Effect of Thinning on the Tural Regeneration of Broad-leaved Trees in Abies Sachalinensis Plantations

27 Mar Effect of Thinning on the Tural Regeneration of Broad-leaved Trees in Abies Sachalinensis Plantations

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Abstract

トドマツ人工林を多様な広葉樹を含む針広混交林へと誘導する施業法を検討するため，間伐後8∼11年経過した39年生の間伐林分と28年生の無間伐林分において下層植生を調べた。間伐は3段階の異なる強度の列状間伐（1伐4残，2伐3残，3伐2残）を行った。間伐後の林内の相対光合成有効光量子束密度は，間伐強度が強くなるにしたがって明るくなる傾向があった。高木性・亜高木性広葉樹の種の豊富さと個体数は，無間伐区に比べ間伐区で高かったが，間伐区間では間伐強度が強いほど，個体数が少なくなる傾向があった。低木類と草本類の最大植生高と植被率は，間伐強度が強くなるにしたがい増加していた。2伐3残区と3伐2残区では，ウド，アキタブキなどの大型草本の回復が著しく，多くの高木性・亜高木性広葉樹はこれらの草本類により覆われていた。しかし，2伐3残区と3伐2残区では，大型草本の被圧を抜け出した樹高1 m以上の広葉樹が11種あり，その個体数は1 ha当たり2,500∼3,750本存在していたことから，十分な数の更新木が確保できたと考えられた。したがって，間伐によって，トドマツ人工林を多様な広葉樹を含む針広混交林へと誘導することが可能であり，間伐には誘導伐の効果を併せもつことが示された。

English Abstract

To examine the conversion of conifer plantations to mixed forests comprising coniferous and broad-leaved trees, we investigated responses of understory vegetation to variable-intensity thinning in 39-year-old Todo fir (Abies sachalinensis) stands 8 and 11 years after thinning, with 28-year-old unthinned stand for comparison. The following three operatio l thinning levels were used: one-row thinning and four-rows leaving (Th1), two-rows thinning and three-rows leaving (Th2), and three-rows thinning and two-rows leaving (Th3). Understory relative Photosynthetic Photon Flux Density increased with thinning intensity. Species richness and density of broad-leaved trees were positively influenced by thinning. However, in thinned stands, tree density decreased with thinning intensity. The maximum height and cover of both shrubs and herbs were positively affected by thinning, with the highest thinning level resulting in the highest maximum height and cover. The maximum height of herbs was higher than the mean height of trees under the Th2 and Th3, which may have hindered tree growth. However, 2,500∼3,750 seedlings/ha of ≥100 cm in height occurred under the Th2 and Th3 regimes, which may be sufficient for regeneration. Therefore, it may be feasible to apply thinning to convert conifer plantations to mixed forests.