New study investigates how geomorphology shapes salt marsh diversity in the Wadden Sea
A new study commissioned by the Common Wadden Sea Secretariat provides a comprehensive analysis of how geomorphological conditions shape salt marsh vegetation diversity across the Wadden Sea. The report, prepared by Wageningen Marine Research, compares trilateral vegetation data with high resolution geomorphological information, offering essential insights for evaluating progress toward the Wadden Sea Plan target of maintaining ‘a salt marsh vegetation diversity reflecting the geomorphological conditions of the habitat’.
Salt marshes are key habitats of the Wadden Sea World Heritage Site, contributing to biodiversity, coastal defence, and ecological functioning. The results show that vegetation diversity generally continues to increase with area (especially for barrier connected and chenier marshes) and does not level off even at the largest spatial scales analysed. This highlights the importance of large and wide salt marshes for maintaining diverse vegetation communities and their functionality within the ecosystem. Elevation heterogeneity is an important driver of vegetation diversity at intermediate spatial scales, while at very small scales, vegetation diversity is constrained by the limited number of vegetation types that can occur within the area.
Natural geomorphological dynamics can act as important geomorphological drivers that increase topographic heterogeneity and thereby promote more diverse vegetation communities. Marshes with natural creek systems consistently support higher vegetation diversity than those with man made drainage networks. Natural creeks create a more varied micro topography by forming natural depressions, enhancing habitat heterogeneity. Wash overs add small scale elevation variation, specifically on barrier islands.
Management measures such as grazing also influence vegetation composition, but their effects are modest compared to marsh size and geomorphology. Minimum intervention and moderate grazing generally support higher vegetation diversity, indicating that low intensity management maintains structural variety.
A major outcome of the study is the development of a robust and comparable method to assess salt marsh vegetation diversity across the Wadden Sea. By analysing vegetation type accumulation curves, this approach offers a reliable tool for tracking changes in salt marsh vegetation diversity across marshes of different size and geomorphological origin, thereby contributing to a more effective assessment of the Wadden Sea Plan target in future Quality Status Reports.