RESPONSES OF RESTORED GRASSLAND DITCH VEGETATION TO HYDROLOGICAL CHANGES, 1989-1992

The impact of hydrological manipulation of an unfertilised, Dutch peat grassland area on plant species composition on ditch banks and in ditchwater was studied. The hydrological manipulation involved raising the groundwater level by admitting nutrient-poor, Ca-rich groundwater in one compartment, and by retaining precipitation in another compartment. A third compartment served as control. The plant species composition showed significant correlations with the following hydrochemical parameters: in bank Vegetation with K+ concentration, and winter and summer groundwater levels, water depth and elevation; in aquatic vegetation with pH, the concentrations of Cl- organic-C and NH4+ water temperature and elevation. The number of terrestrial plant species increased after compartmentation from 97 to 122; 16 submerged and floating species were found. Most 'new' terrestrial species probably emerged in response to cessation of fertilization and biomass removal, since they showed no preference for any compartment. Five species showed preference for the groundwater compartment and two for the rainwater compartment. Of the 'new' terrestrial plants, seven were relatively rare: Carex panicea, C. oederi, C. pallescens, C. vesicaria, Galium uliginosum, Juncus acutiflorus and Stellaria uliginosa. Of the aquatic plants, eight were relatively rare: Chara globularis, two Callitriche spp., four potamogetonaceae and Ranunculus circinatus. This study indicates that hydrological manipulation of grassland systems in which fertilisation has ceased has profound effects on the vegetation in the ditches of these systems. Bank vegetation responds more slowly and to other hydrochemical factors than aquatic vegetation. Short-term responses in terms of increase in diversity of vegetation pattern and species richness are promising. Long-term responses are not yet known.