Habitat account - Raised bogs and mires and fens


7140 Transition mires and quaking bogs 

Background to selection

Distribution of SACs/SCIs/cSACs with habitat 7140 Transition mires and quaking bogs.  Click image for enlarged map.
Distribution of SACs/SCIs/cSACs with habitat 7140 Transition mires and quaking bogs. Click image for enlarged map.

Description and ecological characteristics

 

The term ‘transition mire’ relates to vegetation that in floristic composition and general ecological characteristics is transitional between acid bog and 7230 Alkaline fens, in which the surface conditions range from markedly acidic to slightly base-rich. The vegetation normally has intimate mixtures of species considered to be acidophile and others thought of as calciphile or basophile. In some cases the mire occupies a physically transitional location between bog and fen vegetation, as for example on the marginal lagg of raised bog or associated with certain valley and basin mires. In other cases these intermediate properties may reflect the actual process of succession, as peat accumulates in groundwater-fed fen or open water to produce rainwater-fed bog isolated from groundwater influence. Many of these systems are very unstable underfoot and can therefore also be described as ‘quaking bogs’.

 

Transition mires and quaking bogs can occur in a variety of situations, related to different geomorphological processes: in flood plain mires, valley bogs, basin mires and the lagg zone of raised bogs, and as regeneration surfaces within mires that have been cut-over for peat or areas of mineral soil influence within 7130 Blanket bogs (e.g. ladder fens).

 

The following NVC types form the core of transition mire vegetation in the UK:

 

     

  • M4  Carex rostrata – Sphagnum recurvum mire
  • M5  Carex rostrata – Sphagnum squarrosum mire
  • M8  Carex rostrata – Sphagnum warnstorfii mire
  • M9  Carex rostrata – Calliergon cuspidatum/giganteum mire
  • S27  Carex rostrata – Potentilla palustre tall-herb fen.

 

However, this list is not exhaustive: forms of M2 Sphagnum cuspidatum/recurvum bog pool community, M14 Schoenus nigricans – Narthecium ossifragum mire and M29 Hypericum elodes – Potamogeton polygonifolius soakway are also important components on some sites. M21 Narthecium ossifragum – Sphagnum papillosum valley mire is excluded from the Annex I definition, as it is not transitional in a successional sense or in terms of its soil chemistry. Not all examples of M9 Carex – Calliergon mire belong to this Annex I type; where it occurs in more base-rich conditions or in association with other rich fen communities, it may be referable to 7230 Alkaline fens, or, in stands where great fen-sedge Cladium mariscus is dominant, to 7210 Calcareous fens with Cladium mariscus and species of the Caricion davallianae.

European status and distribution

 

Transition mires and quaking bogs have a wide European distribution but appear to be relatively scarce in the Mediterranean region.

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

 

Transition mires and quaking bogs are a widespread but local habitat type in the UK that is ecologically variable and occurs in a wide range of geomorphological contexts.

Site selection rationale

 

Site selection has placed emphasis on covering the range and variability of physiography and mire chemistry, and the site series includes examples from a range of different hydrological and geomorphological situations. Extensive examples have been favoured as these generally have the best conservation of structure and function, particularly of hydrological features. The selected sites span the wide geographic range of the habitat and include both upland and lowland examples. Particular NVC categories are not strongly or exclusively associated with a given mire type, and most selected sites support a range of vegetation communities.


Site accounts

Altnaharra Highlands and Islands
Although relatively small, this floodplain site in northern Scotland supports a wide range of communities including M4 Carex rostrataSphagnum recurvum mire, M9 Carex rostrataCalliergon cuspidatum/giganteum mire and S27 Carex rostrataPotentilla palustris tall-herb fen. Altnaharra is also one of only two sites in the UK to support the rare string sedge Carex chordorrhiza.
Border Mires, Kielder - Butterburn Cumbria, Northumberland and Tyne and Wear
Border Mires, Kielder – Butterburn is made up of several individual sites running north-east from Carlisle. Collectively, these sites contain a wide range of bog-moss Sphagnum species, for example 11 on Caudbeck alone, along with an almost equally large number of Carex species. The transition mire element of these sites is relatively small, but is an important component of one of the least-damaged and more valuable species-rich mire complexes in England.
Breney Common and Goss and Tregoss Moors Cornwall and Isles of Scilly

Although possibly the site of a former raised bog, this site lying either side of the A30 trunk road and encompassing the River Fowey is now recovering from an intensive period of china clay and gravel extraction. H7140 Transition mire has developed in the hollows between ridges and mounds on which dry heathland forms a mosaic with acid grassland. Wet heath merges into Sphagnum-dominated fen vegetation with common cottongrass Eriophorum angustifolium, round-leaved sundew Drosera rotundifolia, bog-myrtle Myrica gale, bog asphodel Narthecium ossifragum, black bog-rush Schoenus nigricans and bog pimpernel Anagallis tenella. Of particular note are the nationally scarce plants yellow centaury Cicendia filiformis, marsh clubmoss Lycopodiella inundata and pillwort Pilularia globulifera.

