Propagule deposition along river margins: linking hydrology and ecology

1. Research has highlighted the importance of buoyancy for hydrochorous propagule dispersal, but recently the river bed has been identified as a significant store of viable propagules. 2. Over four consecutive 4-month periods, deposited propagules (predominantly seeds) and sediment were sampled at 78 river bed, bank face and bank top sites within three river reaches in two catchments. Species of deposited propagules were compared with the propagule bank and standing vegetation to identify 'new' species. 3. Forty-four percent of deposited propagule species were not present in the vegetation and the largest proportions were deposited in winter. New propagules showed a higher proportion of species that establish long-term seed banks and produce light seeds, a higher proportion of R-strategists, and a lower proportion of C-strategists than species in the vegetation. 4. Ordination of propagule species abundance data revealed differences in samples between reaches; bed and bank locations; and sampling periods. Within depositional samples there were significant positive correlations between average C scores and the proportion of species producing relatively heavy, short-lived seeds and scores on an ordination axis describing a gradient from channel bed to bank top. 5. Correlations and multiple regression models between species richness and abundance of deposited propagules (total and new), and the quantity and calibre of river-deposited sediment, demonstrated a direct link between propagule deposition and hydraulic conditions during inundation. 6. Synthesis. Riparian connectivity of river flows, varying hydraulic conditions and temporary storage of propagules are all complex components of hydrochorous propagule dispersal in the study reaches. Flood flows can transfer sediment particles and non-buoyant, viable propagules from the river into the riparian zone. Propagules can pass in and out of storage within the channel bed and margins through a variety of dispersal processes and can then settle out of the water in suitable depositional environments generating distinct spatial and temporal patterns in total propagule deposition. New propagules show greatest floristic similarity to propagule bank and bed samples. They are preferentially smaller, lighter and form longer term seed banks, making them particularly suitable for long distance transport over prolonged time periods and for widespread dispersal across river margins.