Rehabilitation of acidified floating fens by addition of buffered surface water

Floating fens are species-rich succession stages in fen areas in the Netherlands. Many of these fens are deteriorating due to acidification; Sphagnum species and Polytrichum commune build 10-25 cm thick moss carpets, and the species diversity decreases. Earlier experiments in wet ecosystems indicate that successful restoration of circum-neutral and mesotrophic conditions requires a combination of hydrological measures and sod removal. In an acidified fen recharged by rainwater in the nature reserve Ilperveld (The Netherlands), a ditch/trench system was dug for the purpose of creating a run-off channel for acid rainwater in wet periods and to enable circum-neutral surface water to enter the fen in dry periods. Moreover, the sod was removed in part of the fen. Ditch/trench creation or sod cutting had no effect individually, but a combination of the two measures led to a change in the abiotic conditions (higher pH and Ca), and in turn to an increase of species-richness and the reestablishment of a number of characteristic species. Reestablishment of rare vascular plant species and characteristic bryophytes might be a long-term process because of incomplete recovery of site conditions and constraints in seed dispersal.