Investigating transport and deposition of plant seeds in an alpine braided floodplain

Fluvial seed dispersal is a key mechanism controlling riparian vegetation dynamics along rivers. Riparian plant species produce a large number of lightweight seeds that are well adapted to be transported by wind and water over long distances both along channels and across floodplains. Water-mediated dispersal (hydrochory) is particularly sensitive on how water flows into secondary channels, low-flow velocity areas where seeds can land and eventually germinate. River-floodplain connectivity is thus a key factor to understand the spatial and temporal distribution of riparian vegetation and its modifications due to morphological changes.

Despite our general understanding of hydrochory, the specific relationship between water discharge, river morphology and seed dispersal at the reach scale remains largely unexplored. To quantify this relationship, we investigated the transport and deposition mechanisms of plant seeds in a 1 km long reach of the Moesa river, Switzerland. The reach is characterized by a relative wide gravel bed floodplain that has been activated by multiple flood events in the past years. Vegetation patches and wood debris are sparsely located across the floodplain, creating a mosaic of morphological structures. We combine field experiments with mimics and numerical modelling to understand the influence of such structures and discharge variability on plant seed dispersal.

We will show how specific river planform structures, such as confluences, riffles, pools, and log jams affect particles travel times, long-distance dispersal, and deposition patterns. We will be able to show how daily and seasonal fluctuations in the water discharge may influence these processes, considering some key seed morphological traits. Our results will be analyzed with connectivity metrics applied at the reach scale but at different temporal scales to disentangle the role of each underlying process. Overall, this study is expected to aid river managers to mitigate impacts of flow discontinuities such as dams and physical barriers on riparian vegetation dynamics and design better e-flows regulations favoring plant biodiversity and ecosystem resilience in the face of climate change.