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

本研究旨在阐明洪水后河床输沙传播种子对新沉积沉积物植被发育的作用。测定了生长在多摩河漫滩上的草本植物种子的落水速度。利用种子萌发法对冲积物中的种子进行了检测，并与洪泛区发育的植物群落的物种组成进行了比较。因此，得到以下结果：70种草本植物中55种种子无浮力，在24小时内沉入水中。非浮力种子的落水速度与极细沙（0.075 ~ 0.125mm）和细沙（0.125 ~ 0.25mm）泥沙颗粒的落水速度一致。从洪水后取样的植物残骸中萌发了60种幼苗。这意味着残骸中包括一些植物种子。多数种子浮力大，有棘或冠突。洪水过后，在河漫滩上采集的沉积物中萌发了76种植物幼苗，表明沉积物中含有大量的植物种子。极细沙和细沙含量较高的样品，其萌发苗数和种数较高，说明有较多种子随沉降速度相同的泥沙颗粒积累。河漫滩上先锋植物群落的组成种有50 ~ 70%是由沉积物样品萌发的。这表明冲积物中所含的植物种子对新沉积沉积物上植物群落的发育有重要贡献。为了对作为输沙系统的流域进行准确的输沙管理，有必要开展种子在输沙过程中的传播研究。

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.