Hydrology, shore morphology and species traits affect seed dispersal, germination and community assembly in shoreline plant communities

Abstract

1. Seed dispersal and germination are two primary processes influencing plant community assembly. On freshwater shores, water levels regulate both processes. However, it is still unclear how water levels, shore morphology and species traits interactively affect seed dispersal and germination, and how these interactions determine plant community assembly. We hypothesize that a drawdown water regime enhances seed establishment compared to a year-round stable water level, that this increases species richness and diversity, and that this is modulated by species traits and shore morphology. 2. Germination of 20 wetland plant species with different dispersal capacities (floating capacity expressed as seed floatation half-time) and soil moisture preferences for germination (Ellenberg F) was tested on artificial shores in 24 outdoor ponds in two complementary experiments over 8 weeks. The 'dispersal experiment' tested the effect of water regime on recruitment of hydrochorously dispersing seeds. The 'seed bank experiment' tested the effect of water regime on germination from a sown seed bank, on steep and gradual shores. 3. In the dispersal experiment, the drawdown regime increased recruitment and species richness. Longer floating species colonized a larger shoreline section. Soil moisture preference for germination did not determine colonization patterns. 4. In the seed bank experiment, the drawdown regime increased the number of seedlings on gradual sloping shores, but not on steep shores. The number of germinating seedlings corresponded to the area subjected to the drawdown regime in both shore types. Species richness was not affected by water regime or shore morphology, and species traits did not determine shoreline colonization. Most seeds germinated in moist soil conditions for all species. 5. Synthesis. A spring drawdown instead of stable water regime stimulates establishment of hydrochorously dispersing seeds in temperate wetlands, leading to higher species richness and diversity. Germination from the seed bank is more affected by water regime and shore surface than by the tested species traits. Species traits, water levels and shore morphology together determine wetland plant community assembly, with dispersal as the main driver of seedling community diversity. Water-level regulations and shore morphology can be used to influence plant communities in wetland restoration.