Mosaic fen communities with many rare plant species were outcompeted by fast-growing bryophyte stands (Sphagnum flexuosum/Sphagnum fallax and Polytrichum commune). Eutrophication and acidification were the main causes. Management experiments were carried out to set back this succession by restoring the former abiotic conditions (high pH, low nutrients). The management options investigated were: sod removal, lime application, drainage improvements (digging ditches and trenches) and combinations of sod cutting and liming, respectively, with drainage improvements. Four pilot projects are discussed, one in the brackish (oligohaline) peat area, one freshwater site with floating Carer-dominated fens and two freshwater sites with purple-moor grassland (Cirsio dissecti-Molinietum) in the holocenic peatlands of The Netherlands. Plant species, soil and water chemistry were monitored. The evaluation after five years showed that some of the desired plant species had returned, e.g. Viola palustris, Hydrocotyle vulgaris, whereas others had increased in cover. e.g. Scirpus lacustris spp. tabernaemontani (brackish site), Pedicularis palustris, Calliergonella cuspidata (fen site) and Gentiana pneumonante and Carex panicea (C. dissecti-Molinietum peatland). This happened, however, only in restricted strips along trenches and ditches. In the combined measure of drainage improvement and sod cutting, the Sphagnum species declined permanently, but after sod cutting only, they recovered within three years to the previous cover of ca. 80%. The sod-cutting measure and the control treatment showed no differences in the presence of a rain water lens, whereas drainage and combination treatments showed narrow (0.5-2 m wide) zones with intrusion of surface water. Soil and water pH and Electric Conductivity (EC) showed a sharp decrease within 0.5 m from the trenches (pH from ca. 6 to ca. 4). Also the cations Ca2+ and Mg2+ and the anions Cl- and SO42- showed this sharp decline. Applying lime to initiate a rapid rise of the pH was only successful in combination with drainage where water conditions had been improved and the peatmoss carpet had been removed. Remarkable was that the Sphagnum carpet neutralised 1500 kg ha(-1) within 3-6 month in the liming treatment of intact vegetation. In conclusion: rehabilitation was successful only in areas where the top water layer was removed by drainage through trenches. This effect occurred within narrow strips bordering the trenches, not in vast areas due to the enormous resistance of peat against through flow and the floating character of the fen. (C) 2001 Elsevier Science B.V. All rights reserved.
