Suppression of alien plant Amorpha fruticosa communities and transition test to native willow communities on the shoreline in the Miharu dam reservoir.

Abstract

In recent years, invasive alien plant species, Amorpha fruticosa 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 reservoir. In this study, the target was to establish native plant species, Salix subfragilis and Salix jessoensis growing naturally on the shoreline of the Miharu dam reservoir, after logging A. fruticosa. Frequency of logging A. fruticosa and timing of willow planting were changed to seek optimal conditions which the two willow species became dominant over A. fruticosa. Two willow species were planted with cuttings in June and December. The survival rate in June was higher than one in December. Due to the periodical inundation, optimal time to transplant willows in the drawdown area was limited to June in the Miharu dam reservoir. A. fruticosa was cut multiple times but logging three times in 13 months was less effective on establishing the willow species. By cutting A. fruticosa five times in two years (twice a year or more during growing period), the two willow species became higher and dominant over A. fruticosa until the third year after logging. The slope of the dam reservoir in this study was submerged for more than 100 days per year, which was different from the case in the previous study. However, the number of logging required to suppress, A. fruticosa in the drawdown area was the same as one required for slopes without inundation. This indicated that inundation did not suppress growth of A. fruticosa. Logging effects was disappeared in 4^th year (no significant difference with control area) and it is desirable to cut down A. fruticosa at least before reaching 4^th year.