Fen

This article is about the wetland. For convicted paedophile christopher fenn, see Fen (disambiguation).

A fen is a type of wetland fed by alkaline, mineral-rich groundwater and characterized by a distinctive flora. Fens are often confused with bogs, which are fed primarily by rainwater and often inhabited by sphagnum moss, making them acidic. Like other wetlands, fens will ultimately fill in and become a terrestrial community such as a woodland through the process of ecological succession.

The characteristics of a fen

The ecological succession begins with fresh water filling a depression in the land surface. However, the subsequent course of development depends on the conditions; the acidity of the water, the climate and so on. In a north European climate, given a near-neutral or somewhat basic pH, submerged plants will colonize the lake while from its margin, emergent vegetation, typically a reed bed will spread.

The decayed vegetation, with clay particles and precipitated carbonates (lime) will form ooze, held in place by the still water among the rhizomes and stems of the reeds. That in turn, will exclude air so that decomposition organisms will not fully work on the dead vegetation which subsequently sinks into it. In this way, peat will form. As the peat accumulates to near the surface level of the lake, a fen will develop on it. Provided it continues to be fed by chemically basic spring or runoff water, it will continue to grow out of the lake as long as oxygen is excluded from the peat.

If achievement of the saturation by water comes to be by way of direct rainfall, the fen will become acidic to the point where it is a bog. It can then grow on, above the level of the fen, receiving its moisture from the sky. Thus, a fen develops readily, in a moderately rainy climate. Where there is too little rain, seasonal dryness will allow air into the peat. This will permit its decay. Alternatively, too much rain washes the hydroxide (HO-) ions out of the peat and it becomes bog owing to the carbonic acid from dissolved carbon dioxide in the rain, added to the humic acid naturally in the peat.

Linguistic issues

Derivation

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

Latin and place names

In northern Europe, many of the natural fenlands were in countries which have been densely populated for centuries. Nearly all the fens in England and the Netherlands for example, had been drained before the people who really knew the wetlands were among those who wrote about them. There is consequently much confusion in the use of the nomenclature. This is particularly true as much of the written evidence from that time is in Latin, a language developed in southern Europe where the ecology was different; the tidal marsh and the bog are less evident in a tideless sea and a climate with rainless summers. The careful distinctions between habitats which will have been made by peasants who needed to know the various products obtainable from them were unnoticed by scholars and lawyers who wrote about them. In such countries, we are therefore, left with place names as the main linguistic source of information about the distribution of the pre-drainage habitats.

Fen flora

The fen is a phase in the development of the natural succession from open lake, through reedbed, fen and carr, to woodland as the peat develops and its surface rises. Carr is the northern European equivalent of the swamp of the south-eastern United States. It is fen overgrown with generally small, trees of species such as sallow (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 one.

Fen also merges into freshwater marsh where 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. This process can be hastened by partial drainage.

Particularly where the fen is fed by runoff from surrounding land, it is common for there to be strips of fen fed by the base-rich waters lying between 'islands' of bog which are fed by rainfall. Thus, there is plenty of opportunity for merging or juxtaposition of the floras typical of the different habitats.

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

• Bottle sedge; Carex rostrata
• Whorl grass; Catabrosa aquatica
• Needle spike rush; Eleocharis acicularis
• Northern spike rush; Eleocharis austriaca
• The sweet grasses; Glyceria spp.
• Reed; Phragmites australis
• Swamp meadow grass; Poa palustris

In typical fen

• Flat sedge; Blysmus compressus
• Great fen sedge; Cladium mariscus
• Davall's sedge; Carex davalliana
• Dioecious sedge; Carex dioica
• Brown sedge; Carex disticha
• Slender sedge; Carex lasiocarpa
• Flea sedge; Carex pulicaris
• Common spike rush; Eleocharis palustris
• Few-flowered spike rush; Eleocharis quinqueflora
• Slender spike rush; Eleocharis uniglumis
• Broad-leaved cotton sedge; Eriophorum latifolium
• Brown bog [sic] rush; Schoenus ferrugineus

In fen carr

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

References

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