The relationships between soil seed bank, aboveground vegetation and disturbances in old embanked marshlands of Western France
Résumé
The study deals with the influence of land use and abandonment on species composition of vegetation and seed bank in grasslands and oldfields. We wanted to explore: (1) How the seed bank changes when agricultural practices cease? In convergence with proposals in the literature, we addressed in particular the following two questions that have been proposed by Symonides (1986), Pickett & McDonnell (1989) and Roberts & Vankat (1991) for seed bank characteristics under secondary succession: (i) Does species richness and species diversity in the soil seed bank decrease during succession? (ii) Does the density of buried seed decline during succession? (2) What is the role played by seed bank in the recolonisation of plots disturbed by experimental disturbances ? We studied species composition of vegetation and seed bank in an experiment with grassland and oldfield plots in old embanked marshlands (called "Marais Poitevin"). In these wetlands, artificial disturbances (mowing) and natural disturbances (cattle, roebucks, coypus, voles) are very frequent. In order to mimic disturbances, experimental disturbances were generated in spring after the end of the winter flooding and emerged seedlings counted three months later. Data about the seed bank, the undisturbed vegetation and seedlings emerging in disturbed quadrats were sampled. Detrended Correspondence Analysis (DCA) of the undisturbed quadrats, disturbed quadrats and seed bank samples showed significant differences of species composition. Similarity between seed bank and undisturbed aboveground vegetation was low and not very different between grassland and oldfield. Very few seedlings emerged in the undisturbed vegetation both in grassland and oldfield, which potentially indicates the importance of gaps for seed bank expression. In Marais Poitevin, the seed bank contributed very little to the seedling flora, and vegetative regrowth clearly predominated recolonisation after disturbances. In the seed bank, few species lost after succession from grassland to oldfield vegetation were still present as seeds in the soil, but in most cases species lost were not recorded in the seed bank. The results have shown that species richness and species diversity in the seed bank decrease during succession. On the other hand, the density of buried seeds did not decrease significantly from grassland to oldfield