Species composition of soil seed bank and seed rain of abandoned wet meadows and their relation to aboveground vegetation

Abstract

Until the late fifties of this century species rich wet meadows were characteristic of the swampy alluvial plains in SchleswigHolstein (north-western Germany). Today many of these meadows undergo successional changes due to abandonment. The vegetation development after abandonment can be characterised as a sequence of different successional stages. After an initial phase(successional stage I) follows a phase of clonal expansion of highly competitive species (successional stage II) and a phase of immigration and establishment (successional stages III and IV). In the course of succession species richness decreases and highly productive vegetation-stands develope. Species contributing to successional changes can either be present in the initial aboveground vegetation, in the soil seed bank or in the seed rain. The species composition of the seed bank and of the seed rain of different successional stages of abandoned wet meadows was investigated. Furthermore the relation between aboveground vegetation and seed bank and between aboveground vegetation and seed rain were determined using a cluster analysis. Seeds of some typical wet meadow ( Lychnis flos-cuculi ) and forage meadow species ( Cardamine pratensis, Cerastium holosteoides, Ranunculus repens, Poa trivialis ) are regulary present in the seed bank of later successional stages, whereas they are absent in the aboveground vegetation there. Thus the aboveground vegetation of permanent plots of the early successional stage I clusters with the seed bank of all permanent plots in the cluster analysis. On the other hand the aboveground vegetation of the permanent plots of the later successional stages II and III constitute cluster. The results of this study support the view, that even if the seed bank of later successional stages contains some species of earlier stages, the possibility to re-establish species rich meadows from species poor fallows is relatively low. This is due to the fact, that seedling-densities of meadow species are decreasing exponentially during succession. Furthermore, rare or endangered plant species are scarcely ever found in the seed bank. The seed rain contains almost exclusively species of the aboveground vegetation present on the permanent plots and the surrounding area. No diaspores of characteristic meadow species were trapped in the seed traps, if these species were absent from the aboveground vegetation or did not produce seeds during the investigation period. Thus, the similarity of the aboveground vegetation of all permanent plots with the seed rain is very high; cluster analysis indicates that the aboveground vegetation of each permanent plot clusters with the seed rain of the same plot. The great similarity of the aboveground vegetation of all permanent plots with the seed rain is caused by the small dispersal distances of most of the occurring species. Both diaspores of herbaceous ( Urtica dioica, Galeopsis tetrahit agg.) and woody ( Alnus glutinosa, Betula pubescens, Salix alba ) invasive species were trapped in seed traps in some cases. It can be concluded that the seed rain of abandoned wet meadows has an essential impact on progressive secondary succession (transition between successional stage II and III and between successional stage III and IV), while it has only a subordinate role in restoring species rich wet meadows from abandoned sites, that is in regressive succession.