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

We studied the quantitative composition of the vegetation at 208 stream sit es distributed throughout Denmark with the purpose of identifying distinct plant communities, using fluster analysis, and their relationship to enviro nmental conditions using indirect gradient analysis. Six plant communities were defined in the analysis, although the differences in species compositi on among them were small. The plant communities were mainly related to diff erences in water alkalinity with a Potamogeton-community being associated w ith high alkalinities, Sparganium-, Callitriche- and Batrachium-communities with medium alkalinities and a Myriophyllum alterniflorum-community associ ated with low alkalinity. Also stream size turned out to be an important va riable to the separation of plant communities, with the Callitriche-communi ty mainly found in small streams and the Potamogeton-community and a commun ity of amphibious helophytes mainly found in larger streams. The most commo n species were Sparganium emersum and Elodea canadensis forming the wide sp read Sparganium-community at half of the stream sites throughout Denmark, a pattern most likely stimulated by the common practice of weed cutting. Tho ugh communities of stream plants could be defined, the overlap of species a mong them was very substantial. Communities are less distinct in Danish str eams compared to streams in other regions including wider environmental gra dients of temperature, current velocity, substratum, alkalinity and nutrien t richness. (C) 2000 Elsevier Science B.V. All lights reserved.