Limited seed retention during winter inhibits vegetation establishment in spring, affecting lateral marsh expansion capacity

Abstract

Coastal systems worldwide deliver vital ecosystem services, such as biodiversity, carbon sequestration, and coastal protection. Effectivity of these ecosystem services increases when vegetation is present. Understanding the mechanisms behind vegetation establishment in bio-geomorphic systems is necessary to understand their ability to recover after erosive events and potential adaptations to climate change. In this study, we examined how seed availability affects vegetation establishment in the salt marsh-intertidal flat transition zone: the area with capacity for lateral marsh expansion. This requires vegetation establishment; therefore, seed availability is essential. In a 6-month field experiment, we simulated a before and after winter seed dispersal at two locations, the salt-marsh vegetation edge and the intertidal flat, and studied seed retention, the seed bank, and the seed viability of three pioneer marsh species: <i>Salicornia procumbens</i>, <i>Aster tripolium</i>, and <i>Spartina anglica</i>. During winter storm conditions, all supplied seeds eroded away with the sediment surface layer. After winter, supplied seeds from all three species were retained, mostly at the surface while 9% was bioturbated downwards. In the natural seed bank, <i>A. tripolium</i> and <i>S. anglica</i> were practically absent while <i>S. procumbens</i> occurred more frequently. The viability of <i>S. procumbens</i> seeds was highest at the surface, between 80% and 90%. The viability quickly decreased with depth, although viable <i>S. procumbens</i> seeds occurred up to 15 cm depth. Only when seeds were supplied after winter, many <i>S. procumbens</i> and some <i>S. anglica</i> individuals did establish successfully in the transition zone. Viable seed availability formed a vegetation establishment threshold, even with a local seed source. Our results suggest that, although boundary conditions such as elevation, inundation, and weather conditions were appropriate for vegetation establishment in spring, the soil surface in winter can be so dynamic that it limits lateral marsh expansion. These insights can be used for designing effective nature-based coastal protection.