Seed arrival and persistence at the tidal mudflat: identifying key processes for pioneer seedling establishment in salt marshes

Salt marshes are highly valuable ecosystems that provide numerous important ecosystem services. Given the global marsh decline, there is a pressing need to understand the natural bottlenecks and thresholds to their establishment and long-term ecological maintenance. Seed presence in the right place and time is a prerequisite for pioneer establishment. We performed field surveys and manipulative seed-bank experiments on 2 mudflats with different levels of exposure in the Westerschelde, The Netherlands, to identify patterns of seed arrival and persistence and their role in determining seed bank abundance. Seed arrival decreased with distance from the seed-source area, i.e. the salt marsh, resulting in seeds being deposited on the mudflat close to the marsh edge. Once arrived, most of the surface seeds were washed away during subsequent tides, with retention of <20%. However, seed burial enhanced seed retention nonlinearly with increasing burial depth. Predation was not a major cause of seed loss. These results point to the importance of hydro dynamic and morphodynamic processes to both lateral seed dispersal and vertical seed burial on tidal flats and, therefore, suggest that saltmarsh establishment and expansion are closely linked to the physical dynamics of the entire estuarine system.