Tree species composition of riparian forests and their habitat conditions along the Kinugawa River, central Japan

Eighty two plots were set at the riparian forests along the Kinugawa River and its tributaries in Tochigi Prefecture and relationships between tree species composition and site conditions were investigated. By PCA, site conditions of the plots were characterized mainly by the axis representing valley width and altitude, and secondarily by the axis representing relative elevation from water surface. By the cluster analysis, tree compositions were classified into nine: Aesculus type, Zelkova type, mixed type, Alnus-Cercidiphyllum type, Pterocarya type, Toisusu-Salix type, Toisusu type, Ulmus type, and Salix type. At the riparian area with narrow floodplain, Toisusu and Toisusu-Salix types developed on the lower floodplain, and Aesculus type on the higher floodplain and adjacent hill-slope. The montane areas (800-1,400 m in alt) were characterized by wider riparian area and by diverse microtopographic units, resulting in six of the nine types observed there. Furthermore, major types at the montane areas tended to distribute at lower altitude along the tributaries than the main river. This was attributed to the similar topography of the tributaries to the main river at higher altitude. Thus, diverse microtopographic conditions were realized within wide range in altitude when considering both the main river and tributaries, and should enable diverse riparian forests to establish in riparian habitats of the Kinugawa River basin.