Abstract
This study examined the environmental conditions that were required to maintain a threatened species, Salix hukaoana. Twenty-five plots were set up in the riparian zone along the Yubiso River, and forest structure, topography, and forest age were investigated. The riparian forest in the area was classified into the following five forest types by dominant species: Salix sachalinensis, Toisusu urbaniana, Salix hukaoana, Pterocarya rhoifolia, or Fagus crenata. The Salix sachalinensis type was found only in the lower floodplain. The Salix hukaoana type and Toisusu urbaniana type were found in both the lower and the higher floodplain. The Pterocarya rhoifolia type was found in the lower floodplain and on hillslopes in the higher floodplain. The Fagus crenata type was found on both terraces and hillslopes. All the stands of Salicaceae forest types were less than 45 years old. Among the five forest types, the stand age of Salix sachalinensis type was lower than the other types by 27 years, while those of Pterocarya rhoifolia and Fagus crenata types aged more than 88 years. The Salix hukaoana type was likely to become established on open areas in the floodplain after a fluvial disturbance and was maintained until its replacement by the Pterocarya rhoifolia type. In the Yubiso River valley, a mosaic structure consisting of various types of forest might be formed due to the diverse disturbance regime and environmental conditions that are peculiar to the wide valley floor. Therefore, it is crucial to maintain this variation in disturbance regime and site conditions for conserving the S. hukaoana population together with the riparian vegetation and along the Yubiso River.
