Hydrochorous seed transport in a small river in Northern Germany as trait-dependent filter of plant dispersal and recruitment

We studied seed transport in the Upper Eider River (Northern Germany). Our main questions were: Diaspores of what species are transported in the river and which of them remain viable after the drift? Could functional species traits be used as predictors for the survival of plant propagules in course of water transport? The water body of the Upper Eider River was sampled for plant diaspores at two bridges with each four traps changed weekly during the whole year. Samples were separated into two equal groups. Each two samples per bridge and week were dried and seeds were counted manually. The other two samples were spread on sterilised soil for germination. Species composition and community weighted trait means were compared for dried samples (total transport) and seedlings (germinated after drift). About half of the species and 1/10 of the seeds were able to survive hydrochorous transport. Species traits (community weighted means) were not reliable predictors for survival of species during the hydrochorous transport, but the majority of traits reflect the differences between the transport pools of seeds and seedlings. Small seed size, ruderal life strategy, and high light preference correlate positively with germinability after the drift, while large-seeded species adapted to endozoochory tend to lose viability during hydrochory. Dispersal of terrestrial plants with running water in the studied small river system is a highly stochastic event. We did not find evidence that specific adaptations to hydrochory significantly contribute to its success. Nevertheless, a few functional traits can increase the probability for the species to pass through the ecological filter “hydrochory.” Among those traits are (i) small seed size (less vulnerability for mechanical stress), (ii) generalist dispersal mode, less dependent on other particular vectors, and (iii) ruderal life strategy.