a large number of wetland classification systems that each define wetlands and wetland types in their own way.[14] However, many classification systems include four broad categories that most wetlands fall into: marsh, swamp, bog, and fen.[14] While classification systems differ on the exact criteria that define a fen, there are common characteristics that describe fens generally and imprecisely. A general definition provided by the textbook Wetlands describes a fen as "a peat-accumulating wetland that receives some drainage from surrounding mineral soil and usually supports marsh like vegetation."[17]

Three examples are presented below to illustrate more specific definitions for the term fen.

Canadian Wetland Classification System definition

In the Canadian Wetland Classification System, fens are defined by six characteristics:[19]

  1. Peat is present.
  2. The surface of the wetland is level with the water table. Water flows on the surface and through the subsurface of the wetland.
  3. The water table fluctuates. It may be at the surface of the wetland or a few centimeters above or below it.
  4. The wetland receives a significant amount of its water from mineral-rich groundwater or surface water.
  5. Decomposed sedges or brown moss peat are present.
  6. The vegetation is predominantly graminoids and shrubs.

Wetland Ecology: Principles and Conservation (Keddy) definition

In the textbook Wetland Ecology: Principles and Conservation, Paul A. Keddy offers a somewhat simpler definition of fens as "A wetland that is usually dominated by sedges and grasses rooted in shallow peat, often with considerable groundwater movement, and with pH greater than 6."[14] This definition differentiates fens from swamps and marshes by the presence of peat.

The Biology of Peatlands (Rydin) definition

Fens are defined by the following criteria:[15]

  1. The wetland is not flooded by lake or stream water.
  2. Woody vegetation 2 meters or taller is absent or canopy cover is less than 25%.
  3. The wetland is minerotrophic (it receives its nutrients from mineral-rich groundwater).

A further distinction is made between open and wooded fens, where open fens have canopy cover less than 10% and wooded fens have 10-25% canopy cover. If tall shrubs or trees dominate, the wetland is instead classified as a wooded bog or swamp forest depending on other criteria.


Carr is the northern European equivalent of the wooded swamp of the southeastern United States,[20] 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.[citation needed]

Temporary flooding by beavers can have negative effects on fens.[21]

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."[22]


See also

Specific fens


  1. ^ Rydin, Hakan and John K. Jeglum. The Biology of Peatlands, 2nd edn. Oxford: OUP, 2013. p. 11. ISBN 978-0-19-960299-5.
  2. ^ Godwin et al. (2002).
  3. ^ Anderson, Dagmar (January 7, 2013). "Cost-effective assessment of conservation of fens". Applied Vegetation Science. 16 – via Wiley Online Library.
  4. ^ a b Keddy (2010), p. 8.
  5. ^ Wheeler & Giller (1982)
  6. ^ Keddy (2010), Chapter 9.
  7. ^ Charlton & Hilts (1989)
  8. ^ Slack et al. (1980)
  9. ^ Schröder et al. (2005)
  10. ^ Godwin et al. (2002), Table 3.
  11. ^ Sheail & Wells (1983)
  12. ^ Keddy (2010), Chapter 13.
  13. ^ a b c Loisel, Julie; Bunsen, Michael (2020). "Abrupt Fen-Bog Transition Across Southern Patagonia: Timing, Causes, and Impacts on Carbon Sequestration". Frontiers in Ecology and Evolution. 8. doi:10.3389/fevo.2020.00273. ISSN 2296-701X.
  14. ^ a b c d e Keddy, Paul A. (2010). Wetland ecology : principles and conservation (2nd ed.). Cambridge: Cambridge University Press. ISBN 978-1-139-22365-2. OCLC 801405617.
  15. ^ a b c d e f g h i j Rydin, Håkan (2013). The biology of peatlands. J. K. Jeglum (Second ed.). Oxford, UK. ISBN 978-0-19-150828-8. OCLC 861559248.
  16. ^ a b van Diggelen, Rudy; Middleton, Beth; Bakker, Jan; Grootjans, Ab; Wassen, Martin (November 2006). "Fens and floodplains of the temperate zone: Present status, threats, conservation and restoration". Applied Vegetation Science. 9 (2): 157–162. doi:10.1111/j.1654-109x.2006.tb00664.x. ISSN 1402-2001.
  17. ^ a b c d Mitsch, William J. (2007). Wetlands. James G. Gosselink (4th ed.). Hoboken, N.J.: Wiley. ISBN 978-0-471-69967-5. OCLC 78893363.
  18. ^ "bayou". dictionary.cambridge.org. Retrieved 2021-02-23.
  19. ^ Canada Committee on Ecological Land Classification. National Wetlands Working Group (1997). The Canadian wetland classification system. Barry G. Warner, C. D. A. Rubec (2nd ed.). Waterloo, Ont.: Wetlands Research Branch, University of Waterloo. ISBN 0-662-25857-6. OCLC 43464321.
  20. ^ Bug Life Archived 2010-03-04 at the Wayback Machine
  21. ^ Reddoch & Reddoch (2005)
  22. ^ William Shakespeare (2008). "King Lear, Act II, Scene IV, line 162". Penguin Books. Retrieved 5 September 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)
  • Keddy, P. A. (2010). Wetland Ecology: Principles and Conservation (2nd ed.). Cambridge, UK: Cambridge University Press.
  • Reddoch, Joyce M.; Allan H. Reddoch (2005). "Consequences of Beaver, Castor canadensis, flooding on a small shore fen in southwestern Quebec". Canadian Field-Naturalist. 119 (3): 385–394.
  • Schröder, Henning K., Hans Estrup Andersen & Kathrin Kiehl (2005). "Rejecting the mean: estimating the response of fen plant species to environmental factors by non-linear quantile regression". Journal of Vegetation Science. 16 (4): 373–382. doi:10.1111/j.1654-1103.2005.tb02376.x. JSTOR 4096617.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. ISBN 9780444420046.
  • Slack, Nancy G., Dale H. Vitt & Diana G. Horton (1980). "Vegetation gradients of minerotrophically rich fens in western Alberta". Canadian Journal of Botany. 58 (3): 330–350. doi:10.1139/b80-034.CS1 maint: uses authors parameter (link)
  • 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. JSTOR 2259872.

External links

  • Media related to Fens at Wikimedia Commons

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