The vegetation of artificial drainage channels within grazing marshes in the UK: how does its composition correspond with described communities?

Large areas of British coastal and riverine floodplains support grazing marsh, a landscape representing an intermediate stage in the reclamation of natural wetlands toward intensive agriculture. Once disregarded for nature conservation, in the late twentieth century it was realised that grazing marshes were significant for wetland birds and invertebrates, as well as the key refuge for aquatic vegetation in lowland Britain. The Centre for Ecology and Hydrology (CEH) surveyed this vegetation, focussing on drainage channels and their banks. In multivariate analysis, vegetation composition was related to channel dimensions, soil type, land use, channel management and the drainage function of the channel. CEH attempted to relate the channel vegetation assemblages to those aquatic and swamp types described within the British National Vegetation Classification and the wider literature. Although we sometimes achieved this aim channels often appeared to contain intimate concatenations of types that could not be simply separated into recognised aquatic and swamp associations. Thus, channel vegetation may be represented either as combinations of described communities or as unique multilayered associations, comparable with woodlands. The degree to which aquatic and swamp elements occur in predictable combinations is discussed. Despite being artificial habitats, drainage channels have intrinsic biodiversity value, meriting recognition within European systems for the protection of biodiversity.