針葉樹人工林における林分構造のばらつき(II) : 生育段階別の樹高曲線の形状特性

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針葉樹人工林の樹高曲線における，生育段階の進行に伴う形状の変化および各生育段階での形状のばらつきについて調査した．大分県および熊本県内のスギ挿木（225林分），スギ実生（99林分）およびヒノキ（94林分）人工林（林齢はいずれも15～70年）の収穫調査データを用いた．樹高曲線を平均直径を境に二分し，平均直径以上および平均直径以下の曲線部分の傾きについて，生育段階別に解析した．樹高曲線の平均直径以上の曲線部分の傾き（S2）は，生育段階の進行に伴い，スギで減少，ヒノキでほぼ一定となった．これは生育段階の進行に伴う直径差（最大直径―平均直径）の増加，樹高差（最大樹高―平均樹高）の不変性を反映していた．一方，樹高曲線の平均直径以下の曲線部分の傾き（S1）は，生育段階の進行に伴いスギ実生で減少，スギ挿木およびヒノキではぼ一定となった．これは直径差（平均直径―最小直径）の増加，樹高差（平均樹高―最小樹高）の微増を反映していた．また，各生育段階内のS1およびS2は，平均直径の増加に伴い減少した．The objective of this study was to investigate the changing pattern in the form of DBH-height curve for coniferous even-aged stands with an increase in top height (H), and to investigate the variation of the form of the DBH-height curve within each H class (2m octave). Data from 225 Cryptomeria japonica D. Don stands oriented from cutting (Cj-cutting), 99 Cj stands oriented from seedlings (Cj-seedling) and 94 Chamaecyparis obtusa S. et Z, stands (Co), varying from 15- to 70-year-old, in Oita and Kumamoto prefectures, was utilized in this study. The changing rate of DBH-height curves for trees larger than and smaller than stand mean DBH, were compared respectively between H classes and within each H class for each stand group. The changing rate of curve for trees larger than stand mean DBH (S2) became gentle for Cj, and approximately constant for CO, with an increase in H class. These results were due to an increase in size difference between maximum and mean DBH, and a consistency between maximum and mean height, with an increase in 13 class. Also, the changing rate of the curve for trees smaller than stand mean DBH (S1) became gentle for Cj-seedlings, and approximately constant for Cj-cutting and Co, with an increase in H class. These results were due to an increase in size difference between mean and minimum DBH, and a slight increase in difference between mean and minimum height, with an increase in H class. The above results were discussed in relation to competition between trees and growth of DBH and total height. The changing pattern in S2, with an increase in H class, was thought to be induced by size-dependent growth in DBH and size-independent growth in height among trees larger than mean DBH. On the other hand, the changing pattern in S1 was thought to be induced by size-dependent growth in DBH and height among frees smaller than mean DBH, because competition for light, water and nutrient between trees smaller than mean DBH was generally asymmetric. Furthermore, within a given H class, S1 and S2 decreased with an increase in mean DBH for each stand group.
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針葉樹人工林における林分構造のばらつき(II) : 生育段階別の樹高曲線の形状特性

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