Seeding Dynamics from a Local Seed Mixture on a Bioengineered Riverbank Protection Structure

Restoration of riverbanks through soil bioengineering techniques allows managers to combine riverbank stability and riparian ecosystem functioning. This restoration often involves the sowing of a seed mixture, which helps develop herbaceous vegetation. This development and sufficient vegetation cover are essential for protection against erosion and for hosting biodiversity, two of the main goals of riverbank bioengineering. Restoration aims at recreating ecosystems closer to an undisturbed state; choosing seed mixtures of local provenance is therefore encouraged. In this study, we investigated the local seed mixture sown on bioengineered riverbanks and the conditions influencing the first steps of plant development, so as to delineate the setting favoring restoration. We focused on the composition of the seed mixture and germination capacity as well as the effect of sowing density and soil quality on vegetation cover and diversity. We tested four sowing densities: 5, 10, 15, and 30 g.m^-1. The seed mixture presented considerable diversity and germination rates were heterogeneous. Sowing density had a positive impact on vegetation cover and diversity, and high cover up to 100% was rapidly reached. Soil quality did not affect vegetation diversity but had a significant effect on vegetation cover, with the nutrient content, notably nitrogen, most probably involved.