Salix hukaoana is an endangered riparian pioneer tree that is distributed within a restricted area of Japan. Microsatellite genetic variations and genetic structures were investigated in 80 subpopulations patchily distributed within five river basins with varying degrees of habitat fragmentation. The correlation between geographic distance and genetic distance based on the Bayesian assignment test was significant across relatively intact riparian habitats, with steeper slopes of regression for more densely grouped subpopulations, suggesting restricted gene flow. However, the correlation became less apparent with increasing spacing of the habitat patches. These contradictory results are attributed to the increased chance of long-distance dispersal of sexual propagules among more isolated habitat patches. The observed accumulation of genetic diversity with increasing distance downstream along a few, but not all, of the rivers and the results of assignment tests suggested a downstream directionality of gene flow. The results of this study illustrate the patterns of genetic diversity and contemporary dispersal of S. hukaoana, and provide important insights into understanding the gene dispersal of riparian trees and into the conservation of genetic resources for this species.

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.