VEGETATION SUCCESSION IN A FLOATING MIRE IN RELATION TO MANAGEMENT AND HYDROLOGY

The vegetation succession in a floating mire was studied in relation t o management and hydrological conditions in a former peat cutting area in the northern part of the Netherlands. An existing map showed that in 1956 the vegetation consisted mainly of meadows, reedbeds and rich fen vegetation while a recent survey revealed that this picture had co mpletely changed in 1989. The area had mainly become woodland but part had remained open, due to the continuation of a mowing regime. The ti ming of the mowing appears to be critical, especially during the earli er phases of the succession. Winter-mowing favours the development of eutrophic reedbeds, while summer-mowing promotes mesotrophic sedge com munities. In the course of time these succession lines converge increa singly towards acidic vegetation types. The development from open wate r to embryonic bog differed greatly between sites with a similar manag ement regime. Mostly this took only a few decades but in a few sites t he vegetation had hardly changed since 1956. Whereas the former sites were acid throughout the profile, the latter showed a high pH from top to bottom. It is obvious that acidification does not occur as long as alkaline surface water can move freely underneath the floating mat. A s soon as a given site becomes disconnected from the surface water the depletion of bases by infiltrating rainwater is no longer compensated . A rough estimate of the acidification rate shows that in an infiltra tion area three decades are sufficient for a floating mat of 40 cm to become completely decalcified. This corresponds well to the observed s uccession from rich fens to embryonic bogs in the same period. It is s uggested that the best way to preserve rich fen vegetation is to start the succession anew by digging turf ponds.