FENS 1

astdragon.org
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 DrepanocladosCampylium 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 (CallunaErica 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.

FENS TWO

lastdragon.org
BIODIVERSITY REFERENCE
British habitats
 
   Fens   
 
(page 2)

ACCESS: DROVES AND WATERWAYSAs already indicated, most large fens have a past history of extensive management. Small fens or those in valley mires may be relatively natural, but a history of peat cutting or sedge harvesting requires routes of access. Water levels in the same fen systems have required control. Consequently the geographical Fenland has an ancient system oflodes, navigable waterways, some apparently dating back to Roman times. Subsidiary to these are dykes, which when clear may be able to take small boats, and additional drainage ditches. Cut routes through the fen are droves and again some are at least a few hundred years old. They provide a continuing short turf habitat and may support a flora of small sedges, rushes and herbs that cannot compete in taller vegetation.

Wicken Lode, an ancient, continually navigable waterway, now mainly used by holiday barges. It is bordered by the Lode Bank, a drove which itself is of substantial age. It is higher and dryer than most of the Fen droves, which are often temporarily flooded in the winter months. Wicken Fen, Cambridgeshire, May 1998.

The waterways are of great importance for their aquatic fauna and flora. The smaller dykes and ditches are readily colonised by submerged and floating-leaved aquatic plants, including, in rich fens, populations of local or rare stoneworts (Charophyta). The invertebrate fauna may be equally diverse. Further colonisation by taller aquatic macrophytes such as the Common Reed (Phragmites australis) is likely and the probability is that the dykes will become clogged with vegetation and silt. At length they may be crossable on foot and their value for water management, and indeed for their fauna and flora is lost. A cyclic programme of clearance maintains the waterways and ensures continuing habitat for rare species.

Water Violet (Hottonia palustris) amongst other aquatic macrophytes colonising a drainage dyke. Wicken Fen, May 1998.

CONSERVATION

Open water and flood-plain mires have probably been more extensively destroyed by man in Britain than those of any other class. This is especially true of the once vast swamps and marshes of the East Anglian Fenlands, occupying the large shallow basin of the three main rivers draining to the Wash. Many others have been reclaimed for agriculture, since the richer types of fen peat give a very fertile soil when dried out, and a great many have been partly drained or modified by human activity.
(Goode & Ratcliffe, in Ratcliffe, 1977)

Ratcliffe does go on to make the point that the Norfolk Broads owe their present existence to human activity. Large scale cutting of peat in the Middle Ages formed extensive hollows that have flooded and undergone succession, now forming important habitats from open water to rich fen and carr.

Mire surfaces are often sensitive to disturbance and uncontrolled public access can cause considerable damage to vegetation and to ground nesting birds. Trampling damage can be reduced by provision of boardwalks, which also have the effect of controlling the route taken by casual visitors.

Wicken Fen, being (mostly) owned by the National Trust, is managed to allow public access, which must then be balanced against conservation necessities. A circular nature trail via boardwalks takes visitors through most of the main habitats, permits access to the windpump and bird hides and ensures minimal damage to the Fen. Effectively the boardwalk is also a bridge across much of the wetter ground and (usually) means that waterproof footwear is not necessary. Wicken Fen, July 1998.
This particular section of the boardwalk has been replaced and re-routed since the taking of this photograph.

The dramatic loss of fenland habitat has taken its inevitable toll with regard to the specialist plants and animals of such habitats. Senecio congestus, the Marsh Fleawort, was once a plant of fen ditches in the Lincolnshire and East Anglian fenlands but is long extinct. A second Senecio species, S. paludosus, the Great Fen Ragwort, was also thought long extinct until a very few plants appeared on a cleared ditchbank in Cambridgeshire in 1972, evidently from long-buried seed. The population has been maintained and plants have been transferred to a safer location, though it is reasonable to expect that the surviving British stock is genetically impoverished. Other plants such as the Fen Orchid, Liparis loeselii and the Fen Violet, Viola persicifolia are critically endangered.

Predictably there are a number of birds confined to fen habitats. The East Anglian fens are the main British stronghold of the Bearded Tit (Panurus biarmicus), a species of reed-beds. Savi’s Warbler (Locustella luscinoides) is another reed-bed species that once bred in the Fenlands before becoming extinct in Britain. It has re-established itself in Britain since the 1950s, with Wicken Fen being a key site. The Marsh Harrier (Circus aeruginosus) is arguably the special bird of prey of the East Anglian fens. After coming close to extinction from use of cyclodiene agricultural pesticides, populations are now recovering following banning of these chemicals from agricultural use.
THE RÔLE OF THE ENTOMOLOGISTS

It was, however, the entomological interest of the East Anglian fens that first led to preservation of surviving fragments as nature reserves. The strong interest, particularly in butterflies and moths, during the 1800s meant that not only were the fens much visited, but also that sedge cutters were able to supplement their incomes by finding and selling larvae of the more sought after species. As the old Fenlands were turned so rapidly into productive agricultural land, it was the entomologists who first seem to have realised the value of holding onto and preserving some remnant of the old watery wilderness. The Swallowtail butterfly became a symbol of such efforts, though as it turned out, “preservation” of its remaining Cambridgeshire location was not enough, conservation through management was also needed and was sadly lacking in the early years of the 1900s. Other species, such as the Large Copper butterfly and the Reed Tussock moth were already extinct in Britain before any thought was given to conserving the Fens.

