ENVIRONMENTAL-CONDITIONS AND FEN VEGETATION IN 3 LOWLAND MIRES

The abiotic conditions and fen vegetation in three lowland mires were analysed. Two of these mires are in the Netherlands. They have deterio rated considerably as a result of human pressure. One mire complex is in Poland. Its hydrology is almost undisturbed. The variation in the w ater 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 i nto the fens. The 26 vegetation types ranged from poor fen to eutrophi c reedland. Most types were typical for only one region. There were cl ear differences between the three regions. Both the species compositio n and the water quality for the vegetation types indicated that the fe ns in the Vecht river plain are eutrophicated, whereas the fens in Nor thwestern Overijssel suffer from acidification. The Biebrza valley fen s are well developed and are mainly fed by fresh calcareous groundwate r. Stepwise logistic regression performed on 28 fen species revealed t hat the hydrochemical variables explained only a part of the variance: the regional variable 'area' explained a considerable amount of the v ariance for most species. In 27% of the cases it was possible to fit a n optimum curve for the species response to relevant hydrochemical var iables. Monotonic curves could be fitted in 64% of the cases; mostly t hey described the response to variables in a way that is supported by ecological literature. It was concluded that the dataset should be con structed differently to enable the calculation of generally applicable standards.