Habitat account - Raised bogs and mires and fens

7210 Calcareous fens with Cladium mariscus and species of the Caricion davallianae  * Priority feature

Background to selection

Distribution of SACs/SCIs/cSACs with habitat 7210 Calcareous fens with Cladium mariscus and species of the Caricion davallianae.  Click image for enlarged map.
Distribution of SACs/SCIs/cSACs with habitat 7210 Calcareous fens with Cladium mariscus and species of the Caricion davallianae. Click image for enlarged map.

Description and ecological characteristics


This Annex I type comprises the more species-rich examples of great fen-sedge Cladium mariscus fen, particularly those stands enriched with elements of the Caricion davallianae (i.e. small-sedge fen with open low-growing sedge vegetation). Davall’s sedge Carex davalliana itself is extinct in the UK. Such stands occur in the following situations:



  1. sites with a mixture of closed, species-poor Cladium beds, which at their margins have transitions to species-rich small-sedge mire vegetation;
  2. sites where Cladium beds retain their species-richness owing to management; and
  3. situations where Cladium fen is inherently species-rich, possibly owing to the fact that conditions do not allow the Cladium to grow vigorously and dominate the vegetation.


The NVC types which are associated with (but not necessarily confined to) this Annex I type, where they support abundant C. mariscus, are:



  • S2  Cladium mariscus swamp and sedge beds
  • S24  Phragmites australis – Peucedanum palustris tall-herb fen
  • S25  Phragmites australis – Eupatorium cannabinum tall-herb fen
  • M9  Carex rostrata – Calliergon cuspidatum/ giganteum mire
  • M13  Schoenus nigricans – Juncus subnodulosus mire
  • M14  Schoenus nigricans – Narthecium ossifragum mire
  • M24  Molinia caerulea – Cirsium dissectum fen-meadow
  • SD14  Salix repens – Campylium stellatum dune slack community
  • SD15  Salix repens – Calliergon cuspidatum dune slack community


At most sites several of these types are found as complex mosaics with other fen types, and in most cases the species-rich stands are less extensive than species-poor Cladium vegetation.

European status and distribution



This habitat type is generally rare, but has a wide European distribution.

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


Calcareous fens are rare in the UK, having a restricted and discontinuous geographical range with two main centres of distribution: the Broadlands of East Anglia and, to a lesser extent, the fen systems of Anglesey. This habitat type is very scattered and local elsewhere in the UK.

Site selection rationale


Site selection for this priority habitat type takes account of the fact that calcareous fens are rare in the UK. The sites selected include a very high proportion of the total UK resource of the habitat type, including the largest surviving examples.


Site selection has favoured those sites where Cladium stands are most extensive and which show the widest range of habitat conditions in which species-rich Cladium vegetation occurs (see above). Selection has also taken account of the variation in hydrological situations, from flood plain mires and isolated spring-fed systems through to valley fens, and the geographical range of the Annex I type in the UK. Small, isolated, species-poor habitat fragments where Cladium is found, for example drainage ditches in grassland sites and in heathlands (where stands are more extensive than at grassland sites, but are more species-poor) have not been selected.

Site accounts

Corsydd Môn/ Anglesey Fens West Wales and The Valleys
The Anglesey Fens complex supports the second-largest area of calcareous fens in the UK. In some parts the low vigour of Cladium accounts for the species-richness of the vegetation, but elsewhere management prevents the development of monodominant stands, enabling the persistence of communities referable to the Caricion davallianae. The juxtaposition between species-poor stands of Cladium and areas with a more diverse floristic composition and structure is widespread, with characteristically species-rich contact zones between the two. Anthropogenic disturbance is believed to have been instrumental in the develop of various facies of a CladiumMolinia community, a particular feature of the rich fens of north-west Wales.
Crymlyn Bog/ Cors Crymlyn East Wales
Crymlyn supports the largest area of Cladium-dominated vegetation in south Wales. Many of the stands in which great fen-sedge Cladium mariscus occurs as sole dominant are typically species-poor, but other areas display a more diverse vegetation in which tufted-sedge Carex elata, royal fern Osmunda regalis and a range of tall-herb fen species are prominent. Some of these stands are unique in a Welsh context and are strongly reminiscent of NVC type S24 Phragmites australisPeucedanum palustre tall-herb fen, which is otherwise largely confined to eastern England.
Fenland East Anglia
The individual sites within Fenland cSAC each hold large areas of calcareous fens, with a long and well-documented history of regular management. There is a full range from species-poor Cladium-dominated fen to species-rich fen with a lower proportion of Cladium and containing such species as black bog-rush Schoenus nigricans, tormentil Potentilla erecta and meadow thistle Cirsium dissectum. There are good transitions to purple moor-grass Molinia caerulea and rush pastures, all set within a mosaic of reedbeds and wet pastures.
Holme Moor and Clean Moor Dorset and Somerset
This is a relatively small site but it is important as an outlier of calcareous fens in south-west England, where Cladium is a local and rare species. The site occupies an unusual ecological situation on the spring line at the foot of a scarp slope. Although not currently managed, management continued until comparatively recently and in part explains the high species-richness of this site. There are important species-rich transitions from Cladium fen to mire with black bog-rush Schoenus nigricans and blunt-flowered rush Juncus subnodulosus, as well as to fen-meadow vegetation with purple moor-grass Molinia caerulea and meadow thistle Cirsium dissectum.
The Broads East Anglia
This flood plain mire site in East Anglia has the largest example of calcareous fens in the UK and possibly the largest occurrence in the EU outside Sweden. The Cladium habitat occurs in a diverse set of conditions that maintain its species-richness, including managed Cladium fen, contacts between Cladium beds and small sedge mires, and situations where Cladium occurs at the limits of its ecological range. The habitat type forms large-scale mosaics with other fen types, open water and woodland, and important associated plant species are the Annex II 1903 Fen orchid Liparis loeselii (found at Upton Fen), marsh helleborine Epipactis palustris, lesser tussock-sedge Carex diandra, slender sedge C. lasiocarpa and fibrous tussock-sedge C. appropinquata.
Waveney and Little Ouse Valley Fens East Anglia
This site occurs in the East Anglian centre of distribution of calcareous fens and contains very extensive Cladium beds, including managed examples, as well as stands in contact zones between small sedge mire and species-poor Cladium. The habitat type here occurs in a different hydrological situation to the Broads – spring-fed valley fen rather than flood-plain mire.

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

Asby Complex Cumbria
Corsydd Llyn/ Lleyn Fens West Wales and The Valleys
Dorset Heaths Dorset and Somerset
Dorset Heaths (Purbeck and Wareham) and Studland Dunes Dorset and Somerset
Magheraveely Marl Loughs Northern Ireland
Morecambe Bay Pavements Cumbria, Lancashire
Norfolk Valley Fens East Anglia

Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.