Influence of plant traits on biogeomorphic patterns of gravel bed rivers

Feedbacks between riparian vegetation and river morphodynamic processes are pivotal for predicting river morphology in the face of a changing climate and anthropogenic pressures. The effects of vegetation on flow and sediment transport, which ultimately contribute to shape distinct landform structures, depend on plant morphological traits that often reflect plant’s own strategy to cope with fluvial disturbances. Recent observations show that canopy biomechanics and root structure in Populus nigra seedlings tend to adapt depending on hydro-morphological conditions. However, quantitative understanding on how plasticity in plant traits influences river morphology is still limited.

Here, we propose a novel numerical model coupling river morphodynamics and vegetation dynamics that specifically accounts for above- and below-ground plant traits and their effects on morphodynamic processes.  We performed a series of numerical experiments simulating the co-evolution of alternate bars and vegetation under a sequence of flood events and qualitatively compared the results with satellite image observations in the Alpine Rhine river in Switzerland. In particular, we tested the influence of plant traits on the observed reach-scale biogeomorphic patterns by considering different vegetation configurations in which we varied the relative growth of above- and below-ground plant biomass.

Results show that vegetation cover extended over time at a rate that depends on vegetation traits, bar morphology, and the hydrological regime. On more stable bars, which experience little riverbed modification during floods, a clear signature of plant traits in plant survival to floods was observed after a long disturbance-free period, which enable plants to develop enough to interact with flow and sediment transport. As expected, plants that allocate more biomass below-ground were able to resist uprooting, while plants with taller canopies to avoid sediment burial. Along bars where riverbed scour was more pronounced during floods, vegetation was not able to develop as downstream bar migration caused extensive plant uprooting.

These results qualitatively agree with the observations in the Alpine Rhine river, where vegetation has been found to develop only on steady (more stable) bars and not on migrating bars. Our results suggest that the time required by vegetation to modify flow and sediment transport, i.e. biogeomorphic feedback window, can be associated with the time needed for plants to develop specific morphological traits. Moreover, they indicate that bar morphodynamics is able to mute or favor the emergence of plant trait-signature on biogeomorphic patterns.

This study provides a first quantitative-mechanistic understanding of the processes underlying feedbacks between vegetation and river morphodynamics highlighting the importance of plant traits, with potential implications for management purposes and river restoration projects.