立木の樹幹と根株の力学的バランスを表す指標LT比

抄録

A standing tree generally must have a strong stem and a solid root system to withstand mechanical injuries that it is likely to encounter, for example snow break and wind damage. The strength of the stem and roots, therefore, should be well-balanced as the breakage will occur in the weaker one of them. To determine the strength balance of a standing tree, the load-height tree-height (LT) ratio concept is proposed. Load height refers to the height of the lateral load which, if acting, will result a stem breakage or breakage at the roots (uprooting) having equal chances of occurring. A tree whose stem is stronger than its roots will show a large LT-ratio, and in the case of a stem weaker than its roots, the ratio will be smaller. Also discussed is the range of LT-ratios of a model sugi (Cryptomeria japonica D. DON) tree by varying the conditions such as stem curve and diameter. LT-ratios may be useful as a standard for thinning and selective-cutting operations, that is, trees to be cut or left standing can be judged effectively by their LT-ratios.