Effect of Thinning on the Thickening Growth of Stems and Development of Epicormic Shoots in a Secondary Forest of Quercus Crispula and Fagus Cre Ta

27 Mar Effect of Thinning on the Thickening Growth of Stems and Development of Epicormic Shoots in a Secondary Forest of Quercus Crispula and Fagus Cre Ta

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Abstract

鳥取大学付属フィールドサイエンスセンター「三朝の森」の45年生のミズナラやブナを主とする二次林で, 間伐後21年目 (一部, 20年目) の生育状況を調査し, 間伐効果を検討した。間伐は, 上層間伐方式を採用した。胸高断面積合計の年平均成長量および成長率は, 対照区に比べて間伐区や間伐+施肥区で多く (高く) なる傾向があった。しかしながら, 施肥の効果はみられなかった。単木ごとの間伐直後の胸高直径と胸高断面積の年平均成長量や成長率の関係から, 小中径木ほど間伐効果が高い傾向が明らかで, 対照区に比べて間伐区 (間伐+施肥区) で胸高直径25 cm以上の大径木が増加した。間伐時に大きな直径階での間伐割合が高い林分で, 大径木の本数が増加した。樹種別では, ミズナラ, ブナ, ミズメなどで間伐効果がみられたが, コシアブラやミズキでは間伐効果はみられずかえって間伐区で枯死木が多かった。ミズナラの後生枝の平均本数と間伐後21年目の林分材積の関係は, 有意な負の相関を示した。林冠を構成する大きな胸高直径階での均等で高い間伐率は, 単木ごとの肥大成長を促進すると同時に, 林分材積が増加することで後生枝の少ない林分へ誘導するものと考えられる。

English Abstract

Effects of thinning for 21 years (or 20 years) in a 45 year-old secondary forest domi ted with Quercus crispula and Fagus cre ta were examined in Misasa forests of Field Science Center, Faculty of Agriculture, Tottori University. In this experiment, we adopted a method of crown thinning. The mean growth increment (GI) or rate (GR) per year of total basal area was higher in thinning or thinning+fertilization plots than in the control (non-thinned) plots. However, the fertilization alone was not effective. From the relationship between the diameter at breast height (DBH) just after thinning and mean GI or GR of the basal area of individual stems, the effect of thinning was clear in small or middle sized stems in which the big stems with a DBH of more than 25 cm increased after thinning (thinning+fertilization) compared with the control plots. The high-rate thinning of the stems in the large DBH class contributed to the increase of big stems. The effect of thinning on the growth of Quercus clispula, Fagus cre ta and Betula grossa was clear, but not in the case of Acanthopa x sciadophyloides or Cornus controversa. Several stems of the latter 2 species died in the thinning and thinning+fertilization plots, but not in the control plots. In the Quercus crispula, the mean number of epicormic shoots per stem and the stand volume after 21 years of thinning showed a significant negative correlation. It is concluded that high- and even-rate thinning of stems in the large DBH species comprising the crown canopy promoted the thickening growth of individual stems and total volume in the stand contributing to control of the development of epicormic shoots.