Vegetation development in dune slacks: the role of persistent seed banks

The soil seed bank composition was determined at four sites in the dune sla ck 'Koegelwieck' on the Dutch Wadden Sea island of Terschelling. At three d ifferent sites in the slack, where sod-cutting experiments down to the mine ral sand had been carried out, the established vegetation and seed bank wer e assessed after 5, 9 and 39 yr of undisturbed development, respectively. I n addition, a fourth site in the slack was investigated, where vegetation d evelopment had proceeded for 80 yr since plant colonization of bare soil an d where nowadays a vegetation dominated by Calamagrostis epigejos and Salix repens occurs. Together these four sites can be regarded as a chronosequen ce of dune slack formation. Clear time sequences were detected in the seed bank data. Many late success ional species showed a significant increase in the number of seeds during t he succession. Some of the early successional basiphilous pioneer species s uch as Anagallis minima, Centaurium littorale, Littorella uniflora, Radiola linoides and Samolus valerandi, showed either a decrease during the time o f succession or an optimum in the vegetation while remaining present in the seed bank in low but detectable numbers. They could, therefore, play a rol e in re-establishment of the vegetation after sod-cutting. One of the targe t species, Schoenus nigricans, established within a few years after removal of the sod. However, no seeds of this species have been detected in the so il below either of the successional stages. Based on the species disappeara nce from the established vegetation and based on the independent data of Th ompson et al. (1997) an estimation of seed longevity could be made for seve ral Red List species of wet dune slacks.