Feeding into the Wash from the Midlands and East Anglia are four great rivers, the Welland, Witham, Nene and Ouse, the last of which flows through the west of Norfolk. The lower catchments and floodplains of these rivers make up the once huge wetland known as the Fens. Confusingly the Fens also historically contained much habitat known as fen. In its simplest definition a fen is a marsh, rich in specialised plants and other organisms, which is fed by alkaline water from the underlying chalk. Fen habitat is found in many parts of Norfolk, principally in the Fens, Broadland, the Brecks and along major river valleys such as the Wensum.
Like many of Norfolk’s habitats, fen occurs as a result of both natural conditions and human management. Where vegetation on low-lying, frequently flooded alkaline soils has been grazed for centuries or regularly cut for hay, thereby stalling succession to wet woodland known as carr, a diverse fen flora has often developed. Typical plants, among many species found in this habitat, include hemp agrimony, purple loosestrife, greater bird’s-foot trefoil, southern marsh orchid, fen bedstraw and great water dock. Many dragonflies, including the rare Norfolk hawker, are found in fen, as are lesser marsh grasshoppers, short-winged coneheads and numerous other invertebrates. Fens in Broadland are the last habitat in the UK of the magnificent swallowtail butterfly, whose larvae feed exclusively on rare milk parsley plants. Water voles, otters and, especially in winter, bitterns may also be found along ditches in fens throughout the county. Marsh harriers will breed in fen and several birds such as grasshopper warblers and Cetti’s warblers are found where scrub is developing.
A structurally simple form of fen, dominated by common reed, is known as reed-bed. Though it is of less botanical interest than more complex fens, reed-bed is of critical importance as nesting habitat for bitterns, marsh harriers, bearded tits and migrant passerines such as reed and sedge warblers.
Just as fen and reed-bed have developed through grazing and cutting, today their preservation depends on active management, in order to prevent encroachment by scrub. Fens on nature reserves and private land are cut to remove biomass and prevent succession to scrub. They may also be grazed, where conditions are appropriate, by tough livestock such as highland cattle and konik ponies.