Thesis Defence // Léo Rasse

Riverine aquatic vegetation often forms plant canopies (patches and stands) that interact with water flow and sediments. Understanding these two-way interactions is essential because they strongly influence the morphology of stream and river channels, as well as vegetation dynamics. Despite advances in characterising the effects of plant canopies on hydro-geomorphological processes, estimating their response to flow remains challenging due to the difficulty in scaling up individual shoot responses to the canopy level, in determining the spatio-temporal variability of the factors involved in plant – flow interactions, and in studying canopy responses to water flow over large spatial and temporal scales. This thesis aimed to investigate the spatial and temporal responses of aquatic plant canopies to water flow, as well as the spatio-temporal variability in the factors influencing these responses. By using laboratory experiments, field surveys and remote sensing approaches, the properties of aquatic plant canopies involved in plant – flow interaction were measured on patches and stands across multiple spatial and temporal scales. These properties included canopy drag and anchorage forces, canopy size and shoot density, and plant functional traits. The results showed that, in addition to plant functional traits, patch size and density are key determinant factors contributing to their drag and anchorage forces, indicating that models based solely on individual shoots may not reliably estimate patch uprooting risk. At larger scales, the properties of aquatic plant stands involved in plant – flow interactions varied over the seasonal cycle, with distinct trajectories between reaches with contrasting hydro-geomorphological conditions. These patterns highlight that stand responses to flow events should vary across reaches and seasons, and that modelling efforts must consider this spatio-temporal variability. Lastly, high spatial and temporal resolution PlanetScope satellite imagery (3 m, global and ~ daily acquisition) enabled the detection of large changes in stand surface area. By advancing the understanding of canopy responses to water flow and providing new opportunities to monitor riverine aquatic vegetation at large scales, the findings of this thesis provide a basis for proposing hierarchical frameworks to address scaling issues in aquatic vegetation dynamics, while also supporting practitioners in decision-making processes.