Population structure and rejuvenation potential of Schoenus nigricans in coastal wet dune slacks

Populations of Schoenus nigricans L. in (formerly) wet dune slacks of the Wadden Island of Schiermonnikoog and the coastal dunes in North-Holland were analysed. The diameter of tussocks, as an indicator of plant age, was normally distributed only in areas with a periodic disturbance (Wadden Island), but strongly skewed in areas with strong soil fixation (coastal dunes). Rejuvenation was only observed after perturbation. Damage of fruits by the larvae of the microlepidopteran Glyphipteryx schoenicolella was low in brackish and high in freshwater sites. Defoliation by rabbits was strongest in dry sites. Nuts of S. nigricans germinated after a dormancy period of several months, under field conditions in April and May. Seedling growth was very slow, with two leaves after 3 months. The survival of seedlings was low during the first year (12%) and followed a Deevy Type III curve. First flowering was observed in experimental populations after 3 years. The decrease and local extinction of the S. nigricans population is discussed in relation to dune management and to the facilitation model of Connell & Slatyer (1977).