The fate of Salicaceae seedlings related to the dynamics of alluvial bars during floods: differentiating bed erosion, uprooting and burying.

Abstract:

Riparian vegetation is a key factor of the morphological evolution of river. In Europe, riparian Salicaceae is declining following the loss of potential recruitment areas associated with river management. As an exception for lowland rivers, the Loire River (France) shows, in its middle reaches, an efficient sexual regeneration of Populus nigra and Salix alba on bare sediments deposited during flood events.

In the literature, the influence of hydrological patterns as a key factor of the seedlings survival is well documented. Some studies focused on seedlings ability to withstand flood constraints and detailed the effect of duration and intensity of floods but few studies characterized precisely the processes applied on seedlings. As a working hypothesis, we consider that three types of flood stresses can induce mortality of seedlings: (i) uprooting by drag applied on the seedlings without sediment erosion, (ii) erosion of the recruited areas and (iii) burying. The distinction of these three processes allows identifying the importance of survival factors due to a strong sediment dynamics (e.g. erosion height > root height) or to the anchorage and resprouting ability.

The main issues are: what are the governing processes (type and intensity) determining survival or death of seedlings and which factor (fluvial dynamics vs. own characteristics of seedlings) controls their survival?

In-situ measurements were performed on a forced alluvial bar (20.000 m²) to detail the bar dynamics (bathymetry, topography, scour/fill processes, grain size surveys, flow velocity) and to survey the associated recruitment. On 48 plots (1.410 m²) the density, height and species (P. nigra and S. alba) were surveyed the year of recruitment (after dry period) and the next year after flood period.

We highlight the following phases of processes during floods. The erosion of substrate dominates at the beginning of the rising limb. The erosion or uprooting processes depend of the balance between available bed shear stress and sediment size. Then the deposit occurs on the back of the bar before the peak discharge and protects them against uprooting by burying during the higher energy of flow. At the end of the falling limb, sediments are reworked, decreasing the burying height of seedlings and allowing possible uprooting (drag) or erosion of sediments.