Distribution: Throughout the world wherever there is sufficient rainfall to support wetland communities. Scattered throughout Britain and Ireland, famously in East Anglia, but often in fragmentary state.
An immediate problem in providing any brief summary of fens is that ecologists themselves differ in how they use the term. Firstly it is necessary to distinguish "fen" as an ecological term from "Fenland" as a geographical entity.
Fenland, a geographical area of East Anglia
Little of this ancient area now exists. Most is now arable farmland, a flat landscape of huge fields and drainage dykes, with few natural hedges and with scattered, surprisingly isolated human communities. A few fragments remain to provide most of our finest British examples of "fen" as an ecological concept. These include Chippenham Fen and Wicken Fen in Cambridgeshire and Holme and Woodwalton Fens in Huntingdonshire, all of which are National Nature Reserves, though Wicken Fen, the most accessible of these, is owned and managed by the National Trust.
Although this present account is wider in concept, examples and photographs here are currently all from the geographical Fenland. An excellent account of the history, ecology and present/past management of fens by Godwin (1978) refers entirely to Fenland.
Though outside Fenland as defined here, there are a number of named fens stretching further east in Norfolk across to the Norfolk Broads, generally as parts of valley mire complexes. Many of the fenland rarities are or were to be found in both areas.
Fens as an ecological concept
The East Anglian fens are characterised by alkaline conditions resulting from water draining from chalk and other calcareous rock formations. They (and similar examples elsewhere) may be distinguished as "rich fen", though there is often a general understanding that a "fen" will be relatively eutrophic (nutrient rich). A classic plant of rich fen is Saw-sedge (or simply "Sedge") (Cladium mariscus), which may be a key species in past or present management (see below). The definition of "fen" however often also covers sites with much lower mineral input and corresponding higher acidity; such areas may be described as "poor fen" and are commonly characterised by extensive development of Sphagnum moss carpets.
(It may be noted in passing that "poor fen" should not be confused with a "Poor's Fen", which at Wicken and elsewhere was a name given to a specific area of a fen where the village poor, lacking other rights, were still able to cut peat or harvest sedge.)
They consider fen to be defined by a pH generally above 6.0 and with relatively high levels of calcium and bicarbonate ions. The vegetation of such mires tends to be rich in herbs and 'brown mosses' (they cite Drepanoclados, Campylium and Scorpidium, and Cratoneuron also should be included).
By contrast, bog is defined by a pH generally below 5.0, with low levels of calcium ions, and with chloride and sulphate ions as the main anions. Vegetation includes members of the heather family (Calluna, Erica etc.), cotton-grasses (Eriophorum) and other calcifuge ("calcium-avoiding") members of the sedge family, and often an abundance of Sphagnum mosses.
THE PLANT COMMUNITIES
A detailed account of British fen types and their plant communities is provided by Wheeler (1984). Only outline summaries are given here.
Herbaceous fen (including open reed and sedge fens)
Traditional management of many of the East Anglian fens has been the cutting of 'Sedge' (Cladium), primarily for thatch. Sedge is more flexible and durable than reed and was used for ridging reed-thatched roofs or, less often, for thatching entire roofs. It also had uses as kindling material and as a tough litter for floor coverings. Large areas were maintained for this purpose and the survival of undrained fen fragments no doubt has been due to the value of sedge as a crop. However, there is now much less demand and former sedge fens have in many cases been colonised by bushes to form carr. This has meant the local disappearance of many plants requiring these open conditions and is implicated in the extensive loss of invertebrates such as the Swallowtail Butterfly (see below). While there is still a small commercial demand, cutting sedge is now largely a conservation exercise. Management aims at, e.g. Wicken Fen, are not only to maintain sedge fens but also to reclaim areas from carr and extend the open communities.
The areas dominated by grasses such as Calamagrostis species or Molinia were traditionally cut as "litter" for animal bedding. Like the sedge fens they tend to be herb-rich and continued cutting on an annual or biennial cycle is required to maintain floristic diversity.
As noted above, poor fens (if considered "fens" at all) are likely to have a moss layer including or dominated by Sphagnum species. Purple Moor-grass (Molinia caerulea), Bottle Sedge (Carex rostrata) and the smaller sedges, such as C. echinata and C. nigra may predominate amongst the vascular plants.
Carr (wooded fen)
Bushes and trees forming carr must be capable of growing in waterlogged conditions with little oxygen supply to the roots. Willows and sallows (Salix species) may predominate, including Grey Sallow (S. cinerea subsp. cinerea), which often dominates East Anglian fens and occurs more locally north to central Scotland.
Two buckthorn species, the Common or Purging Buckthorn (Rhamnus cathartica) and the Alder Buckthorn (Frangula alnus) are also locally abundant carr species in some of the East Anglian Fens. Common Buckthorn is not confined to fen carrs; it can be a component of scrub on dry chalk slopes. Alder Buckthorn is more restricted to damper habitats.
At Wicken, both were abundant in the first part of the 20th century and Rhamnus still is, locally being the dominant carr species. Frangula, however, suffered a decline through apparent fungal attack in the 1930s and has suffered other misfortunes since (Friday, 1997). It is currently a typical pioneer species in young carr and appears to be recovering its former status.
Alder (Alnus glutinosa) is a larger tree that may come to dominate richer fens in some localities. It is notable as possessing rood nodules containing a symbiotic nitrogen-fixing actinomycete. Nitrogen may otherwise be deficient during development of fen woodland.
On poorer sites, Downy Birch (Betula pubescens) is likely to become the dominant tree species, sometimes forming more or less pure woodland on the thicker peats.