The Swallowtail (Papilio machaon) is now confined in Britain to the Norfolk Broads, though it was once also a classic butterfly of the geographical Fenland and persisted at Wicken Fen until the 1950s. Its distribution is necessarily limited by that of its foodplant, the Milk Parsley (Peucedanum palustre), which itself is a nationally scarce species, virtually confined to the old Fenland and the Norfolk Broads and extinct in most of the northern part of its range.

The Milk Parsley is a plant of rich fen, growing in sedge and litter fields and dependent on suitable cutting regimes. There have been several attempts to reintroduce the Swallowtail to Wicken Fen and management of the sedge and litter fields has included maintenance of its foodplant as a priority. However, past encroachment of carr has reduced the suitable area and it seems that at the present time the remaining area is too vulnerable to both drying and flooding for the Swallowtail population to remain viable. Clearance of some of the carr is a management priority, though it may be noted that a successful introduction from Norfolk will not replace the loss of genetic diversity. Dempster et al. (cited by Heath et al., 1984) carried out a museum study that suggested that Wicken butterflies were genetically somewhat different from those of Norfolk, a not unlikely result of the fragmentation of the old Fenland habitat.

It should also be noted that there are early records of the Swallowtail in Britain that suggest a much wider distribution in southern England and perhaps use of a broader spectrum of foodplants such as the relatively common Wild Carrot (Daucus carota) (Morris, 1870). Whereas the endemic British subspecies, Papilio machaon britannicus, is strictly a fenland butterfly feeding only on Milk Parsley, the widespread continental subspecies, Papilio machaon bigeneratus, is less restricted in its diet. It is possible these early records simply represent occasional vagrants from the continent, but it was collected in some numbers at least in Dorset. The Swallowtail had been locally abundant in the Fenlands of Huntingdonshire and Cambridgeshire but there were already concerns for its survival in the latter part of the 1800s. As the fens became the last refuge of this butterfly in Britain, perhaps isolation then further defined our endemic subspecies?
The illustration is taken from Morris (op. cit.) and was based on an 1851 Cambridgeshire specimen.


[To be added: accounts of the Large Copper & Reed Tussock. Ecotypic adaptation and genetic consequences of human disturbance in the Fen Nettle.]

FURTHER INFORMATION

Wicken Fen is the best documented fenland site, as well as being an area in which the author has a research interest, so has been much cited in this web profile. Much more is to be found at:

Wicken Fen Vision
http://wicken.org.uk
The official site (National Trust) of the Wicken Fen National Nature Reserve, with the management plan and plenty else on this area of ancient fenland. There was an older site, formerly at http://www.demon.co.uk/ecoln/wicken_fen/index.html (English Nature) which contained a wealth of information. Though sadly gone, it is archived at the Wayback Machine (insert the URL and wait …).
Fungi of Wicken Fen
http://www.lastdragon.org/wicken/wickenfungi.html   (link temporarily unavailable)
Primarily of research and specialised interest, but gives some extra background to habitats in the Fen.

For a very comprehensive account of Wicken Fen, its history, management and natural history, see Friday (1997). It has much of great relevance to fens in general. A detailed account of the history and past management of Wicken Fen is also given by Rowell & Harvey (1988).

Godwin (1978) gives an excellent account of the history of Fenland, with much on past and present management and conservation. Goode & Ratcliffe (in Ratcliffe, 1977, chapter 8) give an excellent overview of British peatlands including fens, their diversity and conservation criteria. Volume 2 of the same work summarises key British fenland sites selected as grade 1 or grade 2 SSSIs. A valuable compendium of detail, if somewhat outdated now, was provided by Tansley (1939). For a modern overview of mires, with a rigorous examination of terminology, see Wheeler & Proctor (2000).

References:

  • Friday, L., (ed.) (1997). Wicken Fen: the making of a wetland nature reserve, Harley Books, Colchester.
  • Godwin, H., (1978). Fenland: its ancient past and uncertain future, Cambridge University Press, Cambridge.
  • Heath, J., Pollard, E., & Thomas, J., (1994). Atlas of butterflies in Britain and Ireland, Viking, Harmondsworth.
  • Morris, F.O., (1870). A history of British butterflies, 3rd ed., Bell and Daldy, London.
  • Ratcliffe, D.A., (ed.) (1977). A nature conservation review, vol. 1, Cambridge University Press, Cambridge.
  • Rodwell, J.S., et al., (1995). British plant communities. 4Aquatic communities, swamps and tall-herb fens, Cambridge University Press.
  • Rowell, T.A., & Harvey, H.J., (1988). The recent history of of Wicken Fen, Cambridgeshire, England: a guide to ecological development. Journal of Ecology 76: 73-90.
  • Stewart, A., Pearman, D.A., & Preston, C.D., (1994). Scarce plants in Britain, JNCC, Peterborough.
  • Tansley, A.G., (1939). The British Islands and their vegetation, Cambridge University Press, Cambridge.
  • Wheeler, B.D., (1984) British fens – a review, in Moore, P.D., (ed), European mires, Academic Press, London.
  • Wheeler, B.D., & Proctor, M.C.F., (2000). Ecological gradients, subdivisions and terminology of north-west European mires. Journal of Ecology 88: 187-203.

Note: Like much of this site, these pages are likely to be the object of continuing development and review.

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