Irish Fen Habitat - Pollardstown Fen, Co. KildareA fen is a wetland system with a permanently high water level at or just below its surface. It’s principal source of nutrients is from surface or groundwater and the substrate is an alkaline to slightly acidic peat soil. The vegetation is usually dominated by sedges. Fens occur throughout the country, most commonly in the west and midlands of Ireland.

Distribution Map of the Fens of Conservation Importance in IrelandDistribution of Fens in Ireland

Fens tend to occur in limestone regions of Ireland where the water supply is sufficiently rich in minerals. They occur throughout the country most commonly in the west and Midlands of Ireland.

This map shows the distribution of conservation worthy fens in Ireland. Source: Ireland’s Peatland Conservation Action Plan 2020 published by the IPCC in 2009.

Formation of Fens

The formation of fen peatland habitats in Ireland

Fens began to develop 10,000 years ago in depressions occupied by shallow lakes in which anaerobic conditions occurred. Without oxygen the complete decomposition of plant material is prevented.

In time this un-decomposed plant material forms a thick layer of peat that rises towards the surface of the lake. Eventually the surface peat is invaded by sedges to form a fen.

Fen Structure and Hydrology


Irish Fen Types

Fens may be subdivided into two major groups based upon their topography and hydrology, they are; topogenous fens and soligenous fens. There are three types of topogenous fen and three types of soligenous fen.

Topogenous fens form within basins in the landscape where the water fluctuations are in a vertical direction. Water accumulates on account of the topography.

Soligenous fens are formed where sloping terrain provides a continuous supply of flowing water, the horizontal movement of water is more important than the vertical movement of water.

Fens can also be classified, on floristic and chemical grounds, into rich fens and poor fens, although these categories may merge. The main difference in vegetation types that occur on fens depends primarily on whether the fen is described as a “rich” (basic) or “poor” (acid) fen.

Rich fens and flushes are fed by water that is base-rich or calcareous, and are mainly found in areas overlying limestone. The peat substrate is alkaline. Blunt flowered rush (Juncus subnodulosus) is an indicator species in the vegetation of rich fens.

Poor fens and flushes are fed by water that is base poor or acid, but not as acidic as raised bogs. The underlying substrate consists of acid peat. Star sedge (Carex echinata) is an indicator species in the vegetation of poor fens.

Between these extremes are transition or quaking fens, which are in between being base rich and base poor. Lesser tussock sedge (Carex diandra) is an indicator species in the vegetation of transition or quaking fens.

Extent and Utilisation of Fens

The original extent of fens in the Republic of Ireland was 92,508ha (Hammond Peatlands Map of 1979). IPCC monitors the status of the resource on an on going basis.  Our latest assessment in 2009 was published in Ireland’s Peatland Conservation Action Plan 2020. The most serious impact on fens has been their reclamation for agricultural land which involves drainage, fertilisation, reclamation and removal of peat. As a result 77% have been lost to this use, leaving 20,912ha suitable for conservation.

The Utilisation of Fens in the Republic of IrelandA number of these sites are secondary fens created by the activity of man. Examples of these are in cutaway bog areas, the margins of raised bogs, the Shannon callows and lake shores where water levels have been artificially lowered. More recent impacts are from enrichment, infilling and dumping, which can destroy a site or seriously degrade its quality. Fens are particularly sensitive to damage from these activities.

Value and Importance of Fens

  • Fens have a high biodiversity of species and habitats. Up to 1000 species have been recorded in fens.
  • Fens act in a number of ways to regulate the environment. Their functions include water purification, flood prevention and carbon storage.
  • Fens have a high value for sporting, recreation and fishing as they are often intimately associated with freshwater bodies.
  • Fens are important breeding sites and feeding areas for birds.
  • The peat in fens contains an informative record of the past.

Conservation Status of Irish Fens

Under the EU Habitats Directive the National Parks and Wildlife Service (NPWS) are obliged to complete a report every 7 years on the condition of designated habitats. The assessments carried out found that 10 years after the introduction of the habitats directive, no peatland type of priority importance in Ireland is in good conservation status. Fens were given a BAD assessment in this report due to the loss of their area, structure and function. The drivers of the loss of fens include peat mining, draining for cropland, infilling, fertiliser pollution and eutrophication.

Text, Photographs and Images © Irish Peatland Conservation Council, Bog of Allen Nature Centre, Lullymore, Rathangan, Co. Kildare.  Email: bogs@ipcc.ie; Tel: +353-45-860133.