Habitat account - Raised bogs and mires and fens


7230 Alkaline fens 

Background to selection

Distribution of SACs/SCIs/cSACs with habitat 7230 Alkaline fens.  Click image for enlarged map.
Distribution of SACs/SCIs/cSACs with habitat 7230 Alkaline fens. Click image for enlarged map.

Description and ecological characteristics

 

Alkaline fens consist of a complex assemblage of vegetation types characteristic of sites where there is tufa and/or peat formation with a high water table and a calcareous base-rich water supply. The core vegetation is short sedge mire (mire with low-growing sedge vegetation) of the following NVC types:

 

     

  • M9  Carex rostrata – Calliergon cuspidatum/giganteum mire
  • M10  Carex dioica – Pinguicula vulgaris mire
  • M13  Schoenus nigricans – Juncus subnodulosus mire

 

At most sites there are well-marked transitions to a range of other fen vegetation, predominantly, but not exclusively, to M14 Schoenus nigricans – Narthecium ossifragum mire and S24 Phragmites australis – Peucedanum palustre tall-herb fen in the lowlands. Alkaline fens may also occur with various types of swamp (such as species-poor stands of great fen-sedge Cladium mariscus), wet grasslands (particularly various types of purple moor-grass Molinia caerulea grassland) and areas rich in rush Juncus species, as well as fen carr and, especially in the uplands, wet heath and acid bogs.

 

There is considerable variation between sites in the associated communities and the transitions that may occur. Such variation can be broadly classified by the geomorphological situation in which the fen occurs, namely: flood plain mire, valley mire, basin mire, hydroseral fen (i.e. as zones around open waterbodies) and spring fen. Another important source of ecological variation is altitude, with significant differences between lowland fens, which are rich in southern and continental species, and upland fens, which are rich in northern species.

European status and distribution

 

A significant proportion of the Alkaline fens surviving in the EU are believed to occur in the UK and Sweden. Alkaline fen vegetation has declined dramatically in the EU generally in the past century.

UK status and distribution Click to view UK distribution of this habitat

 

Alkaline fens occur over a widely scattered geographical range in the UK, but are unevenly and locally distributed, with important concentrations of the habitat in East Anglia, in northern England, and on Anglesey in north Wales. Alkaline fen vegetation has declined dramatically in the past century in the UK, and in many parts of the country only small, fragmentary stands survive.

Site selection rationale

 

The SAC series reflects the UK’s special responsibility for protection of this habitat type, and the limited extent of surviving Alkaline fens. Site selection has taken account of geographical range and ecological, geomorphological and altitudinal variation. Site selection has favoured sites where the component NVC types are well represented. The SAC series encompasses the larger sites, where the hydrology tends to be least-modified by human activity and which show the most intact structure and function.


