fen

A fen is a type of wetland that is sustained primarily by mineral-rich groundwater and surface water. Compared with bogs, fens are usually less acidic and higher in nutrients, supporting more diverse plant communities and ongoing peat formation. The term reflects its hydrological dependence on groundwater inputs rather than rainfall alone.

Fens form where groundwater and mineral-rich surface waters discharge to the surface, often in valley bottoms, on limestone or other calcareous substrates, or where springs supply nutrients to the wetland. The mineral content raises pH and provides nutrients such as calcium, magnesium, and phosphorus, influencing the plant life and soil chemistry. Soils are typically peat-rich but can include substantial mineral inputs, and water tables remain high enough to create persistent wetland conditions.

Vegetation in fens is dominated by hydrophilic grasses, sedges, reeds, and rushes, with a lower presence of Sphagnum moss compared with bogs. Common groups include Carex species, Calamagrostis, and Juncus, along with a variety of flowering plants adapted to nutrient-rich, moist soils. Fen ecosystems can host a range of invertebrates and bird species that rely on open, wet meadow-like conditions. Ecologists distinguish rich fens, with higher nutrient levels and plant diversity, from poor fens, which are more acid and moss-dominated.

Conservation and management focus on maintaining natural hydrology and limiting nutrient and sediment inputs. Draining, fertilization, peat extraction, and climate change threaten fen extent and function. Restoration efforts aim to reestablish groundwater inputs, reconnect pathways for water flow, and re-create appropriate vegetation communities. Fens occur in many temperate regions around the world, including parts of Europe, North America, and Asia; the fenland landscapes of eastern England are a well-known cultural example.