A fen is a type of wetland fed by mineral-rich surface water or groundwater. Fens are characterised by their water chemistry, which is neutral or alkaline, with relatively high dissolved mineral levels but few other plant nutrients. They support a wide range of animals and plants, many of which are tall marsh plants growing closely together.
Fens are distinguished from bogs, which are acidic, low in minerals, and usually dominated by low-growing plants including Sphagnum and other mosses.
The word "fen" is derived from Old English fenn. from Proto-Germanic *fanja. Cognates include Gothic (fani), Old Frisian (fenne), Dutch (veen) and German (Fenn(e), Venn, Vehn, Feen, Fehn).
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.
Carr is the northern European equivalent of the wooded swamp of the south-eastern United States, also known in the United Kingdom as Wet woodland. 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.
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