A fen is a type of wetland that is supplied primarily by groundwater or surface waters enriched with dissolved minerals. Unlike acidic bogs, fens typically have neutral to alkaline pH and relatively high concentrations of dissolved minerals but low levels of readily available nutrients. These conditions support a distinctive community of grasses, sedges and so-called brown mosses (for example Scorpidium and Drepanocladus), and in many regions a diverse flora that can include carnivorous plants such as butterworts (Pinguicula).

Characteristics and typical species

Fens are classed as minerotrophic wetlands because their chemistry is strongly influenced by mineral-rich groundwater or surface flow. Common physical and biological traits include:

  • Neutral to alkaline water chemistry with higher dissolved calcium and magnesium than bogs.
  • Peat formation, though peat in fens is often thinner or less acidic than in bogs.
  • Vegetation dominated by sedges (Carex species), reed grasses, and a mosaic of mosses — especially brown moss genera — with shrubs or trees present only where water levels drop.
  • High local plant species richness and the presence of certain "fen indicator" species that reveal water regime and nutrient status.

Formation, distribution and variation

Fens form where mineral-rich groundwater reaches the surface or where surface waters flow slowly over mineral soils, such as in valley bottoms, along lake margins and river floodplains, and around springs. Regional climate, geology and hydrology shape their character: some fens are permanently saturated, others experience seasonal fluctuations that create a shifting mosaic of wetter and drier zones. They occur in temperate and boreal regions worldwide and present a broad range of subtypes, from calcareous fens fed by alkaline springs to minerotrophic marshes influenced by river flooding.

Ecological importance and uses

Fens provide many ecosystem services. They offer habitat for rare and specialized plants and animals, support pollinators and birds, store organic carbon in peat soils, and help moderate water flows by absorbing and slowly releasing water. Because fen plant assemblages are sensitive to changes in water chemistry and hydrology, they also act as indicators of groundwater quality and landscape-scale environmental change.

Threats, management and restoration

Historically, fens have been degraded by drainage for agriculture, peat extraction, channelization, and nutrient enrichment from surrounding land. These impacts lower water tables, alter chemistry and promote invasion by woody plants. Modern conservation focuses on restoring natural hydrology and protecting water quality: common measures include blocking drains, reinstating groundwater flows, controlling nutrient inputs, and using grazing or mowing to prevent scrub encroachment. Successful restoration usually requires long-term monitoring of water levels and vegetation.

Further information and resources

Because fens are defined by their hydrology and chemistry rather than by a single list of species, local surveys and hydrological studies are often needed to identify and manage specific fen sites. Conservation success depends on protecting groundwater sources and maintaining the subtle balance of water, minerals and vegetation that gives each fen its character.