Plant communities in lowland Danish streams: species composition and environmental factors

We studied the quantitative composition of the vegetation at 208 stream sites distributed throughout Denmark with the purpose of identifying distinct plant communities, using fluster analysis, and their relationship to environmental conditions using indirect gradient analysis. Six plant communities were defined in the analysis, although the differences in species composition among them were small. The plant communities were mainly related to differences in water alkalinity with a Potamogeton-community being associated with high alkalinities, Sparganium-, Callitriche- and Batrachium-communities with medium alkalinities and a Myriophyllum alterniflorum-community associated with low alkalinity. Also stream size turned out to be an important variable to the separation of plant communities, with the Callitriche-community mainly found in small streams and the Potamogeton-community and a community of amphibious helophytes mainly found in larger streams. The most common species were Sparganium emersum and Elodea canadensis forming the wide spread Sparganium-community at half of the stream sites throughout Denmark, a pattern most likely stimulated by the common practice of weed cutting. Though communities of stream plants could be defined, the overlap of species among them was very substantial. Communities are less distinct in Danish streams compared to streams in other regions including wider environmental gradients of temperature, current velocity, substratum, alkalinity and nutrient richness. (C) 2000 Elsevier Science B.V. All lights reserved.