The construction of upstream dams regulates flood magnitudes, reduces the inundation frequency of the riparian zones and curtails the discharge of sediments to the downstream. Adaptation of a Phragmites japonica colony growing at a rarely flooded sandbar (1.4 Â 0.25 km) was investigated. Above-and belowground biomass was sampled, together with litters on the ground surface of a quadrat and soil in the rhizosphere. Five sampling locations were selected with three locations located in sandy and two in stony areas. The results showed the total biomass and the ratio of above-to belowground biomasses were larger at the sandy sites than the stony sites. However, the fraction of root biomass (i.e. root to rhizome ratio) in the belowground biomass at the stony sites was higher than that at the sandy sites. Higher aboveground and total plant biomass in the sandy areas was because the plants had better access to water and nutrients than plants growing in stony beds. A higher proportion of root biomass, at the expense of rhizome biomass, in the belowground biomass of plant growing in stony sites was due to the difficulty in accessing water and inorganic nutrients. The ratios of nitrogen (N) to phosphorus (P) in the plant tissues were mostly between 5 and 10, indicating that plant growth was N limited. Low ratios of total N (TN) to total P (TP) were also recorded in soil samples, where TN was 300-700 mg/kg and TP was 300-350 mg/kg. A wide phenotypic plasticity observed in P. japonica was an importance factor to maximize the plant survival and fitness in frequently disturbed habitats, and this plasticity was the result of both true adjustment and ontogenetic drift. A mitigation effort, aimed to prevent coarsening of the riverbed by supplying sand to the downstream, can actually advance the development of P. japonica colonies.

Release of trapped sediment from a dam is currently considered to be an effective mitigation method in dammed rivers. However, it has a high impact on the ecosystems downstream. In order to mitigate the impact of the sediment trapped by these dams, sediment gates were constructed in the two most downstream dams-Dashidaira Dam and Unazuki Dam-to release trapped sediment into the downstream. Since synchronous sediment release from Dashidaira Dam and Unazuki Dam was initiated, however, the formerly completely stony beds of the sediment bars and the river channel downstream have become covered with sand, and in the last several years they have become remarkably vegetated. Accumulation of sediment and organic matter in densely and lightly treed areas was investigated on a sediment bar subject to sediment release from upstream. Sediment accumulated in a half cone shape behind trees, and the amount of accumulated sediment was larger at sites with greater tree density. The amounts of accumulated litter were also larger at sites with greater tree density. A higher ratio of autochthonous than allochthonous litter was observed. The ratio of TN to TP was 20-25 in this litter, as opposed to 1.0 in the soil; thus plant growth seems to be limited more by levels of nitrogen than phosphorus. Because sediment and organic matter accumulate in reservoirs over time, it was concluded that the present method of releasing trapped sediment from dams is not suitable, and further modification is required.