Emergent vegetation around the 15 ponds includes water horsetail Equisetum fluviatile, bogbean Menyanthes trifoliata and marsh cinquefoil Potentilla palustris. Many of the transitions include tall fen vegetation with bulrush Typha latifolia, common reed Phragmites australis and bottle sedge Carex rostrata. Other wetland plants found in the pond margins and across the more shallow ponds include marsh St John’s-wort Hypericum elodes, sharp-flowered rush Juncus acutiflorus and ivy-leaved bellflower Wahlenbergia hederacea. Of particular note are the nationally scarce Cornish moneywort Sibthorpia europaea and wavy St John’s-wort Hypericum undulatum. Extensive willow carr has developed over much of the central part of the Goss Moor.

Broubster Leans Highlands and Islands
Broubster Leans has developed on a floodplain beside the River Forss in northern Caithness. The Leans support a diversity of plant communities from open water through to willow carr, rush-pasture and blanket bog. The main transition mire communities are S27 Carex rostrataPotentilla palustris tall-herb and fen, S11 Carex vesicaria swamp (with water sedge Carex aquatilis replacing bladder sedge C. vesicaria, which has not been recorded at this site) and S10 Equisetum fluviatile swamp. These mire communities are notable for the abundance of water sedge C. aquatilis which replaces, or is frequently dominant over, other Carex species more typical of the habitat. A range of additional communities also occur including M9 Carex rostrataCalliergon cuspidatum mire, S9 Carex rostrata swamp and M27 Filipendula ulmariaAngelica sylvestris mire. Small areas of open water occur along the former bed of the River Forss.
Corsydd Eifionydd West Wales and The Valleys
Corsydd Eifionydd embraces two isolated topogenous peatlands situated within the upland-fringe transition between Snowdonia and the Lleyn Peninsula. The site supports a diverse range of oligo-mesotrophic topogenous fen communities which often manifest as strongly quaking rafts, particularly over former peat-cuttings. Areas of wet fen dominated by slender sedge Carex lasiocarpa are a particular feature of Cors Gyfelog. Relatively forb-rich stands are referable to NVC type M9 Carex rostrataCalliergon cuspidatum/giganteum mire but significant areas conform more closely to the concept of mesotrophic bog, with a prominent bog-moss Sphagnum spp. component (including S. denticulatum, S. contortum, S. papillosum, S. subnitens and occasional S. magellanicum) and species such as cranberry Vaccinium oxycoccos, bog St John’s-wort Hypericum elodes and bog asphodel Narthecium ossifragum occurring beneath an open sedge canopy. Bog sedge Carex limosa is probably more abundant at Cors Gyfelog than at any other Welsh lowland site and bog orchid Hammarbya paludosa has recently been noted from hummocks of S. subnitens. Cors Graianog defines the more acidic end of the spectrum of transition mire and quaking bog at this site, with extensive areas of M4 Carex rostrataSphagnum recurvum mire as well as vegetation with affinities to M18 Erica tetralixSphagnum papillosum raised mire.
Crowdy Marsh Cornwall and Isles of Scilly

Crowdy Marsh is one of several valley mires found around the edge of the granite massif of Bodmin Moor. Most of the gently-sloping wide valley is now occupied by a freshwater reservoir, but feeder streams still meander via a network of water tracks between low peaty mounds over the remainder. The H7140 Transition mire includes the water tracks and occasional waterlogged hollows found throughout Crowdy Marsh.

The deep hollows are dominated by the bog-moss Sphagnum [auriculatum] with a mix of typical transition mire species such as marsh cinquefoil Potentilla palustris and bogbean Menyanthes trifoliata. Marsh St John’s-wort Hypericum elodes and bog pondweed Potamogeton polygonifolius are common in the water tracks, often fringed by the rushes Juncus effusus and Juncus bulbosus. Pale butterwort Pinguicula lusitanica is widely distributed along the margins of the fen.

