The role of sediment transportation and deposition for seed dispersal and vegetation development on gravel-bed river floodplain.

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This study aimed to clarify the role of seed dispersal by sediment transportation through riverbed for vegetation development on newly deposited sediments after flooding. The fall velocities through water of plant seeds were determined for herbaceous species that grow on the floodplain of the Tama River. Furthermore, The seeds deposited with the alluvial materials were detected by the seedling germination method, and the composition was compared with the species composition of the plant communities developed on the floodplain. Consequently, following results were obtained.1. 55 herbaceous species in 70 species had non-buoyant seeds that subsided in water within 24 hours. The fall velocities through water of the non-buoyant seeds were corresponded to the fall velocity of sediment particles of very fine sand (0.075-0.125mm) or fine sand (0.125-0.25mm).2. Seedlings of 60 species were germinated from the plant debris sampled after the flooding. It means that debris includes a number of plant seeds. Most of the seeds were buoyant and had spine or pappus.3. Seedlings of 76 species were germinated from the sediment sampled on the floodplain after the flooding, showing that sediments includes many plant seeds. Germinated seedling number and species number were higher in the samples those contain higher ratio of very fine sand and fine sand, suggesting that many seeds accumulated with the sediment particles that had same fall velocities.4. 50-70% of the component species of the pioneer plant communities on the floodplain germinated from the sediment samples. This fact suggested that plant seeds contained in alluvial materials had great contribution to development of the plant communities on the newly deposited sediments.For accurate sediment management of the river basin regarded as a sediment transportation system, it is necessary to progress the study about the seed dispersal through the sediment transportation.