Abstract
The distributions of three dominant species, Fraxinus spaethiana, Pterocarya rhoifolia and Cercidiphyllum japonicum, were investigated in relation to the species composition in a riparian forest in Chichibu, central Japan. This riparian forest varied from V-shaped valley to sedimentary basin of earth and rocks topographically, and was classified into a Chrysosplenieto-Fraxinetum spaethianae association phytosociologically. This association was further classified into three community units each with characteristic topography. The Acer shirasawanum sub-association was distributed in the sedimentary basin of earth and rocks, Deinanthe bifida sub-association Carex morrowii variant in slopes and sub-ridges of V-shaped valley, and D. bifida sub-association typical variant in taluses and the bottom of the V-shaped valley. The dominant tree species F. spaethiana was distributed at all sites, especially in a stable location such as a sedimentary basin of earth and rocks. On the other hand, P. rhoifolia was distributed in the alluvial fan area and sedimentary basin of earth and rocks, and C. japonicum in V-shaped valley. Therefore F. spaethiana was dominant in all areas of the Chrysosplenieto-Fraxinetum spaethianae association. On the other hand, the distribution of P. rhoifolia was roughly identical to that of the A. shirasawanum sub-association, and C. japonicum was distributed in the area of D. bifida sub-association including both C. morrowii and typical variants. This distribution of the dominant species would depend on each regeneration trait adapted to the disturbance in the riparian area. It can be concluded that each vegetation unit reflects the location suitable for regeneration of these dominant species in the riparian forest.
