Fen

Wicken fen.

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A fen is a type of wetland fed by surface and/or groundwater. Fens are characterised by their water chemistry, which is neutral or alkaline. Fens are different from bogs, which are acidic, fed primarily by rainwater (ombrotrophic) and often dominated by Sphagnum mosses.

Derivation

The word "fen" is derived from Old English fenn and is considered to have proto-Germanic origins, since it has cognates in Gothic (fani), Old Frisian (fenne), Dutch (veen) and German (Fenn(e), Venn, Vehn, Feen, Fehn).

Fen vegetation

Fen was once thought to be a phase in the natural succession from open lake, through reedbed, fen and carr, to woodland, or as the peat develops and its surface rises, to bog. Now, it is more generally recognised that fens are persistent habitats whose existence is dependent on the availability of water.^{[citation needed]}

Carr is the northern European equivalent of the wooded swamp of the south-eastern United States. It is a fen overgrown with generally small trees of species such as willow (Salix spp.) or alder (Alnus spp.). A list of species found in a fen therefore covers a range from those remaining from the earlier stage in the successional development to the pioneers of the succeeding stage.

Fen also merges into freshwater marsh, when it develops more in the direction of grassland. This is most likely to occur where the tree species of carr are systematically removed by man for the development of pasture (often together with drainage), or by browsing wild animals, including beavers.

The water in fens is usually from groundwater or flowing sources (minerotrophic) with a fairly high pH (base-rich, neutral to alkaline). Where the water is from rainwater or other sources with a lower pH (more acidic), fen is replaced by vegetation dominated by Sphagnum mosses, known as bog.

Where streams of base-rich water run through bog, these are often lined by strips of fen, separating "islands" of rain-fed bog.

List of fen flora species

The following is a list of plant species to be found in a north European fen with some attempt to distinguish between reed bed relicts and the carr pioneers. However, nature does not come in neat compartments so that for example, the odd stalk of common reed will be found in carr.

In pools

• Beaked sedge; Carex rostrata
• Whorl grass; Catabrosa aquatica
• Needle spike-rush; Eleocharis acicularis
• Northern spike-rush; Eleocharis austriaca
• Sweet grasses; Glyceria species.
• Common reed; Phragmites australis
• Swamp meadow grass; Poa palustris

In typical fen

• Flat sedge; Blysmus compressus
• Great fen sedge; Cladium mariscus
• Lesser tufted sedge; Carex acuta
• Lesser pond sedge; Carex acutiformis
• Davall's sedge; Carex davalliana
• Dioecious sedge; Carex dioica
• Brown sedge; Carex disticha
• Tufted sedge; Carex elata
• Slender sedge; Carex lasiocarpa
• Flea sedge; Carex pulicaris
• Greater pond sedge; Carex riparia
• Common spike-rush; Eleocharis palustris
• Few-flowered spike-rush; Eleocharis quinqueflora
• Slender spike-rush; Eleocharis uniglumis
• Broad-leaved cotton sedge; Eriophorum latifolium
• Reed sweet-grass; Glyceria maxima
• Yellow flag iris; Iris pseudacorus
• Brown bog [sic] rush; Schoenus ferrugineus

In fen carr

• Narrow small-reed; Calamagrostis stricta
• Purple small-reed; Calamagrostis canescens
• Tussock sedge; Carex paniculata
• Cyperus sedge; Carex pseudocyperus
• Wood club rush; Scirpus sylvaticus

See also

References

Rose, F. Grasses, Sedges, Rushes and Ferns of the British Isles and north-western Europe (1989) ISBN 0-670-80688-9