Divergence between riparian seed banks and standing vegetation increases along successional trajectories

Questions: Plant community resilience largely depends on the secondary succession induced by species re-colonization from seed banks. Soil seed bank resilience is, however, poorly understood, especially in regularly disturbed habitats like riparian zones. Two questions were asked: (1) what are the changes in species diversity experienced by riparian soil seed banks along successional trajectories, and (2) to what extent do riparian soil seed banks promote vegetation resilience during secondary succession? Location: Southeast Quebec, Canada. Methods: Soils were collected along five rivers in field edges and riverbanks of post-agricultural riparian zones with three contrasting successional stages (un-planted, planted with trees 15-17 yr prior to sampling, natural riparian forests), and their seed bank composition determined with the seedling emergence method. Species richness in seed banks was assessed along successional trajectories for distinct ecological groups, using LMM. The compositions of soil seed banks and standing vegetation (from botanical surveys) were compared based on NMDS and indicator species analysis. Results: Seed bank species richness decreased along successional trajectories. Tree-planted riparian zones were generally closer to unplanted riparian zones than to natural riparian forests, the latter being more species-rich for natives, trees, shrubs and zoochores, and species-poor for exotics, forbs and stress-tolerators. Likewise, seed bank species composition of unplanted and tree-planted riparian zones was similar but differed from that of natural riparian forests. Conversely, standing vegetation of tree-planted riparian zones was intermediate between early and late successional stages, at least at field edges. For the three successional stages, seed bank composition clearly differed from standing vegetation. Conclusions: The high resilience of riparian plant communities appeared poorly related to the dynamics of their soil seed banks. This species shift between seed banks and standing vegetation during secondary succession is likely due to periodic flooding, leading to the regular turnover of seeds. The resilience of riparian communities might thus be more influenced by spatial dispersal along rivers and across landscapes than by in situ temporal dispersal in soils.