Crymlyn Bog/ Cors Crymlyn East Wales
Transition mires and quaking bogs occur on deep, wet topogenous peats over a relatively small area of this extensive coastal lowland site. Bottle sedge Carex rostrata and bogbean Menyanthes trifoliata are important components of some stands, together with common cottongrass Eriophorum angustifolium, water horsetail Equisetum fluviatile, star sedge Carex echinata, the locally rare mud sedge Carex limosa and, in places, the nationally rare slender cottongrass Eriophorum gracile. The floristic character of some of these stands displays affinities to NVC types M9 Carex rostrataCalliergon cuspidatum/giganteum mire and S27 Carex rostrataPotentilla palustris tall-herb fen, but other expressions of this habitat at Crymlyn include a stronger poor-fen element in which bog-mosses (including Sphagnum squarrosum, S. denticulatum and S. fimbriatum) figure prominently. The transition mire and quaking bog at this site is vulnerable to the continuing expansion of common reed Phragmites australis, encouraged by trends of increasing site wetness, nutrient-enrichment and lack of grazing.
Derryleckagh Northern Ireland
Derryleckagh in Northern Ireland is a large example of an inter-drumlin wetland with well-developed transition mire vegetation of the basin mire type. Although a large part of the site is dominated by tall reedswamp, it still contains extensive areas of M9 Carex rostrataCalliergon cuspidatum/giganteum mire dominated by lesser tussock-sedge Carex diandra, Sphagnum contortum and brown mosses. Within this fen vegetation are well-developed pools and soakways with marsh St John’s-wort Hypericum elodes prominent. The site contains several rare or localised invertebrate species.
Emer Bog Hampshire and Isle of Wight
Emer Bog lies in a wet infilled hollow on the developed eastern hinterland of the New Forest. Apart from scattered willow Salix scrub, it is largely open, and dominated by bottle sedge Carex rostrata and marsh cinquefoil Potentilla palustris, with frequent common cottongrass Eriophorum angustifolium, and occasional pools with bogbean Menyanthes trifoliata. White sedge Carex curta and the bog-mosses Sphagnum fimbriatum and S. squarrosum become common at the edge of the bog, with the rushes Juncus effusus and J. acutiflorus. There are also patches of common reed Phragmites australis. The basin is surrounded by more mature willow Salix woodland and open heathland.
Fen Bog North Yorkshire

This valley mire lies in Newtondale, a deep glacial spillway in the North Yorkshire Moors. The peat deposit is up to 18 metres deep, and is now mostly covered with acidophilous mire vegetation. The following plants are abundant: the bog-mosses Sphagnum papillosum and S. capillifolium, common cottongrass Eriophorum angustifolium, deergrass Trichophorum cespitosum, purple moor-grass Molinia caerulea, cross-leaved heath Erica tetralix, bog-myrtle Myrica gale, round-leaved sundew Drosera rotundifolia, tormentil Potentilla erecta and heath milkwort Polygala serpyllifolia. White beak-sedge Rhynchospora alba is locally abundant.

One of the important features of this site is the development of lateral water tracks containing a plant association more usually characteristic of mires in oceanic regions. A number of species occurring in these communities at Fen Bog do not occur elsewhere in north-east England and are very locally distributed outside western districts. These soligenous mire associations, some of which show the influence of base-rich water, include the bog-mosses Sphagnum [auriculatum] and S. recurvum, the sedges Carex rostrata, C. limosa, C. echinata and C. dioica, bog pondweed Potamogeton polygonifolius, many-stalked spike-rush Eleocharis multicaulis and bogbean Menyanthes trifoliata.

