2019 Volume 21 Issue 2 Pages 159-170
In recent years, invasive alien plant species, Sicyos angulatus has been spreading across the shoreline area especially in the drawdown zone, where the water level fluctuates （8 m） between the flood and non-flood seasons, in the Miharu dam. Particularly, the expansion of the S. angulatus community has been conspicuous since 2009, when the water level rise to the non-flood season has slowed along with the maintenance work for the reservoir shoreline in the end of the flood season. It is hypothesized that decrease in the number of flooding days given to the seeds of S. angulatus during the non-flood season relates to the proliferation of S. angulatus community. Hence, the research was focused on the ecological characteristics such as water tolerance of S. angulatus seeds. It is found that many S. angulatus fruits observed along the shoreline turning their color green to brown in October, leaving seeds to mature. Green and brown fruits had different water tolerant properties. All green fruits sank in the water in 178 days and were decayed, and the germination rates in the next spring were low. Therefore, it was considered that fruit germination could be reduced if the water level was raised to immerse S. angulatus community by the middle of October, when the fruits were ripened. In the case of the Miharu dam, germination of S. angulatus could be inhibited within the range of 2 m above drawn water level in flood season. On the other hand, about 40％ of the brown mature fruits were floating even after the seven months of the flooding period, and more than 20％ germinated in the next spring whether they are floating or sunken. Since the water level during the non-flood season is raised back near the normal high-water level and maintained in the Miharu dam, it was thought that the floating fruits were diffused near the normal high-water level and germinated as seedlings in the next spring. This could be the reason S. angulatus community in the Miharu dam was large near the normal high-water level. In order to suppress S. angulatus, it is crucial to immerse the fruits while immature, cut down S. angulatus before attaching the fruits, and prevent fruits from reaching to the shore.
