Fen

Fens are peatlands that receive their water from both precipitation and groundwater. Depending on the underlying geology, the groundwater feeding fens can be mineral-rich, supporting dense vegetation. Contact with groundwater makes fens less acidic than bogs; however, fens are still slightly acidic environments. Fens are diverse, and their appearance is driven by both their nutrient content and the acidity of the water. They are also transitional habitats, and some lowland fens would develop into wet woodlands or raised bogs over time, if left alone.

Fens exhibit a rich diversity of species and support around a third of all native plant species in the UK, as well as half of all dragonfly species, a range of aquatic beetles and thousands of other insect species. Water voles (Arvicola amphibius) and harvest mice (Micromys minutus) can also be found in fens, as well as birds such as marsh harriers (Circus aeruginosus), warblers and bitterns (Botaurus stellaris). 

Base-rich fens (also known as ‘rich-fens’) are associated with calcareous water (pH 5 or higher) which has been in contact with a base-rich rock such as limestone. They are typically found in the lowlands, and often appear as flat, expansive landscapes covered with dense vegetation interspersed with streams, ponds and ditches. Lowland fens often form in shallow lake basins with poor drainage and a constant inflow of water which causes waterlogging. Such habitats are found in the Norfolk and Suffolk Broads and the Cambridgeshire fens.  

These habitats feature reeds and sedges such as the common reed (Phragmites australis), great fen-sedge (Cladium mariscus), common sedge and yellow sedge (Carex demissa), and a variety of other flowering plants including grass-of-Parnassus (Parnassia palustris), marsh marigold (Caltha palustris) and purple loosestrife (Lythrum salicaria). Orchids such as the southern marsh-orchid (Dactylorhiza praetermissa), common spotted-orchid (Dactylorhiza fuchsii), marsh helleborine (Epipactis palustris) and the rare narrow-leaved marsh orchid (Dactylorhiza traunsteineri) and fen orchid (Liparis loeselii) are also characteristic. 

Base-rich fens can also be found in the uplands. However, Upland fens and flushes are often small in extent, occur on shallow peat and are dominated by lower-stature vegetation, with rich diversity of flowering plants.

Base-poor fens (also known as ‘poor-fens’) on the other hand are associated with acidic water (pH 5 or lower) which has been in contact with sandstone or granite. These fens occur mainly in the uplands and resemble bogs, with Sphagnum, cottongrasses and heather growing on the surface. The largest continuous base-poor fen in the UK covers 300 ha and is found in Insh Marshes on the floodplain of the River Spey in Scotland. 

The nutrient-rich fen peat means that these are some of the most valuable crop growing regions in the UK, which has led to drainage and the adoption of damaging agricultural practices over vast areas of lowland fens. Find out more about threats to peatlands.