The onset of secondary seed dispersal is controlled by germination-features: A neglected process in sudden saltmarsh establishment

Effective seed dispersal is critical for enabling rapid state shift from a bare tidal flat to a vegetated marsh. While tidal currents are the main dispersal vector in coastal environments, biological characteristics that keep seeds afloat have been shown to influence primary seed dispersal, i.e., seed departure from the parent plant. In contrast, secondary seed dispersal processes that move (germinated) seeds trapped within microsites have been largely neglected. Here, we explore the extent to which the coupling between biotic traits and abiotic factors affects secondary seed dispersal, and whether secondary seed dispersal may explain sudden saltmarsh establishment (i.e., rapid colonization of bare tidal flat by large numbers of seedlings in spring). We used two widely spread pioneer species: Scripus mariqueter and Spartina alterniflora. Combined flume and field results demonstrated that: (1) germination stage, current velocity, and sedimentary regime have a marked effect on the probability of secondary seed dispersal by influencing the threshold lift-off velocity of (germinated) seeds within microsites; (2) density and bud length are critical biotic traits that best predict the potential of secondary-dispersal onset after seed germination; (3) S. alterniflora possess a stronger secondary dispersal ability compared to S. mariqueter; and (4) secondary dispersal of germinated seeds in spring may explain observed sudden marsh establishment on bare tidal flats. Our findings provide novel insight relevant to understanding the drivers of seed dispersal and thereby sudden saltmarsh establishment events, with important implications for understanding the effects of climate change on critical state transitions and enabling human-aided restoration.