Soil seed bank composition along a gradient from dry alvar grassland to Juniperus shrubland

Dry alvar grasslands on limestone on the Baltic island of Oland, SE Sweden, are very species-rich as long as the traditional agricultural exploitation of grazing and fire wood collection continues. After abandonment, encroachment of Juniperus communis starts and a closed woodland can develop within 100 yr. A chronosequence, representing a successional series, was used for the comparison of sites still grazed, and sites ungrazed for about 20, 55 and 80 yr, respectively. Out of the 58 characteristic dry alvar grassland species 55 % disappeared from the established vegetation after 80 yr of abandoning, and 80 % also vanished from the seed bank. Arenaria serpyllifolia, Trifolium repens, Agrostis vinealis, Linum catharticum, Polygala vulgaris, Cerastium fontanum, Luzula campestris, Achillea millefolium and Potentilla tabernaemontani were the only species left in the seed bank. More than 75 % of the dry alvar grassland species were classified as having a transient or short-term persistent seed bank. It is concluded that restoration management, by cutting junipers, of overgrown dry alvar grassland cannot rely on the longevity of seeds in the soil. Seeds have to be dispersed by wind or grazing animals.