Research on Structures of Riparian Forest and its Function for Disaster Prevention

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1. Structure of woody communities behind check dams and the influence of stream water on itTwo woody communities on riverbed behind check dams were surveyed to clarify the structure of riparian woods. The growth in height and diameter of Euptelea polyandra and Salix spp., which are pioneer and dominant trees, was related to the width of sedimentary area and had a tendency to show the better condition with higher dencity in lower reach. The ages of dominant trees have agreed with the observed flood years. The analysis results of tree height and species indicated a obvious difference of woods structure between upper and lower reach. Namely, the influence of stream water on woody communities is stronger in upper reach, and frequency of destroying by debris flow is also higher.2. Constitution of woods species in reparian forest related to the flood frequencyThe distribution of riparian woods was compared with the cross section forms of riverbed in order to clarify the influence of flood frequency on woods structure. As the result of surveying and calculation of flood stages, it has been considered that the site, where floods occur sometimes per year, is bare or covered only by willow shrub. In the site, where floods arise once in few years, willow grows as dominant tree, and subordinate trees are consisted of Euptelea polyandra and Weigela hortensis. Where floods appear once for several decades, willow species lack and broadleafed trees develops as riparian woods. It means that traverse structure of riparian woods is devided to several types of tree species, according to the distance from stream and to the specific height.