スギ大径並材生産を目指した強度間伐の多角的評価 : 徳島県における選木育林・早期仕上げ間伐を例として

Abstract

選木育林・早期仕上げ間伐は, スギ大径並材の早期生産を目指し徳島県西部で30年程前に普及が図られた育林体系で, 約15年生時の選木と, 約25年生時の強度間伐を特徴としている.本研究では, 強度間伐から長時間経過した林分と通常の間伐が行われてきた林分との比較を通じ, 強度間伐が林木および林分の成長, 森林生態系, 間伐の採算性に与える影響を検討した.植生調査からは.低木・亜高木層の種多様性が通常間伐林分に比べて選木育林林分で高いことが示され, これは選木育林林分で下層植生が連続的に維持され, 遷移が進んだ結果と考えられた.また毎木調査より, 選木育林林分のほうが通常間伐林分よりも林分蓄積は小さいが単木材積は大きいことが示され, より多くの大径材を早期に生産するという目的はおおむね達成されたことがわかった.さらに樹幹解析と間伐材の売上額予測では, 強度間伐実施林齢を少し上げると, 残存木の肥大成長はやや劣るものの, 間伐材をより有利に販売できる可能性が示された.本施業体系の現代における応用を考えるにあたっては.本研究の結果を踏まえ, 強度間伐の実施時期, 最終伐期, 伐出システム等について再度検討する必要があるだろう.

Senboku-ikurin is a local silvicultural system which was proposed more than 30 years ago in western part of Tokushima prefecture. The noteworthy features are early selection of trees left for the future at stand age of ca. 15 years, and heavy thinning at ca. 25 years. Its goal is early production of large-diameter logs. In this study, we tried to evaluate effects of the heavy thinning on the growth of Cryptomeria japonica, forest ecosystem, and the profitability of thinning. The evaluation was made by comparing characteristics of stands that experienced heavy thinning and those that were treated by normal thinning. The results of vegetation survey showed that the heavy thinning enhanced species-diversity of woody plants. Additionally, stem analysis and estimation of income from heavy thinning suggested that early practice of heavy thinning is superior in tree growth to late practice, but inferior in its profitability. We concluded that the heavy thinning of "Senboku-ikurin" had positive effects for planted trees, forest ecosystems, and forest management. However when we try to put this system to practical use at the present time, we should introduce some operational flexibility by making changes in the timings of heavy thinning and final cutting, and innovate more appropriate logging systems.