The Role of Propagule Banks from Drainage Ditches Dominated by Free-Floating or Submerged Plants in Vegetation Restoration

Dominance by free-floating plants results in a loss of plant species in many waters. An important source for re-establishment of non-floating aquatic plants can be the propagule bank. This study focuses on whether the propagule bank of free-floating plantdominated ditch sediments can serve as potential source for recovery of a diverse plant community. The first objective was to determine differences in propagule germination from sediments of ditches in the Netherlands that differ in vegetation composition through a seedling-emergence experiment. The second objective was to analyze the effect of sediment disturbance on the number of germinating propagules. The results show that, compared to sediments from ditches with submerged vegetation, sediments from free-floating plantdominated ditches produced significantly lower numbers of individuals and species of submerged and emergent plants, while numbers of individuals and species of free-floating plants were higher. These results suggest that sediments from free-floating plantdominated ditches have lower potential to recover a diverse plant community probably resulting from positive feedback mechanisms caused by the vegetation present, maintaining the free-floating plantdominated state. Sediment disturbance strongly favors the germination of free-floating plant propagules, especially from free-floating plantdominated ditch sediments. Ditch maintenance activities such as mowing and dredging will therefore likely favor persistence of the free-floating plantdominated state. To shift from dominance by free-floating plants to a more diverse plant community, alternative maintenance methods should be considered that cause less sediment disturbance together with measures that promote colonization such as temporary drawdown or re-introduction of species.