Abstract
This study investigates the species distribution of willow forests at 23 rivers in Japan. These rivers were categorized into four groups (G1 to G4) by flooding histories and Salix species. G1 and G2 groups are located in Hokkaido and Tohoku/Hokuriku districts, respectively, which are characterized by snowmelt floods in April to June and predominance of northern species fitted to low temperature. G3 group is rivers in Kinki/Chugoku districts, where snow melt floods occur in January to March and moderate temperature species are dominant. Rivers in Kanto/Chubu/Kinki/Shikoku/Kyushu districts, categorized by no snowmelt floods and warm species, are selected into G4 group. The comparison was conducted for the ratio of the willow forests (WF ratio) and the total forests (TF ratio) with respect to the surveyed length (TF) in gently sloped river channels (=1/5000-1/500). The WF ratio was greatest for G1 (72%), followed by G2 (45%), G3 (25%) and G4 (4%). The ratio did not differ significantly between G2 and G3. The TF ratio was greatest for G1, followed by G2, G3 and G4, although there are only slight differences between G1 and G2 and between G2 and G3. The influence of flow regimes on WF ratio was also investigated, indicating that although the number of days of snow melt floods are positively correlated to WF ratio, summer flood days had a negative correlation. These findings indicates that Salix species compositions fall into three regional groups, or four if flood types are considered. The WF ratio differed among those four groups. Thus, the regional factors are important for riparian forest management.
