Contributions of large wood to the initial establishment and diversity of riparian vegetation in a bar-braided temperate river

Abstract

The purpose of this study was to examine the effects of large wood (LW) on the physical environment and the initial establishment of vascular plant species in the Rekifune River, a large bar-braided monsoonal river in Japan. The physical environment and the diversity and composition of plant species were compared in relation to the orientation of LW pieces. We found that shading effects were more prevalent in the immediate vicinity of LW pieces than in quadrats distant from LW. The effect was especially strong at the center of LW jams (the “jam center”). Fine sand and silt were concentrated in the quadrats downstream from the LW pieces. In contrast, cobbles dominated the upstream quadrats. The highest diversity was found in the jam center, while intermediate values were observed in the quadrats surrounding LW. Indicator species analysis detected 21 indicator species only in the jam center. The LW jams favored the deposition of plant fragments and sediment and created shaded areas within and around the structures. Buried seeds may be transported with LW during a flood, and seeds dispersed by wind and stream flows may be trapped by the complex structure of LW jams. The specific environmental conditions and the trapping of seeds and plant fragments result in the early establishment of mid-successional tree species at LW jams. In conclusion, the LW pieces deposited on gravel bars altered the light and substrate conditions and thereby provided specific safe sites for various riparian plant species.