Conditions of establishment for the Salix community at lower-than-normal water levels along a dam reservoir shoreline

摘要

In limited-water-level reservoirs, areas along the reservoir shoreline are often exposed within the drawdown zone when water levels are reduced from normal to limited levels during the flood season. To prevent erosion and conserve the landscape, test plantings and landscaping along the shoreline have been implemented at some dam reservoirs. The establishment of vegetation within the drawdown zone contributes to both landscape and wildlife habitat conservation. Distribution, habitat, and seed-dispersal period of willows such as Salix subfragilis were investigated at Miharu Dam reservoir in northeastern Japan. Eight willow species were found around the dam, but S. subfragilis dominated within the drawdown zone. Field survey results beginning in 1995 indicate that the size of the S. subfragilis community increased after initial impoundment. Although many other Salix species disperse seeds prior to the drawdown period, S. subfragilis disperses during and after drawdown in the spring; thus, when water levels are lower than normal, its seeds are supplied to the newly exposed zones that provide suitable habitat for Salix seedling establishment. Our results suggest that S. subfragilis dominated because of water-resistant properties and timing of the seed dispersal period.