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

UK status and distribution

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.

Click here view UK distribution of this species

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 Cladium – Molinia 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 australis – Peucedanum 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 where this Annex I habitat is a qualifying feature, but not a primary reason for site selection

• Asby Complex Cumbria
• Corsydd Llŷn/ 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