Insh Marshes Highlands and Islands
Insh Marshes in north-east Scotland is the largest transition mire in the UK. This site is representative of the flood plain mire type. The vegetation is a relatively uniform area of S27 Carex rostrataPotentilla palustris tall-herb fen in which Sphagnum is found locally. String sedge Carex chordorrhiza is a rare sedge that occurs at this site and only one other, Scottish, site in the UK. It is more frequently found in this mire type in continental Europe.
Inverpolly Highlands and Islands
This extensive and diverse upland site in north west Scotland supports numerous soligenous mires, readily identified within the wider blanket bog communities by their relative species-richness. Particularly characteristic of the site are areas dominated by black bog-rush Schoenus nigricans, the best example of which occurs within a small oceanic valley mire west of Loch Scionscaig. Other species associated with these areas include bogbean Menyanthes trifoliata, slender sedge Carex lasiocarpa, bog sedge Carex limosa and lesser clubmoss Selaginella selaginoides. Wetter areas support intermediate bladderwort Utricularia intermedia.
Monadh Mor Highlands and Islands
Monadh Mor is the last remnant of an extensive complex of woodland and swamp communities with Scandinavian affinities which once occupied a large part of the summit of the Black Isle ridge in north-east Scotland. The site supports a complex mosaic of well-drained ridges and wet hollows formed by glacial processes with a resultant landscape supporting many areas of open water. The wet hollows support lochans and the streams into which they drain are fringed by extensive stands of M4 Carex rostrataSphagnum recurvum mire which represent relatively extensive examples of Transition mires and quaking bogs. These in turn grade into M18 Erica tetralixSphagnum papillosum mire with scattered trees, the 91D0 Bog woodland habitat for which this site has also been selected.
North West Pembrokeshire Commons/ Comins Gogledd Orllewin Sir Benfro West Wales and The Valleys
Habitat occurrence account not yet available.
Oak Mere Cheshire
Open water and peat deposits lie in this kettle-hole depression within Delamere Forest, and peat-cutting has given rise to additional pools and fens. The water is acidic, but slightly nutrient-rich. There are transitions at the water’s edge with soft rush Juncus effusus, water horsetail Equisetum fluviatile, common spike-rush Eleocharis palustris, marsh pennywort Hydrocotyle vulgaris, the moss Drepanocladus fluitans and bulrush Typha latifolia. Small depressions in the peat are occupied by bottle sedge Carex rostrata, common cottongrass Eriophorum angustifolium, purple moor-grass Molinia caerulea, cross-leaved heath Erica tetralix and round-leaved sundew Drosera rotundifolia.
Rannoch Moor Eastern Scotland, Highlands and Islands
Rannoch Moor in west Scotland is an unusual ecological variant of transition mire developing in the midst of 7130 Blanket bogs, and supporting Rannoch-rush Scheuchzeria palustris at its only known location in the UK. Such vegetation, clearly belonging to the Scheuchzerietalia palustris order, is developed in regeneration surfaces within raised and blanket bogs elsewhere in Europe, but in this site Rannoch-rush is also found in situations where the blanket bog is locally influenced by mineral-rich groundwater. The community containing Rannoch-rush has some affinities to M1 Sphagnum auriculatum bog pool community and is a distinctive and unusual type. Other types of transition mire occur widely, including ladder fens.
Rhos Goch East Wales
The transition mire and quaking bog at Rhos Goch manifests as a suite of poor-fen swamp communities juxtaposed within the context of a lagg zone between 7110 active raised bog and rush pasture. A wide range of communities are present, extending from S27 Carex rostrataPotentilla palustris tall-herb fen and M5 Carex rostrataSphagnum squarrosum mire through to swamp vegetation more strongly dominated by single species such as bottle sedge Carex rostrata, water horsetail Equisetum fluviatile and common spike-rush Eleocharis palustris.
Shortheath Common Hampshire and Isle of Wight
A valley mire forms the focal point of this site in the western Weald which also embraces a wide range of heathland habitats and woodland. The northern strip of the mire is the most mesotrophic and has much grey willow Salix cinerea but also a rich ground-flora with abundant sedges Carex curta and C. rostrata, soft rush Juncus effusus, marsh cinquefoil Potentilla palustris and the bog-moss Sphagnum recurvum. An oligotrophic area to the south is dominated by S. recurvum with cross-leaved heath Erica tetralix, common cottongrass Eriophorum angustifolium, purple moor-grass Molinia caerulea and round-leaved sundew Drosera rotundifolia. It is notable for its high cover of cranberry Vaccinium oxycoccos. Other bog-mosses such as Sphagnum capillifolium and S. papillosum are also present, and the whole forms a floating raft over much of the mire.
Subberthwaite, Blawith and Torver Low Commons Cumbria
This site in south-west Cumbria supports some of the best examples of Transition mires and quaking bogs in the UK, with over 200 mires on a broad hilly plateau. The mires are dominated by tall sedges and rushes with mixed herbs, over a ground layer of bog-mosses Sphagnum spp. and feather-mosses including Calliergon cuspidatum. Twenty-six NVC types are represented, including M4 Carex rostrataSphagnum recurvum mire, M9 Carex rostrataCalliergon cuspidatum/giganteum mire, and S27 Carex rostrataPotentilla palustris tall-herb fen.
Tarn Moss Cumbria
This basin mire has developed in a shallow, elongated hollow in acidic glacial drift. The mire communities are of special interest comprising areas of typical acid bog within a matrix of poor-fen. In comparison to other Cumbrian basin mires, Tarn Moss is remarkable in being almost entirely devoid of tree or scrub cover, as well as being little disturbed with no obvious signs of past peat-cutting.

