Control of Non-migrating Bar Morphodynamics on Survival of Populus nigra Seedlings during Floods

Populus nigra seedling survival to flood stresses during their first stage of development was analyzed in the Loire River, France. We related bar dynamics (assessed from bathymetrical, topographical, flow velocity, scour chains, and sediment grain size surveys) to seedling survival. The study highlights (i) the influence of flood succession and flood stages on seedling survival after establishment, (ii) the spatial distribution of fluvial processes at the bar scale and their relative contribution to seedling mortality, (iii) threshold values for each process. At the bar scale, 28% of mortality is explained by uprooting associated with erosion of sediments. To a lesser extent, uprooting by drag force applied on the stems and burying by sediments also contribute to seedling mortality. The majority of seedling mortality is ultimately due to a combination of erosion and burial (50.6%) during flood events. The relative contribution of each process depends on the combination and phasing of erosion and deposition processes, sediment supply (quantity and grain size), and flow velocity governed by hydrological variations. Based on the results of this study we hypothesize that the survival of seedlings during floods may be a function of local processes involved in bar dynamics.