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BIODIVERSITY REFERENCE
 
British habitats
 
   Fens   
 
(page 1)


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.


 
View across litter fields, Wicken Fen, Cambridgeshire, July 1997. The litter fields here are cut annually or biennially and are dominated by grasses such as Purple Small-reed (Calamagrostis canescens) and Purple Moor-grass (Molinia caerulea, especially subsp. arundinacea). They contain a rich diversity of tall herbs. Creamy-white patches of Meadowsweet (Filipendula ulmaria) are visible in the distance and beyond is carr with Buckthorn (Rhamnus cathartica) and Grey Sallow (Salix cinerea subsp. cinerea) along with Birch (Betula pubescens).
 


 

DEFINITIONS

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
Geographically, Fenland is the ancient area of marshes on the vast floodplain that surrounded the rivers Witham, Welland, Nene and Ouse (and their subsidiaries) that drain much of central and eastern England and flow into the Wash between Norfolk and Lincolnshire. It is or was an area of several hundred square miles (3800 km2 according to Good & Ratcliffe, in Ratcliffe, 1977), and in postglacial times evidently continued much further still into what is now the North Sea. It extended from Lincolnshire in the north to Norfolk in the southeast, encompassing much of the geographical (no longer political) county of Huntingdonshire and part of Cambridgeshire.
Perhaps one indication of the immensity of the area is the myth and legend surrounding Hereward the Wake, an ancient "Robin Hood-like" folk hero or outlaw who operated out of the Fens and who fortunately has not yet been subjected to the same recent historical and cultural media travesties as Robin Hood.

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
Ecologists regularly use the word "fen" but establishing a precise definition is more difficult, reflecting the diversity and continuity of habitat and vegetation types. Generally, however, use of the term equates to the majority of topogenous mires, where local relief results in permanently high water tables, as on flood plains and in shallow depressions, or in transitional zones of vegetation bordering open waters. Smaller areas of fen may also occur within soligenous mires associated with routes of moving drainage water. It is understood here that any type of mire is generally characterised by peat formation, i.e. the build up of incompletely decomposed organic matter under conditions of waterlogging and negligible diffusion of oxygen, though in fact, as Wheeler & Proctor (2000) emphasise, some soligenous fens accumulate little or no peat, though are otherwise similar in hydrochemistry and vegetation.

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.)

 
This variation and often imprecision of terminology has been examined by Wheeler & Proctor (2000), who make a number of recommendations. In examining a wide range of mire types they find a bimodal distribution of pH that backs up the rather vague existing concepts of "fen" and "bog".

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.
On this definition, many examples of so-called "poor fen" or "acid fen" are better considered as "bog" and this gives a more satisfactory treatment of phases and microtopography in complex acid mire systems.


 

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.
The communities recognised in the National Vegetation Classification are given by Rodwell et al. (1995).

Herbaceous fen (including open reed and sedge fens)
The open fens are usually dominated by tall reedlike grasses, such as Common Reed (Phragmites australis), Small-reeds (Calamagrostis species) or Purple Moor Grass (Molinia caerulea), or by tall sedges such as Carex acutiformis or C. disticha, or by Saw-sedge (Cladium mariscus), or locally by the Blunt-flowered Rush (Juncus subnodulosus). A variety of other tall herbs such as Meadowsweet (Filipendula ulmaria) may be present, perhaps along with declining rarities such as the Marsh Pea (Lathyrus palustris). Below the field layer is likely to be a layer of mosses, including Campylium and Cratoneuron species. This may be a transitional phase or may be maintained by appropriate management.


Marsh Pea (Lathyrus palustris)
A nationally scarce herb of rich fens, largely confined to eastern England and East Anglia and scattered in a very few coastal fens elsewhere. Lost through drainage (and perhaps nitrogen enrichment?) from many of its former sites (see Mountford in Stewart et al., 1994). Its narrow leaves are, however, very difficult to spot amongst tall fen vegetation and even when in flower it is often not a conspicuous plant. It may await discovery at other sites.
Photograph: Wicken Fen, July 1997.


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.

Sedge (Cladium) fen, Chippenham Fen, Cambridgeshire, 1980
Saw-sedge (or "Sedge") (Cladium mariscus)
On the edge of a drainage dyke.
Wicken Fen, July 1998.
Conservation volunteers cutting a sedge field at Wicken Fen, July 1997

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)
Succession to carr (wet scrub and swampy woodland) is usually to be expected except where active management maintains the open areas. Lowered water tables caused by drainage within or adjacent to the fen complex may accentuate this and since most surviving fens are surrounded by farmland, this may be a substantial management problem.

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.

 
Grey Sallow (Salix cinerea subsp. cinerea) carr, Wicken Fen, July 1997. The butterfly is a Comma (Polygonia c-album).
 
Comma (Polygonia c-album)
The same individual as shown in the photograph above. The Comma is by no means exclusively a fen butterfly, indeed it is widespread and locally common across much of southern England and Wales, but carr woodlands seem to suit it. No doubt this is partly because its main foodplant, the Stinging Nettle (Urtica dioica) is usually present in some abundance on the carr margins and along the droves (cut pathways).
There is some evidence it is currently increasing its range northwards, with recent scattered records in southern Scotland (including one individual in riverside scrub here in Paisley during the 1990s).

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.

Alder Buckthorn (Frangula alnus) in mixed carr, Wicken Fen, July 1998.

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.
In poor fen, the shrub Bog Myrtle or Sweet Gale (Myrica gale) is no doubt even more important as another species that effects nitrogen enrichment via symbiosis.

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.

 
Birch carr with Downy Birch (Betula pubescens), here with Saw-sedge (Cladium mariscus) dominating the ground vegetation but no doubt suffering from the drying and more shaded conditions. Photograph: Holme Fen, Huntingdonshire, 1967.
 


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© A.J. Silverside
e-mail: alan.silverside@uws.ac.uk
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