The fens which are on the East Anglian rivers Lark, Little Ouse, and Wissey, to the east of the true Fenland, have been called the Breck fens. Most of the region is under grass, but there are scattered fens, carrs and woods.
Two types of fen have been distinguished: headwater fens, found at the sources of streams, with non-silty soil and many springs, and valley fens, found in the main part of the valleys, with silty soil and few springs. There are also a few intermediates. The two types differ from each other considerably in the vegetation of the wet fens, less in that of the dry fens, and very little indeed in that of the alder and ash-alder woods, which are the principal types of woodland in both, or of the grass fields in the two regions.
Headwater fens contain the species characteristic of more nutrient-poor habitats, e.g. Schoenus nigricans and Drosera rotundifolia.
Phragmites communis is the natural dominant of wettest valley fens, and is succeeded, under natural conditions, by alder and then ash-alder woods. Human interference, which was intensive a century ago, and would have prevented tree colonization while the water table was falling and the fens were drying, is probably the cause of dry field layer vegetation, which is now dense enough to render tree colonization difficult. The principal dominants are Epilobium hirsutum, Urtica dioica and Galium aparine.
Cladium mariscus is probably the natural dominant of most wet headwater fens, with Schoenus nigricans in spring areas at the head of fens (the most nutrient-poor habitats) and Carex nigricans in spring areas at the head of fens (the most nutrient-poor habitats) and Carex elata in places where the water table is very variable. Other communities, apart from woodland, are probably caused by human interference.