Effects of fluvial geomorphology on riparian tree species in Rekifune River, northern Japan

Abstract

Classification of riverbed geomorphic surfaces based on flooding frequency was conducted and the relationship between their distribution and river morphology was analyzed, to provide an understanding of the structure and species composition of riparian forests dominated by Chosenia arbutifolia. The channel floors of two contrasting river morphologies (bar-braided and incised meandering channels), were divided into five geomorphic surfaces (gravel bar, lower and upper floodplains, secondary channel, and terrace) based on the water level of a 2-yr and a 20-yr recurrence interval. The environmental variables of the same geomorphic surfaces showed similar trends regardless of braided and meandering channel morphology, but differed significantly among the five geomorphic surfaces, which influenced the dominance of tree species. The geomorphic surface map based on recurrence interval of flood and physiognomical vegetation map based on aerial photos appeared almost identical. Geomorphic surface distribution, determined by river channel dynamics and the sediment transport processes occurring at a larger scale and a longer time frame, played an important role in shaping the structure and composition of the riparian forests. C. arbutifolia dominated gravel bar, and the upper and lower floodplains, because these geomorphic surfaces were characterized by gravelly soils which have lower soil moisture availability than soils of other geomorphic surfaces. Thus, an extensive distribution of C. arbutifolia in the braided channel section can be attributed to the frequent lateral migrations of river channels, which resulted in a high ratio of gravel bars, and lower and upper floodplains. In order to preserve indigenous plant communities in riparian zone, dynamic nature and processes of braided rivers should be maintained.