Studies on the Formation of Compound Storied Forest by Line-thinning

1941年植栽(26年生)のスギ林に対し,3列おきに2列伐採, 3列おきに1列伐採の列状間伐を1967年におこなった.伐採跡地にはヒノキを植栽し,2段林造成を試みた.この研究は,列状間伐によるスギ林の環境とくに光環境の変化と二次植栽木ヒノキの生長についておこなったものである.その結果,次のようなことがわかった.林内の明るさは,列状間伐前には相対照度は10数%であったが間伐後は40～60%とかなり明るくなるがスギ･ヒノキの生長にともなって10数年後には相対照度は再び数%程度まで低下することがわかった.保存列区のスギと二次植栽木のヒノキの生長については,スギは間伐面に接した列の直径生長が優れており,中央列の生長は樹高では差がないが直径では劣っている.また,1列伐採区は列間隔が狭いため伐開後20年を経過すると,左右のスギ樹冠の発達によりかなりうっ閉されており,そのためヒノキは光不足になり,樹高,直径ともに極端に劣っている.一方,2列伐採区ではヒノキの列間は生長にともなって樹冠が接し,枝条の枯れ上がりがみられ樹高は優れているが,直径がやや劣っている.したがって,とくに1列伐採区の二次植栽木ヒノキの生長を促進するためにはスギの枝打,間伐による光環境の改善が必要であることを認めた.The purpose of this paper is to create a two-storied forest of Cryptomeria japonica D. Don in upper story and Chamaecyparis obtusa S. et Z. in lower story. This investigation was carried out in the Cryptomeria japonica D. Don stand in the Kyushu University Forest of MIYAZAKI, where 46-year-old Cryptomeria japonica were line-thinned in 1967, and Chamaecyparis obtusa were planted in the cut-over area. The change in the relative light intensity in the stand and growth of Cryptomeria japonica and Chamaecyparis obtusa after line-thinning were measured periodically. The results obtained are summarized as follows: -Relative light intensity- Relative light intensity in the stand before line-thinning was ten-odd %, but changed to 40～60% after line-thinning. After that, relative light intensity dropped considerably with the growth of Cryptomeria japonica and Chamaecyparis obtusa. It was recognized that the pruning of reserved trees was very effective in improving the light conditions under the canopy of the trees. -Growth of Cryptomeria japonica and Chamaecyparis obtusa- Growth of Cryptomeria japonica showed a little difference in height after line-thinning. However, growth of the individuals facing the thinned side was superior in diameter, and the growth of the individuals in the central row of the reserved trees of the line-thinned plots was inferior. We tried to elucidate the relation between the individuals in terms of the changes in the individual rankings of diameter. Judging from the results, the 1/4 line-thinning was not effective in improving the light conditions in the central row of the reserved trees. Growth of Chamaecyparis obtusa in a row planting plot were extremely inferior as regards both height and diameter growth. It was considered that the branches of the surrounding trees developed toward the thinned side; consequently, the light conditions in the row planting plot grew particularly worse. Accordingly, it was judged that the row distance was narrow. From the above results, it is considered necessary to carry out the pruning or thinning of Cryptomeria japonica and Chamaecypris obtusa in order to improve the light conditions in the stand. In that case, this disposal is expected to carry out carefully without hastily improving the light conditions. It is desired that the relative light intensity in the stand is adjusted at 20 to 40%