Processes contributing to the deposition of buoyant seeds along river margins 

Background/Question/Methods 

Riparian tree vegetation is an integral component to the ecology of fluvial systems. The dispersal and deposition of seeds along a river margin is one process responsible for the structuring of a plant population. Studies based on the dispersal of water transported seeds (hydrochory) focus on travel distance, trait characteristics such as buoyancy, and relationship to dispersal phenology. However, a focus on the deposition seeds remains limited in research due to the complexities that arise along a stream’s edge. At least three depositional mechanisms have been identified: stranding on rough surfaces, capture by the meniscus and infiltration by the bed. The objective of this study is to investigate the mechanisms of deposition, with a specific focus on stranding, by developing a model based on the equilibrium forces exerted on a buoyant seed.

Results/Conclusions

The model is based on the characteristic seed traits of the riparian tree White Alder (Alnus rhombifolia) where the seeds have a density around 200-250kg/m³ and a low Bond number of approximately 0.2 where surface tension effects are considered. Lab experiments were carried out to determine the physical factors involving seed deposition such as bed roughness characteristics, flow regimes and microtopography features on the bed surface. The results suggest that deposition of seeds is a primary function of the frictional forces on the exposed bed surface. Capture by the meniscus was observed and can be explained by the “Cheerios effect” where the upward force of buoyancy results in movement along the curved meniscus. The influence of infiltration as a mechanism for deposition was not tested and merits further study. This research extends to riparian tree vegetation that relies on hydrochory as a primary means for transport and is the seed is fully buoyant in the flow.