Site accounts

Asby Complex Cumbria
Asby is one of two upland sites in northern England where there are extensive flushes of M10 Carex dioicaPinguicula vulgaris mire amidst moorland and grassland. An important example in the fen SAC series of hydroseral fen community occurs on the lake margins in the vicinity of Sunbiggin Tarn. There are also lake-side transitions to reedswamp vegetation. Away from the lake the site has an exceptionally rich flora and contains a number of rare and local northern plant species, such as bird’s-eye primrose Primula farinosa.
Corsydd Llyn/ Lleyn Fens West Wales and The Valleys
Corsydd Llyn consists of a chain of four rich-fen sites running across the centre of the Lleyn Peninsula, north-west Wales. Cors Geirch is the largest component site and consists of a sinuous valley mire system linking with Cors Edern to the north via a narrow isthmus of modified mire on deep peat; the remaining two component sites of Cors Hirdre and Aber Geirch occupy separate hydrotopographical units close to the north Lleyn coast. The alkaline fen manifests as soligenous communities referable to NVC type M13 Schoenus nigricansJuncus subnodulosus mire, together with M9 Carex rostrataCalliergon cuspidatum/giganteum mire in dominantly topogenous settings – stands with elements of both communities are also present. Much of the alkaline fen interest at this site occurs within a matrix of anthropogenically-modified peatland vegetation in which bog myrtle Myrica gale, purple moor-grass Molinia caerulea, blunt-flowered rush Juncus subnodulosus and common reed Phragmites australis occur as prominent components; great fen-sedge Cladium mariscus is also locally dominant. Outstanding floristic features of the alkaline fen at Corsydd Llyn include the nationally rare slender cottongrass Eriophorum gracile at its sole north Wales station, together with significant populations of narrow-leaved marsh orchid Dactylorhiza traunsteineri, grass of parnassus Parnassia palustris and lesser-tussock sedge Carex diandra.
Corsydd Môn/ Anglesey Fens West Wales and The Valleys
This composite site includes four component fen systems supporting a diverse range of short-sedge mires, including the best and most extensive Welsh examples of NVC type M13 Schoenus nigricansJuncus subnodulosus mire and a range of communities referable to M9a Carex rostrataCalliergon cuspidatum/ giganteum mire. These are considered to be of pre-eminent importance in the UK, owing to their extent, biogeographical significance and exceptionally rich assemblage of rich-fen species. The fens are strongly influenced by the underlying Carboniferous limestone and are fed by calcareous groundwater arising from discrete springs and more diffuse zones of seepage. The alkaline fen communities often occur within complex vegetation zonations, and typical contact communities include great fen-sedge Cladium mariscus swamp, fen carr, fen meadow communities dominated by blunt-flowered rush Juncus subnodulosus (M22 Juncus subnodulosusCirsium dissectum fen-meadow) and purple moor-grass Molinia caerulea (M25 Molinia caeruleaPotentilla erecta mire) as well as a range of vegetation types broadly referable to the CladioMolinietum. Gradations to unimproved calcicolous and neutral grasslands also occur. The characteristic mixture of southern and northern floristic elements includes a wide range of nationally or locally scarce species, including fly orchid Ophrys insectifera, narrow-leaved marsh orchid Dactylorhiza traunsteineri, marsh helleborine Epipactis palustris, lesser clubmoss, Selaginella selaginoides and slender sedge Carex lasiocarpa. Examples of M13 mire within Anglesey Fens which are strongly influenced by the discharge of calcareous groundwater provide the sole north Wales locus for the Annex II species 1044 Southern damselfly Coenagrion mercuriale.
Cothill Fen Berkshire, Buckinghamshire and Oxfordshire
This lowland valley mire contains one of the largest surviving examples of alkaline fen vegetation in central England, a region where fen vegetation is rare. The M13 Schoenus nigricansJuncus subnodulosus vegetation found here occurs under a wide range of hydrological conditions, with frequent bottle sedge Carex rostrata, grass-of-Parnassus Parnassia palustris, common butterwort Pinguicula vulgaris and marsh helleborine Epipactis palustris. The alkaline fen vegetation forms transitions to other vegetation types that are similar to M24 Molinia caeruleaCirsium dissectum fen-meadow and S25 Phragmites australisEupatorium cannabinum tall-herb fen and wet alder Alnus spp. wood.
Craven Limestone Complex North Yorkshire
There are large fen systems at Great Close and Ha Mire, principally of the NVC type M10b Carex dioicaPinguicula vulgaris mire, Briza mediaPrimula farinosa sub-community. They are exceptionally species-rich types with frequent bird’s-eye primrose Primula farinosa and grass-of-Parnassus Parnassia palustris alongside rarities such as broad-leaved cottongrass Eriophorum latifolium, hair sedge Carex capillaris, alpine bartsia Bartsia alpina and dwarf milkwort Polygala amarella. Where irrigation is more extensive there are transitions to M9a Carex rostrataCalliergon cuspidatum/ giganteum mire, Campylium stellatumScorpidium scorpioides sub-community. This community is also developed extensively around the lagg of Tarn Moss, where there are transitions with M26b Molinia caeruleaCrepis paludosa mire, Festuca rubra sub-community and W3 Salix pentandraCarex rostrata fen carr woodland. There are also extensive M10 Carex dioicaPinguicula vulgaris spring-fed flush fens throughout the site, typically associated with calcareous grassland and limestone scars.
Cwm Cadlan East Wales
