Effects of root architecture, physical tree characteristics, and soil shear strength on maximum resistive bending moment for overturning Salix babylonica and Juglans ailanthifolia

Effects of root architecture, physical tree characteristics, and soil shear strength on overturning moment due to flooding were investigated using Salix babylonica and Juglans ailanthifolia, exotic and invasive plants in Japanese rivers. Tree-pulling experiments that simulated flood action were conducted, and the resulting damage was examined to assess the effects of physical tree characteristics and root architecture on the maximum resistive bending moment (M (max)) for overturning. In situ soil shear strength tests were conducted to measure soil strength parameters. The effects of species differences on the M (max) were examined by analyzing root architecture. S. babylonica has a heart-root system that produces a greater overturning moment due to the strong root anchorage and the large amount of substrate that must be mobilized during overturning. J. ailanthifolia has a plate-root system that produces a smaller overturning moment. However, trees with the plate-root system may withstand overturning better due to an increased root:shoot ratio. The results of the study show that the M (max) of a tree for overturning had significant (P < 0.05) correlations with a tree's physical characteristics, including height (H), trunk diameter at breast height (D (bh)), D (bh) (2) , height multiplied by the second power of D (bh) (trunk volume index H x D (bh) (2) ), and root-soil plate depth (R (d)), and root-soil plate radius (R (r)). Considering the strategy of J. ailanthifolia to increase the root:shoot ratio for anchoring in the substrate, the trunk volume index (H x D (bh) (2) ) is a better parameter than D (bh) (2) because it indirectly involves the difference in below-ground volume and surface area. Different soil cohesion values were found at different experimental sites, and the average M (max) for overturning each species decreased linearly with increasing soil cohesion.