Fens are minerotrophic because they receive water & nutrients from rainfall, soil, rock & ground water. The water is typically base-rich from neutral to alkaline & fens are more nutrient-rich than bogs.

Key Characteristics

  • Site: Base-rich water, less acidic & more nutrient-rich than bogland
  • Soil: Minerotrophic
  • Main species: Sphagnum moss
  • UK: Unknown
  • NI: 2,950 ha

Habitat Description

  • Fens receive water & nutrient supply from soil, rock & groundwater in addition to precipitation.
  • The NIEA Action Plan (2005) classifies fen into two types: ‘poor-fens’ & ‘rich-fens’. ‘Poor-fens’ occur mostly in the uplands but some are associated with lowland heaths or raised bogs, the water is derived from base-poor rock such as sandstone & granite, short vegetation is a dominant feature, including Sphagnum mosses & the water is acidic. ‘Rich-fens’ receive calcareous water of >pH5 that is mineral-enriched & mainly occur in lowlands but also in uplands where there are base-rich rocks.
  • Vegetation composition differs in ‘poor’ & ‘rich’ fens. Sphagnum moss species dominate ‘poor-fens’, forming a moss layer but they also contain sedges such as bottle sedge Carex rostrata, star sedge C.echinata, common sedge C.nigra & purple moor-grass Molinia caerulea. Vegetation in ‘rich-fens’ is typically more diverse. It includes bog pimpernel Anagallis tenella, meadow thistle Cirsium dissectum, saw sedge Cladium mariscus, marsh helleborine Epipactis palustris, blunt-flowered rush Juncus subnodulosus, grass-of-parnassus Parnassia palustris, common butterwort Pinguicula vulgaris, black bog-rush Schoenus nigricans & bladderworts Utricularia species (NIEA, 2005).
  • Other plant communities which occur in fens include swamp vegetation such as bulrush Scirpus spp, flowering-rush Butomus umbellatus, reed-mace Typha species & a variety of tall sedges.
  • § Many plant communities that are similar to those listed in the NVC of Great Britain grow in fens of Northern Ireland, including: M4 Carex rostrata – Sphagnum recurvum mire (poor fen, transition mire), M5 Carex rostrata-Sphagnum squarrosum mire (transition mire),M6 Carex echinata Sphagnum r
  • ecurvum/auriculatum mire (poor-fen), M9 Carex rostrata-Calliergon cuspidatum mire (transition mire, rich-fen), M10 Carex dioica – Pinguicula vulgaris mire (rich fen, rare in lowlands), M13 Schoenus nigricans – Juncus subnodulosus mire (rich-fen, rare), M22 Juncus subnodulosus – Cirsium palustre rich fen (local) & S27 Carex rostrata – Potentilla palustris fen (transition mire).
  • § Fen & swamp communities are often within a mosaic of various habitats such as raised bog, wet grassland, wet woodland & open water plant communities (NIEA, 2005).

Current Status in UK & Northern Ireland

  • Fens in the UK comprise a large proportion of those in the EU but a precise, published estimate of the extent of these is lacking. A significant proportion of the UK fen resource occurs in NI (NIEA, 2005).
  • Fens are a declining habitat due to drainage and control of water levels.
  • The NICS (2000) estimates that lowland fen covers 2,950 ha, but this is a probable underestimate. Within a ten year period (1988-1998) there was a decline of 18% (484 ha) lowland fen but no change in swamp area.
  • A preliminary evaluation shows that a significant proportion of the fen vegetation is in ‘unfavourable condition’ due to drainage, nutrient enrichment & a general lack of management (NIEA, 2005).

Designated Sites

  • Main Valley Bogs

Further Information