Cwm Cadlan supports an outstanding suite of flushed short-sedge mire communities on glacial drift overlying Carboniferous limestone within the valley of the Nant Cadlan on the southern fringe of Brecon Beacons National Park. Communities referable to NVC type M10 Carex dioicaPinguicula vulgaris mire occur widely, often in close association with flushed examples of purple moor-grass Molinia caerulea meadow (M24 Molinia caeruleaCirsium dissectum fen-meadow), and characteristic species include common butterwort Pinguicula vulgaris, bog pimpernel Anagallis tenella, marsh arrowgrass Triglochin palustris and the moss Campylium stellatum. Other sedge-rich swards are also present which display floristic affinities to both M10 and M24; basophilous elements of this vegetation include tawny sedge Carex hostiana, flea sedge Carex pulicaris and quaking-grass Briza media.
Garron Plateau Northern Ireland
Garron Plateau is one of the largest areas of 7130 blanket bog in Northern Ireland. The site occurs over basalt rocks, and where the peats are thinner, localised flushing by mineral-enriched water results in the formation of alkaline fen vegetation. Although there are many such flushes, their total area is relatively small, since the flushes are generally very small in extent. Nevertheless a combination of good quality and geographical position make this habitat very important here. The vegetation is characterised by M10 Carex dioicaPinguicula vulgaris mire. This is generally very species-rich, with a range of small sedges including dioecious sedge Carex dioica, flea sedge C. pulicaris, tawny sedge C. hostiana and yellow-sedge C. viridula ssp. brachyrrhyncha, herbs such as eyebright Euphrasia spp., marsh lousewort Pedicularis palustris, common butterwort Pinguicula vulgaris, lesser clubmoss Selaginella selaginoides, devil’s-bit scabious Succisa pratensis and marsh arrowgrass Triglochin palustris, and the bryophytes Scorpidium scorpioides, Drepanocladus revolvens, Philonotis spp., Cratoneuron spp. and Campylium stellatum.
Holme Moor and Clean Moor Dorset and Somerset
These sites are situated on the north-facing slope of the upper reaches of a small valley and are fed by a mix of acidic and base-rich springs. The most species-rich example of alkaline fen is on Clean Moor, where black bog-rush Schoenus nigricans and blunt-flowered rush Juncus subnodulosus have many associates including the moss Scorpidium scorpioides, small sedges such as Carex pulicaris, C. panicea and C. viridula ssp brachyrrhyncha, and other low growing species such as lousewort Pedicularis palustris and the orchids Gymnadenia conopsea, Dactylorhiza fuchsii and D. praetermissa. In addition to NVC type M13 Schoenus nigricansJuncus subnodulosus mire around the base-rich seepages there is also species-poor swamp with great fen-sedge Cladium mariscus and hemp agrimony Eupatorium cannabinum, and fen meadow on Holme Moor. Holme Moor & Clean Moor is important as a south-westerly site for alkaline fen.
Ingleborough Complex North Yorkshire
Spring-fed flush fens of NVC type M10 Carex dioicaPinguicula vulgaris mire are extensive across Ingleborough, commonly associated with calcareous grassland types, but also found amidst acid grasslands and heathland communities. They are often species-rich communities, in which rare or locally distributed species such as bird’s-eye primrose Primula farinosa, black bog-rush Schoenus nigricans, few-flowered spike-rush Eleocharis quinqueflora and flat-sedge Blysmus compressus are frequent.
Lecale Fens Northern Ireland
The Lecale area in the south-east of Northern Ireland contains a number of inter-drumlin hollows with alkaline fen vegetation that is characteristic of the more undisturbed wetlands in the area. The vegetation is variable, but generally very species-rich, and includes examples of M9 Carex rostrataCalliergon cuspidatum/giganteum mire in addition to M13 Schoenus nigricansJuncus subnodulosus mire, which is very rare in Northern Ireland. Some stands consist of tussocks of black bog-rush Schoenus nigricans with scattered blunt-flowered rush Juncus subnodulosus and common reed Phragmites australis. Smaller herbs, and mosses including Campylium stellatum, Calliergon cuspidatum, C. giganteum and Cratoneuron filicinum, grow between the tussocks. A number of additional species occur which give the vegetation a distinctive character. Notably, these include lesser water-parsnip Berula erecta, slender sedge Carex lasiocarpa, lesser tussock-sedge C. diandra, greater tussock-sedge C. paniculata, great fen-sedge Cladium mariscus, many-stalked spike-rush Eleocharis multicaulis, knotted pearlwort Sagina nodosa and least bur-reed Sparganium natans. Other prominent bryophytes include Calliergon cordifolium, Plagiomnium elatum, Riccardia multifida and Scorpidium scorpioides.
Lendalfoot Hills Complex South Western Scotland
On the Lendalfoot Hills there is a well-developed series of Alkaline fens belonging to M9 Carex rostrataCalliergon cuspidatum mire and M10 Carex dioicaPinguicula vulgaris mire. They are developed over base-rich serpentine rocks and are representative of this habitat in southern Scotland. The fens occur in a range of hydrological situations such as topogenous basin fens, soligenous tracks or soakways in valley fen or wet heath and as spring-fed fens. Some of the CarexPinguicula mires have an abundance of black bog-rush Schoenus nigricans. The Alkaline fens grade to a range of base-poor fens, wet heaths and species-rich grasslands.
Magheraveely Marl Loughs Northern Ireland
Magheraveely Marl Loughs consists of a cluster of six low-lying lakes in the catchment of the River Finn in Northern Ireland. These occur over an area of Carboniferous limestone bedrock. The lakes are surrounded by an inundation zone containing significant stands of alkaline fen vegetation. This is generally composed of a sward that is very rich in sedges and herbs. Characteristic species include the sedges lesser tussock-sedge Carex diandra, long-stalked yellow sedge C. viridula ssp. brachyrrhyncha and glaucous sedge C. flacca. Other frequent species include marsh arrowgrass Triglochin palustre, quaking-grass Briza media and more notably, marsh helleborine Epipactis palustris, grass-of-Parnassus Parnassia palustris, knotted pearlwort Sagina nodosa and fen bedstraw Galium uliginosum. The latter are all scarce species in Northern Ireland.
