To move or not to move: determinants of seed retention in a tidal marsh

1. The effects of moisture conditions, seed morphology, vegetation structure and hydrodynamic variables on seed retention were examined in a system where the dominant dispersal agent is water. Experiments were conducted in a tidal salt marsh and in a flume facility where hydrodynamic variables could be controlled. 2. Moisture condition of seeds greatly influenced which factors were most important in determining seed retention. Seed type (buoyancy) was the most important factor when seeds were dry with seeds possessing very low floating capacity (Plantago maritima) being retained in greater numbers than seeds with intermediate floating capacities (Suaeda maritima and Elytrigia atherica). 3. In contrast, hydrodynamic variables dominated retention processes when seeds were waterlogged. The application of waves in addition to flow velocity dislodged more seeds than flow velocity alone. 4. Vegetation structure influenced retention in both dry and wet conditions but less so than other factors. Denser, less rigid vegetation types retained greater numbers of seeds than more open, more rigid vegetation types. 5. Results suggest that buoyancy traits appear to determine whether seeds move in the drier summer and autumn months after initial detachment from parent plants but the intensity of wave action will determine whether waterlogged seeds stay in a microsite during the wetter months of late autumn to early spring.