Environmental conditions and fen vegetation in three lowland mires

The abiotic conditions and fen vegetation in three lowland mires were analysed. Two of these mires are in the Netherlands. They have deteriorated considerably as a result of human pressure. One mire complex is in Poland. Its hydrology is almost undisturbed. The variation in the water composition in the fens was associated with the variation in the amount of regional groundwater discharge originating from the uplands, and to a lesser extent by the infiltration of polluted water pumped into the fens. The 26 vegetation types ranged from poor fen to eutrophic reedland. Most types were typical for only one region. There were clear differences between the three regions. Both the species composition and the water quality for the vegetation types indicated that the fens in the Vecht river plain are eutrophicated, whereas the fens in Northwestern Overijssel suffer from acidification. The Biebrza valley fens are well developed and are mainly fed by fresh calcareous groundwater. Stepwise logistic regression performed on 28 fen species revealed that the hydrochemical variables explained only a part of the variance: the regional variable 'area' explained a considerable amount of the variance for most species. In 27% of the cases it was possible to fit an optimum curve for the species response to relevant hydrochemical variables. Monotonic curves could be fitted in 64% of the cases; mostly they described the response to variables in a way that is supported by ecological literature. It was concluded that the dataset should be constructed differently to enable the calculation of generally applicable standards.