Changes in alder fens following a decrease in the ground water table: results of a geographical information system application.
Large-scale decreases in the groundwater table in western parts of the lower Rhine area, Germany, have led to changes in species composition of Alnus glutinosa fens and indications of intensive peat mineralization. The chemistry of peat and the distribution patterns of vegetation were studied in an affected area fen of 6.6 ha. Groundwater level, pH of the peat, exchangeable cations, total nitrogen, organic carbon contents and herb layer composition were assessed at each intersection of a rectangular 25 × 25 m grid. The evaluation of individual grid values was performed with a geographical information system (GIS). Isolines and overlays for selected data sets were used to assess extent and direction of change. The peat of the drained areas showed decreased and low pH values. In contrast, bulk density and exchangeable aluminium increased in the mineralized peat. Though more than 800 g/m2 of nitrogen was still found in the upper 10 cm in the wet areas, nitrogen stores diminished to about half of this amount in drained peat. The decline in pH values and in calcium and magnesium contents was mainly attributed to H+ production related to nitrogen mineralization. The evaluation of overlays for selected variables showed a strong relation between the decrease in groundwater level and declining cover of Carex acutiformis as well as peat acidification. Increased in exchangeable aluminium were also closely related to peat acidification. The investigation showed that spatial evaluation of ecological data can be useful for assessment of man-induced ecosystem changes.