Moor House - Upper Teesdale Cumbria, Tees Valley and Durham
This is one of two upland sites in northern England selected for Alkaline fens. Spring-fed flush fens of NVC type M10 Carex dioicaPinguicula vulgaris mire are widespread on the moors amidst calcareous grassland, limestone scars, heath and bog, in enclosed pastures amidst a range of acid and calcareous grasslands and in meadows, often as part of complex vegetation mosaics. The site has an exceptionally important rare plant flora associated with flush vegetation, including species such as bird’s-eye primrose Primula farinosa and Scottish asphodel Tofieldia pusilla. On the highest and coldest parts of the site fen grades into Annex I type 7240 Alpine pioneer formations of the Caricion bicoloris-atrofuscae, and intermediate examples occur.
Mortlach Moss North Eastern Scotland
This small base-rich basin fen, lying upon igneous rock, is representative of Alkaline fens in north-east Scotland. Typical species include broad-leaved cottongrass Eriophorum latifolium, lesser and greater tussock sedges, Carex diandra and C. paniculata respectively, black bog-rush Schoenus nigricans and grass of Parnassus Parnassia palustris. Bladderwort Utricularia sp. is abundant.
Newham Fen Northumberland and Tyne and Wear
Newham is important as a lowland short sedge fen in north-east England, a part of the UK in which Alkaline fens are rare. The site is an example of basin fen, developed from the hydroseral succession of a small lake. The main fen community is M13 Schoenus nigricansJuncus subnodulosus mire and M9 Carex rostrataCalliergon cuspidatum/giganteum mire, and there are transitions to S25 Phragmites australisEupatorium cannabinum tall-herb fen, MG1 Arrhenatherum elatius grassland and W2 Salix cinereaBetula pubescensPhragmites australis woodland. A number of rare species occur at this site, including narrow-leaved marsh-orchid Dactylorhiza traunsteineri, coralroot orchid Corallorhiza trifida, dark-leaved willow Salix myrsinifolia and round-leaved wintergreen Pyrola rotundifolia.
Norfolk Valley Fens East Anglia
Norfolk Valley Fens is one of two sites selected in East Anglia, in eastern England, where the main concentration of lowland Alkaline fens occurs. This site comprises a series of valley-head spring-fed fens. Such spring-fed flush fens are very rare in the lowlands. Most of the vegetation at this site is of the small sedge fen type, mainly referable to M13 Schoenus nigricansJuncus subnodulosus mire, but there are transitions to reedswamp and other fen and wet grassland types. The individual fens vary in their structure according to intensity of management and provide a wide range of variation. There is a rich flora associated with these fens, including species such as grass-of-Parnassus Parnassia palustris, common butterwort Pinguicula vulgaris, marsh helleborine Epipactis palustris and narrow-leaved marsh-orchid Dactylorhiza traunsteineri.
North Fetlar Highlands and Islands
Alkaline fens fed by base-rich water from serpentine rocks are widespread across North Fetlar. They represent the habitat on Shetland and in the far north. The NVC type is M10 Carex dioicaPinguicula vulgaris mire. Transitions occur to base-rich swamp communities.
Rassal Highlands and Islands
Rassal in the north-west Highlands is representative of Alkaline fens developed from flushing through Durness limestone. The main type of fen are the more closed mires rich in Carices and other calcicolous small herbs (M10 Carex dioicaPinguicula vulgaris mire). Open, stony flushes (M11 Carex demissaSaxifraga aizoides mire) also occur, and are transitional in flora to 7240 Alpine pioneer formations of the Caricion bicoloris-atrofuscae. These are spring-fed mires and are numerous across the site. Black bog-rush Schoenus nigricans, broad-leaved cottongrass Eriophorum latifolium and the moss Scorpidium scorpioides are abundant locally in the fens.
The Broads East Anglia
The Broads is one of two sites selected for Alkaline fens in East Anglia, in eastern England, where a main concentration of lowland fen occurs. There are areas of short sedge fen (both M13 Schoenus nigricansJuncus subnodulosus mire and M9 Carex rostrataCalliergon cuspidatum/giganteum mire), which in places form a mosaic with S24 Phragmites australisPeucedanum palustris fen. There are complex zonations present and many differences exist between the individual fens that comprise the site. The fens are principally of the flood plain mire type. The site contains a range of rare and local plant species, including the Annex II 1903 Fen orchid Liparis loeselii, lesser tussock-sedge Carex diandra and slender sedge C. lasiocarpa.
Tulach Hill and Glen Fender Meadows Eastern Scotland
This site in the southern Scottish Highlands has extensive spring-fed Alkaline fens associated with limestone-rich drift. This is the most extensive and species-rich alkaline fen system in Scotland. The main NVC type present is M10 Carex dioicaPinguicula vulgaris mire, with some M9 Carex rostrataCalliergon cuspidatum/giganteum mire developed as basin mire in hollows. The predominant CarexPinguicula mires occur in mosaics with local open flushes of M11 Carex demissaSaxifraga aizoides mire. The fens occur in a complex topography, with drier and wetter habitats and well-developed zonations to dry and flushed calcareous, acid and neutral grasslands, acidic species-poor fen, ericaceous sub-shrub heath and species-rich fen-meadow. It is representative of the northerly variants of the habitat type and supports a rich assemblage of rare northern fen species, such as Scottish asphodel Tofieldia pusilla and false-sedge Kobresia simpliciuscula. Other arctic-alpine and northern species, including hair sedge Carex capillaris, alpine meadow rue Thalictrum alpinum and marsh hawk’s beard Crepis paludosa, are also represented.

