Does zonation reflect the succession of salt-marsh vegetation? : a comparison of an estuarine and a coastal bar island marsh in The Netherlands

Traditionally, vegetation succession in salt-marshes has been inferred from the zonation of the vegetation. However, long-term observation of species composition in salt-marshes has led to the recognition that the change of the vegetation does not always proceed according to the succession schemes derived from zonation. The hypothesis that the zonation of the vegetation reflects its succession is based on a model which assumes that the relief in the marsh is formed by sediment trapped by the vegetation. According to this model the present high marsh developed from a low marsh and the low marsh of today will rise to higher levels. In this study we investigated the sedimentary record in a coastal bar island marsh and in an estuarine salt marsh, in order to detect whether the formation of the relief in these two marshes corresponds to this geomorphogenetic model. The sedimentary record in the estuarine marsh supports the hypothesis that the relief was formed by sediments deposited in a marsh environment. The formation of the relief in the marsh at the coastal bar island, however, could not be attributed to marsh sediments solely. The surface relief is mainly determined by the relief of the sandy subsoil. Historical evidence indicates that the relief of the sandy subsoil had been formed by aeolic processes in a beach environment one century ago, and has subsequently been fossilized. The vegetation displayed a marked zonation in both marshes. The present zonation would suggest that succession in the estuarine marsh was initiated by Spartina anglica, which was confirmed by 50-year-old aerial photography. It was concluded that historic succession could be inferred from zonation in the estuarine marsh, but not in the marsh at the coastal bar island.