Summary 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.

Question: In the past decades, the tall native invasive grass, Elytrigia atherica, has been increasing in frequency and dominance on salt marshes along the Wadden Sea coast. Is this rapid expansion an outcome of natural succession or is it driven by anthropogenic eutrophication resulting from atmospheric deposition?Location: Salt marshes on four back-barrier islands, Wadden Sea on the coast of the Netherlands and Germany.Methods: We used a combination of time series of vegetation maps and chronosequence data of four naturally developed salt marshes to address our questions. These salt marshes have not been grazed by livestock or subject to other management regimes. By comparing development within and between four different salt marshes, we were able to study the spatial and temporal dynamics of the community dominated by E. atherica on natural salt marshes.Results: The expansion rate of the E. atherica community was highest on young salt marshes (up to 30 yr old) with vertical accretion rates of 0.35 cmÁyr À1 . The rate of expansion decreased on older marshes and the direction reversed, becoming negative, on the oldest marshes (around 90 yr old), which have no vertical accretion and are under waterlogged conditions. Conclusions:The expansion of E. atherica on natural, back-barrier islands along the Wadden Sea coast is more influenced by the age of the salt marsh and patterns in vertical accretion of soil than by uniformly spread atmospheric deposition.Dynamics of Elytrigia atherica on salt marshes R.M. Veeneklaas et al.

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Summary 1.Astroturf mats can effectively trap diaspores dispersed by tidal water. 2. Within four tidal inundations, up to 745 propagules per m 2 and between three and eight different species per astroturf mat were trapped. Overall, 15 different species were collected on the astroturf mats, 10 of which were also present in the local vegetation. The other five species must have come from further away, making the method also suitable for studying long-distance dispersal. 3. The seed-retaining ability of the astroturf did not differ significantly for species with different seed characteristics. 4. The method will be useful for assessing actual seed dispersal at specific sites and over specific periods of time, and has particular relevance in predicting and evaluating the success of ecological restoration projects.

Question: Community structure may be influenced by patterns of dispersed seeds (seed rain) because they contribute to the template of plant community development. We explored factors influencing seed rain in a system dominated by tidal water, where direction and magnitude of water flow are difficult to predict, unlike many other hydrochorous systems where water flow is directional. We posed three main questions: (1) are patterns in seed rain linked to effects of hydrodynamic variability; (2) do these patterns in seed rain reflect distribution of seed sources and seed production; and (3) what are the implications for the assembly of tidal communities? Location: Salt marshes on the Wadden island of Schier‐monnikoog, The Netherlands. Methods: Species compositions of vegetation, seed rain, seed production and driftlines along a chronosequence of communities were compared. We also studied seed movement by sowing Astroturf® mats with seeds and checking for seeds remaining after a single tidal inundation. Results: Storm surges had a significant effect on seed‐rain patterns as the highest density and diversity of captured seeds were found during a stormy period. Seed rain of the youngest communities was more influenced by storms than that of older communities. Patterns in seed rain generally followed similar patterns in the distribution of established plants, and seed production. Conclusions: Results suggested mostly local dispersal of seeds. However, there was some evidence of long‐distance dispersal occurring during storm surges in younger communities that are regularly inundated with tidal water. The possible role of seed retention in constraining community development, rather than dispersal per se, is further discussed.