What is it?
Lowland fen is often found in small fragments, isolated by intensively managed farmland, but there are also more extensive areas in wetlands that have escaped drainage. The vegetation generally comprises typical wetland species such as reeds, rushes and sedges, including either tall robust species such as great fen-sedge and common reed, or a suite of small sedges such as common sedge and yellow sedges.
On managed sites, the structure of fen can be very diverse with open pools, wet hollows, mown areas, tussocky areas with deep litter and transitions to swamp and wet woodland, and this makes for a very diverse habitat. Around one third of all plant species native to the UK can be found in fens (although not necessarily specific to the habitat), together with half of the dragonfly species and thousands of other invertebrate species, notably beetles and flies.
Why is it like this?
Fen vegetation forms in shallow valleys and lake basins where drainage is poor and a constant flow of water or periodic flooding causes waterlogging. It is essentially a transitional habitat that would, over time, develop into wet woodland or, if the growth of peat is sufficient to isolate the vegetation from the groundwater, into raised bog. Traditional use of fenland, including mowing, grazing and peat digging, stalled this process and created diverse areas of open fen rich in wildlife. Human activities have also played a role in the formation of fens – in the Norfolk and Suffolk Broads, fen is a secondary habitat that has developed on areas of flooded medieval peat diggings.
Fens are influenced by the fertility and pH of the water and substrate. Small-sedge fens near springs and seepages that are very infertile and lime-rich support black bog-rush and blunt-flowered rush with small sedges and herbs in runnels. The tall-herb fens of East Anglia growing on more fertile and less lime-rich peat support bulky reeds and sedges including common reed, saw sedge, reed canary-grass and reedmace with wild angelica, purple loosestrife, hemlock water-dropwort, meadowsweet, yellow iris and hemp agrimony among other species.
Such is the diversity of fen types that some are considered to be part of other habitats: particularly species-rich areas known as fen meadow can result from regular mowing and grazing in slightly drier areas, and are described within purple moor-grass and rush pasture. ‘Poor fens’ are fens with fewer species found in more acidic conditions and include heathland valley mires, which are described under wet heath.
Distribution in the UK
Widely scattered across the lowlands but restricted in distribution. Extensive areas are confined to East Anglia, Anglesey and the Lough Erne system in Northern Ireland.
What to look for
Look out for several orchid species, including early and southern marsh-orchids, common spotted-orchid and marsh helleborine – some sites also support the rare narrow-leaved marsh orchid or fen orchid. The small and delicate white flowers of grass-of-Parnassus can be found along mown rides or in small-sedge fens. In the Norfolk and Suffolk broads, look out for very tall nettles that are stingless and scarce species such as cowbane and greater water-parsnip. Also in the Broads, look for the spectacular swallowtail butterfly laying its eggs on tall, prominent milk-parsley plants and the Norfolk hawker dragonfly hunting over reed-fringed, unpolluted, freshwater ditches.
Although fens provide suitable habitat for a variety of mammals, including water vole and harvest mouse, these tend to be hard to see, but keep an eye out for Chinese water-deer, introduced to the Cambridgeshire fens and Broads in the 19th century and now considered vulnerable in its native East Asia.
The decline of traditional management practices has, combined with drainage, resulted in the loss of significant amounts of fen. The quality of remaining areas is often compromised by water extraction nearby or the prevention of flooding from nearby rivers and by the nutrients from agricultural run-off. At some key sites, attempts have been made to maintain higher water tables, including irrigation and the installation of an impermeable membrane. Linking and buffering existing sites is a vital step and is being carried out in large landscape-scale projects such as the Great Fen Project.