Abstract
Field observation was performed on settlement and enlargement of plant community of Robinia pseudoacacia L. and Salix subfragilis on and around a sand bar in Watarase River. The observation site was divided into three areas form view point of river topography. The age and position of all individuals were mapped on the aerial photograph, and the enlargement process of Robinia pseudoacacia L. and its competition process with Salix subfragilis were analyzed. Moreover, allometric relationships of tree height and crown width against the tree age of both species were examined and compared. As a result, it was clarified that the transition process of the plant community is different in the three topographic areas. Particularly, in the area on the sand bar where the two species are in competition, the community of Salix subfragilis has a tendency to reduce its area, meanwhile, the community of Robinia pseudoacacia L. has expanded to the boundary between the main channel and floodplain, and will be enlarged hereafter.
