粗放仕立てのスギ壮齢林分の構造と生長

Abstract

一般に, 日本における有用針葉樹の皆伐施業による造林は, 自然環境や生産目標が各地域で異なるにもかかわらず, 画一的でしかも集約な育林技術体系によって行われている。しかし悪化する経営条件を改善し, 林地を保全するには, 粗放な技術体系をも確立させる必要がある。本報告はこのような主旨から, 1986年1月, 高槻市北方郊外の樫団地区に存在する粗放仕立てによって成林した40年生スギ林の構造と生産力を解析したものである。調査林分は第二次大戦直後にスギを造林した後, 下刈り, 除伐等の保育はほとんど行われなかったため, 全層にスギが多数成立するほか, 上層に天然生のアカマツを, 中, 下層にヒノキ, 広葉樹を混交する複層林である。上層の樹冠密度は大きいが, 林内の平均相対照度は約2. 8％で比較的明るかった。樹幹解析からもとめた樹高生長 (資料木A - Dは1987年1月測定) から判断すると, 優勢木の生長は劣勢木より著しく良好で, 年平均樹高生長量は40cm以上もあった。また下層には造林後天然更新したスギが成立している。したがって本林分のスギは耐陰性の高い系統のように思われる。胸高直径と樹高の本数分布はほぼL型で, 柱適寸 (16cm) 以上のスギは300本/haほどである。スギ上層木のH/Dは80 - 70 (同単位) で, スギ保育林分とほぼ同様である。D_2Hに対するV_sの相対生長関係からみると, 上層木のスギはうらごけの傾向がある。全層の立木本数は極めて多く (約4, 000本/ha), 胸高直径と樹高の平均値は小さいが, 上層木のみでは他の林分と比較し地位下程度の値となっている。林分材積は上層木のみの合計で, 310m_3/ha以上もあるが, そのうちスギは約40％であり, アカマツの占める割合が多い。なおヒノキと広葉樹の林分材積は著しく少ない。これに反し林分の冬期における全葉量はスギ単純林と類似するが, 相対的にスギの葉量の比率は高い。以上のことからアカマツのぬき伐りによってスギの生産量を高めることは可能であり, 粗放な育林法も一つの施業技術となりえよう。

In spite of the differences in the natural environment and productive target in each region, the intensive and uniform silviculture system is used for the afforestation of the useful conifer by the clear cutting system in Japan. Howevere, the establishment of an extensive system is also necessary for the betterment of bad forest management and the conservation of forest land. This paper discusses the growing structure and the productivity of a 40-year-old Sugi (Cryptomeria japonica) stand cultivated with extensive tending at Kasida region located in the northern part of Takatsuki City investigated in January, 1986. This stand was cultivated by less weeding since the Sugi seedlings has been planted, which was immediately after the Second World War. Therefore, the existing stand was formed of a multi-storied forest of stratified mixture type with Sugi constituting the whole stratum, Akamatsu(Pinus densiflora) occupying the upper stratum and Hinoki (Chamaecyparis obtusa) or broad leaved trees composing the middle and lower stratum (Fig.1). The crown density of upper stratum was high (Fig.2), but the relative light intensity on ground vegetation had a high value of about 2.8%. Judging from the analysis of the annual rings of the sample trees(A-D were investigated in January, 1987), the growth of the dominant trees was much more vigorous than that of the dominated trees, and its mean height increment so far was estimated to be about 40cm/year (Fig.3). Furthermore, it was recognized that the Sugi trees naturally regenrated from seeds of neighboring trees after planting in the lower stratum (Fig.3). Therefore, this Sugi is presumed to have high tolerance. The distribution of the diameter at breast height and the tree height of Sugi in the research stand resembled those of the L type, and the number of Sugi trees with a pillar size larger than the optimum size(D:16cm<) was about 300/ha (Fig.4). However, Akamatsu could not grow in the lower stratum, and Hinoki and broad leaved trees could not grow in the upper stratum. The ratio of height to diameter(H/D: same unit) of Sugi trees in upper stratum nearly equaled that of the Sugi artificial forest well tended, i.e., 80 - 70 (Fig.5). Viewing from the allometric relation of the stem volume to D2H, the stem of Sugi dominant trees tended to be tapered (Fig.6). As the total number of the whole stratum was large, i. e., about 4000/ha, the mean diameter and the mean height were very small (Table 1), but only the mean diameter and height of Sugi in the upper stratum were similar to those of the other Sugi stands having the lowest site class. The total stem volume of the upper stratum in the research stand was about 310m3/ha, but the existing volume ratio of Sugi was about 40%. Therefore, the ratio of Akamatsu was large, and the stem volume of Hinoki and broad leaved trees was remarkably small (Table 1). On the other hand, the biomass of foliage in winter tended to be nearly equal in value to that of other pure forest of Sugi, but the ratio of the leaf dry weight of Sugi in the canopy was relatively high (Table 1). As mentioned above, the productivity of Sugi may be high if the number of Akamatsu is adequately controlled by thinning. Therefore, the extensive silviculture system can be used as one possible working technique.