Salt marshes form natural barriers between the sea and the land and they provide numerous ecosystem services. In this thesis, we studied the establishment of pioneer vegetation at the edge of the salt marsh. The transition zone from salt marsh to intertidal flat contains diverse and harsh conditions like frequent inundation with salt water, waves, massive temperature variations and a dynamic sediment layer. Annually, plants have to establish is this zone to maintain the salt marsh surface area. Establishment of these plants is affected by tiny worms that crawl around in the soil, dislodging the roots of pioneer plants but also influencing the distribution of seeds in the soil. Additionally, tidal patterns and weather conditions impact the start and length of the growing season, which can influence recruitment success (available seeds) for the following year. Lastly, a dynamic bed-level reduces seed retention and hinders the establishment of plants in spring. The limited availability of seeds in spring was found to be a major constraint for marsh expansion.