The poor-fen is the most extensive and best developed community at Tarn Moss. It is characterised by the dominance of Sphagnum bog-mosses and sedges Carex species, the latter including C. curta, C. echinata, C. rostrata and the very local northern species, C. magellanica. Other species include Hydrocotyle vulgaris, marsh cinquefoil Potentilla palustris, water horsetail Equisetum fluviatile, lesser bladderwort Utricularia minor, marsh violet Viola palustris, common marsh-bedstraw Galium palustre, lesser spearwort Ranunculus flammula, bog asphodel Narthecium ossifragum and cranberry Vaccinium oxycoccos. Small nuclei of more acid vegetation occur throughout the surface and in places merge to form larger patches of acid mire, dominated by heather Calluna vulgaris and cross-leaved heath Erica tetralix over bog mosses. Other species include bog-rosemary Andromeda polifolia and crowberry Empetrum nigrum.

The Broads East Anglia
The Broads contain examples of transition mire in a flood plain in the south-eastern part of the UK, where the habitat is rare. The areas of transition mire, mainly of M5 Carex rostrataSphagnum squarrosum mire, M9 Carex rostrataCalliergon cuspidatum/giganteum mire and S27 Carex rostrataPotentilla palustris tall-herb fen, are relatively small, having developed in re-vegetated peat-cuttings as part of a complex habitat mosaic of fen, carr and open water.
Turmennan Northern Ireland
Turmennan is one of the best examples in Northern Ireland of a transition mire. The edaphic conditions are influenced by the position of the groundwater table in the surface peat layer, which is partly related to the extent of past peat-cutting over the site. The main plant community of the mire surface is formed by an extensive mixed sedge sward dominated by bottle sedge Carex rostrata and including species such as common sedge C. nigra, star sedge C. echinata, ragged-robin Lychnis flos-cuculi, marsh pennywort Hydrocotyle vulgaris and water horsetail Equisetum fluviatile. These species grow over a carpet of brown mosses dominated by Calliergon cuspidatum and C. cordifolium. In places where the water is more acidic, bog-mosses such as Sphagnum squarrosum and S. subnitens replace the brown mosses. Much of the central area of the site is dominated by common reed Phragmites australis. Other, more localised fen communities include a mixed sedge sward dominated by lesser tussock-sedge Carex diandra, floating mats of bogbean Menyanthes trifoliata and ‘soakways’ with the notable marsh St John’s-wort Hypericum elodes prominent.
West Midlands Mosses Cheshire, Shropshire and Staffordshire
West Midlands Mosses represents Schwingmoor vegetation. Floating rafts of Sphagnum-dominated vegetation have developed over semi-liquid substrates within basins. In the UK this type of Sphagnum-dominated vegetation with a scatter of sedges Carex species and cranberry Vaccinium oxycoccos is confined to this part of England and mid-Wales.
Whitlaw and Branxholme Eastern Scotland
Within this site in south-east Scotland transition mires occur as a part of valley mires and basin mires, in complexes of vegetation containing bog and fen vegetation. The vegetation, which is mainly of NVC type M9 Carex rostrataCalliergon cuspidatum/giganteum mire, is therefore not only floristically intermediate between fen and bog, but occupies transitional positions within vegetation zonations. The site provides excellent representation of transition mire, being transitional both in vegetation types (especially the occurrence of bog-moss Sphagnum with other, more fen-type, species such as lesser tussock-sedge Carex diandra) and in hydrological characteristics.

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

Berwyn a Mynyddoedd de Clwyd/ Berwyn and South Clwyd Mountains East Wales, West Wales and The Valleys
Cairngorms Highlands and Islands, North Eastern Scotland
Caithness and Sutherland Peatlands Highlands and Islands
Cors Caron West Wales and The Valleys
Dunkeld - Blairgowrie Lochs Eastern Scotland
Garron Plateau Northern Ireland
Gweunydd Blaencleddau West Wales and The Valleys
Lendalfoot Hills Complex South Western Scotland
Loch of Isbister Highlands and Islands
Mointeach nan Lochain Dubha Highlands and Islands
Muir of Dinnet North Eastern Scotland
Pitkeathly Mires Eastern Scotland
River Wye/ Afon Gwy East Wales, Gloucestershire, Wiltshire and Bristol/Bath area, Herefordshire, Worcestershire and Warwickshire, West Wales and The Valleys
Sligachan Peatlands Highlands and Islands
South Pennine Moors Cheshire, Derbyshire and Nottinghamshire, Greater Manchester, Lancashire, North Yorkshire, Shropshire and Staffordshire, South Yorkshire, West Yorkshire
The New Forest Gloucestershire, Wiltshire and Bristol/Bath area, Hampshire and Isle of Wight
Woolmer Forest Hampshire and Isle of Wight
 

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