林木の枝量とその生長量 (II)

Abstract

1967年8月に京都大学農学部附属芦生演習林の2つのスギ林分 (24年生, 60年生) からえた2本の試料木 (S. No. 2, No. 845) について, 枝材積と生長量にかんし, 年軸解析法および相対生長法による検討をおこなった。個々の枝の基部直径D_EOKと長さL_BKのあいだには, 逆数式 (1), (2) であらわされる関係がたしかめられた (図1)。[D_EOK2 L_BK]と皮つき枝材積V_BOK, 皮なし枝材積V_BIKの関係を第I報のデータとあわせ検討し, 前者が林分分離のないかなりひろい範囲にわたる両対数直線的な関係を, 後者は前者を漸近線とする曲線の一部とみられる関係をもつことをあきらかにし, これらをV_BO, V_BIの計算に利用した (図2, 3, 表2)。両者の差を樹皮容積V_BBとした。皮なし材積生長量⊿V_BIKとV_BOKのあいだにみとめられる両対数直線関係は, 個体ごとにかなりバラバラの分離をする (図4)。これにより⊿V_BIが計算された。 -相対生長法による枝材積および生長量。枝の緑色部および褐色部の皮なし材積V_BIKと樹皮容積V_BBKをくらべると, 緑色部では後者が, 褐色部では前者が大きく, V_BIKに対するV_BBKの関係は両対数直線的で勾配は1より小さい (図5)。枝がおおきくなるとともにV_BBK/V_BIKは小さくなる。樹皮断面積は枝の上部では, 材断面積より大きいが, 緑・褐色部の境附近でひとしくなり, 下部では逆に材断面積が大きくなる。緑・褐色部の境での皮つき直径D_EOと皮なし直径D_BIの比が√<2>にちかいことは, このことをうらづけている (表3)。幹にそって一定の長さ (50cm) にとった層のなかにふくまれる枝の平均年軸長l_ij 平均年軸中央直径 (皮つきD_BOijと皮なしD_BIij) および平均年軸年間皮なし直径生長量⊿D_BIijを測定した (図6, 7, 8, 9)。これを利用し各年軸が円筒形であると仮定し (図10), i層j年軸の枝材積V_BOij, V_BIij, V_BBij および生長量⊿V_BOij, ⊿V_BIij, ⊿V_BBijを計算した (表4)。 -年軸解析による枝材積および生長量。枝材積, 生長量の2つの方法による値をくらべてみると, S. No. 2ではほぼ一致し, S. No. 845では多少のひらきがあった (表5)。年軸解析の方法によれば, 枝量の樹冠内の分布がくわしくわかるうえ, 樹皮の生長量が計算できる。が一方調査測定がかなりめんどうであること, 外観的な年軸のみわけに限度があること, したがって樹種によっては適用することができない, などの問題が指摘されうる。

Quantitative characteristics of branch dimensions of Cryptomeria japonica D. Don were investigated at the Ashiu School Forest of Kyoto University, Kyoto, in August, 1967. From two even-aged Cryptomeria stands, 24 years old and abont 60 years old, two sample trees, one from each stand, were selected for branch dimension analysis. The allometric relations between two of the branch components were the subjects of examination. The branch length (LBk) has a reciprocal relationship to the branch diameter over bark just above butt (DBk), and are given as equations (1) and (2) (Fig. 1). The relationships of the branch volume over bark (VBOk) and without bark (VBIk) to DBOk 2LBk were examined, taking the results of foregoing and present works into account: the former has a logarithmic linear relation covering a wider range without any segregation between stands or individuals, while the latter is considered to be a part of a gentle curve taking the former as asymptote, also showing no evidence of segregation (Figs. 2 and 3). The logarithmic relationship between branch without bark volume increment (ΔVBIk) and branch volume over bark shows segregation by individuals (Fig. 4). These allometric relationships were utilized in the calculation of VBO, VBI and ΔVBIk. The difference between VBO and VBI was deemed the branch bark volume (VBB) (Table 2). At the upper part of the branch, where the outside bark is greenish, the branch bark volume (VBIk) is smaller than the branch volume (VBBk); the rate of increase of the former exeeds the latter with the increase of branch age (Fig. 5). The larger the size, or the older the branch, the smaller the ratio of bark volume to branch without bark volume becomes. The crown is divided into layers of 50cm depth along the trunk. Two to four sample branches from each layer were taken and were divided by annual shoots. The mean of annual shoot length, diameter over bark and without bark at the middle, and diameter increment of each layer were measured. (Figs. 6, 7, 8 and 9). Assuming each annual shoot to be a cylinder, volumes in terms of VBOij, VBIij, VBBij, and annual increments, ΔVBOij, ΔVBIij, ΔVBBij, were calculated (Table 4). The values of the branch volumes and increments calculated by these two different methods of allometry and annual shoot analysis showed fairly good correspondence, at least in one of the two sample trees (Table 5). The annual shoot analysis has the advantage of giving detailed branch dimension distribution in the crown. Disadvantages of the field measurements are that it is laborious and troublesome, and is limited in its application to those tree species in which the annual shoot can be distinguished by the naked eye from the outside.