Effects of streamflow variability on the vertical root density distribution of willow cutting experiments

The use of riparian tree cuttings for river rehabilitation relies on a good understanding of cutting survival and growth responses to environmental variables. A series of experiments aimed at studying the effects of water table fluctuation induced by streamflow variability on the development of below-ground biomass of riparian tree cuttings was performed on the Thur River (Switzerland). White willow (Salix alba) cuttings were installed in gravel bar plots at various locations to develop an understanding of how topography and flow variability control cutting root development strategies, which in turn can lead to observed different vertical root density distributions in response to varying forms of tropism. In particular, we show that the transient root growth dynamics, eventually create a distribution reflecting the hydrologic conditions experienced towards the end of the growing season. A scaling relationship is presented that is useful to predict the expected depth with the highest root density as a function of soil topography and river discharge statistics. The results have implications for predicting cutting survival and growth, along with a corresponding strengthening of the alluvial sediment, in different flow regimes. This approach is therefore useful to incorporate in morphodynamic models covering the ecological and hydraulics responses of below-ground biomass to floods.