Thinning Effect on Tree Growth in Secondary Betula Ermanii Stands on Scarified Sites

27 Mar Thinning Effect on Tree Growth in Secondary Betula Ermanii Stands on Scarified Sites

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Abstract

本研究では，かき起こし跡地の16年生のダケカンバ二次林で除伐試験を行い，除伐後20年間の林分および個体成長を比較し，除伐効果を検討した。除伐試験は北海道大学雨龍研究林で行われ，除伐後に密度が約1,000, 2,500, 5,000本/haとされた3プロットと，無除伐区を設定した。除伐20年後の平均胸高直径は8.4～13.1 cmであり，除伐強度の順に大きく，除伐効果が認められた。1000本区では除伐後12年間は成長促進効果が大きかったが，その後は個体間競争が強くなった。2500本区と5000本区は除伐後12年で収量比数0.9以上の混んだ状態となった。胸高直径成長速度と期首直径の関係は，除伐後はプロット間で差がみられたが，成長に伴い収束する傾向にあった。これらの結果から，ダケカンバ二次林では，若齢時の除伐であれば，非常に疎な状態となる強い除伐であってもその後の個体成長に問題はなく，除伐コストを考えると強い除伐が推奨できる。

English Abstract

We conducted a thinning experiment in secondary Betula ermanii stands on scarified sites and examined the thinning effect on stand and tree growths after the thinning. The experiment was conducted in tural B. ermanii stands in the Uryu experimental forest of Hokkaido University, and 3 plots thinned to approximately 1,000, 2,500, 5,000 trees/ha and an unthinned, control plot were established at 16-year-old stand age. After 20 years of the thinning, mean diameter at breast height was 8.4-13.1 cm and larger in the order of thinning intensity, and therefore, thinning effect was recognized in the thinning experiment. In 1000-tree-plot, tree growth was apparently accelerated in 12 years after thinning, and thereafter, tree competition was enhanced. In 2500- and 5000-tree-plots, the relative yield indices were more than 0.9 at 12 years after thinnings, and they were considerably crowded. Relationships between mean diameter increment and tree diameter differed among plots after the thinning, but the differences diminished with tree growth. In conclusion, we recommend an intensive thinning in tural B. ermanii stands in young growth stage, because tree growth would be enhanced and thinning cost would be small in such a thinning system.