SACs/SCIs/cSACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection

Beinn a' Ghlo Eastern Scotland
Ben Heasgarnich Eastern Scotland, Highlands and Islands
Ben Lawers Eastern Scotland
Ben Lui Eastern Scotland, Highlands and Islands
Cadair Idris West Wales and The Valleys
Caenlochan Eastern Scotland, North Eastern Scotland
Dam Wood Highlands and Islands
Dorset Heaths Dorset and Somerset
Dorset Heaths (Purbeck and Wareham) and Studland Dunes Dorset and Somerset
Durness Highlands and Islands
Eryri/ Snowdonia West Wales and The Valleys
Exmoor Heaths Devon, Dorset and Somerset
Glen Coe Highlands and Islands
Gweunydd Blaencleddau West Wales and The Valleys
Hoy Highlands and Islands
Inchnadamph Highlands and Islands
Invernaver Highlands and Islands
Lake District High Fells Cumbria
Morrone Birkwood North Eastern Scotland
North Pennine Moors Cumbria, North Yorkshire, Northumberland and Tyne and Wear, Tees Valley and Durham
Onich to North Ballachulish Woods Highlands and Islands
Peak District Dales Derbyshire and Nottinghamshire, Shropshire and Staffordshire
Preseli West Wales and The Valleys
Rum Highlands and Islands
Strath Highlands and Islands
Stromness Heaths and Coast Highlands and Islands
The New Forest Gloucestershire, Wiltshire and Bristol/Bath area, Hampshire and Isle of Wight
West Fermanagh Scarplands Northern Ireland
Whitlaw and Branxholme Eastern Scotland
 

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