SOIL SEED BANK, SEEDLING RECRUITMENT AND ACTUAL SPECIES COMPOSITION IN AN OLD AND ISOLATED CHALK GRASSLAND SITE

A soil seed bank study was conducted in an old and isolated chalk grassland site (area ca. 0.05 ha) with a high species richness in The Netherlands, over a two-year period. Each soil seed bank sample was divided into two halves: one part was sown immediately in trays with a layer of sterilized sand and put in a greenhouse, whilst the other half received a chilling period before being sown. Plant recruitment in the field was studied during the same period in permanently marked plots. Seeds of species which had become extinct during the period 1944-1970 were not found in the soil seed bank in the late eighties. One group of species still present in the actual vegetation neither showed any signs of recruitment from seed nor had a soil seed bank. Species in this group that do not recruit vegetatively can be considered as directly threatened by extinction. A group in the actual vegetation did not show any recruitment, but nevertheless had germinable seed present in the soil. Among this group, there are some annuals; however, the majority of the species are characterized by the potential to recruit vegetatively. The majority of the species present in the actual vegetation (45%) demonstrate seedling recruitment and are also present as seeds in the soil, although there is some quantitative discrepancy between these categories. The seed bank did not reflect former stages of the actual vegetation. Based on the results, predictions can be made on the future development of the vegetation and on the fate of a number of species. Adjustment of the management regime is aimed at maintenance of the species richness of the site. This is important for nature preservation purposes since a high number (ca. 27%) of the species in the chalk grassland of this small site are on the Dutch Red List.