数量的間伐に関する生態学的研究

Abstract

従来の間伐のほとんどは樹型級区分によって行なわれて来たが, 樹型級区分による間伐では作業前に残存すべきあるいは間伐すべき材積が数量的に表示できず, 本数管理を系統的に量的に明示されえない欠点がある｡ そこで量的数的に表示されるような間伐方法が最近いろいろ考えられてきた｡ 量的, 数的な間伐を行なうためには林分やそれを構成する個体の一般的な生長の状態を知るとともに, 林分の生産, 消費を解析して林分の生産構造を明きらかにする必要がある｡ 一般にあまり極端に立地が違わなければ単位面積あたりの葉量は, 林分が閉鎖していれば樹種別にほぼ一定である｡ とすると葉の同化能率は樹種によってほぼ決っているから単位面積あたりの総同化生産量は同一樹種でだいたい一定になる｡ 林分の純生産量は葉による総同化生産量と, 呼吸, 枯死その他による消費との差で示されるが, haあたりの年間純生産量は針葉樹林で5 ~ 20ton, 落葉広葉樹林で3 ~ 10tonと推定されている｡ 同化生産物の幹への配分は, 立木密密度によって影響をうけ, 立木度が高いほどhaあたりの幹量は多く逆に枝はすくなくなる｡ しかしあまり立木密度が高いと幹は細長となり自然間引による枯損が多くなる｡ 数樹種について立木密度に関する特性曲線 (平均幹材積と立木密度の関係における林分が保持しうる最大密度曲線) を求め, さらにこれに競争密度効果線の代りに用いた等平均樹高線を組みあわせて, 平均樹高を媒体として約450種類の間伐モデルを例をスギにとって想定し, その主間伐合計幹材積を計算によって求めて比較した｡ その結果, 閉鎖が極端に破られないように林分が管理される限り, またその林分が放置されている状態で主伐時に特性曲線に至る程度以上の植栽本数があれば, 主間伐合計幹材積は間伐の経路 (間伐開始の遅速, 間断年数の長短, 植栽密度, 主伐密度の多少など) にかかわらず, あまり大きな差を示さないことが明きらかとなった｡ そしてこの計算に用いた諸因子を応用して, スギ, ヒノキ, アカマツに関する間伐指針表を平均樹高を変数として調製した｡ 林分の物質生産面から考えて, 林分全体の呼吸という消費因子をできるだけおさえるために, 同化生産にあまり関与せず呼吸損失の多い下層木を伐るという考え方で, 物質生産に最も有利な立木密度が決まるはずで, これは今後の呼吸消費量の研究にまたねばならない｡ また, 幹材積の出現頻度や, 個体間の順位変動などを検討した結果, 間伐のみによって均一に大きさのそろった幹材生産を導くことが不可能であることがわかった｡ 均一な大きさのそろった幹材を生産するためには, 上層木ほど強い緑枝打を間伐と併用することが有効であろうと思われる｡

In the past, almost all of the thinnings used to be treated according to Crown-class or stemclass systems, but under these systems it was impossible to have systematic or quantitative informations on the number or volume of removed or reserved stems. Then, the new thinning systems have been developed which can be expressed quantitatively or numerically. To practice quantitative thinning, it is necessary not only to obtain general rules on growth of the stand and that of individual trees, but also to analyse the factors concerning with the production or consumption and to make clear the production structure of the stand. The leaf-amount is closely connected with the growth of stand or of individual trees. The leaf-amount of the same species seems to keep the constant value per unit area regardless of the stand-age, the site quality (within its reasonable range), tree-size or stand density, if the crowns are well closed (Table 1). Supposing the assimilation-rate of leaf is almost constant in the same species, it may be possible to guess that the productions per unit area of the same species will have little variations among closed stands. However, it has to be noticed that the extraordinary changing of the soil conditions, such as by fertilization and ploughing, seems to result in a changed leaf-amount and assimilation-rate. The net production of a stand can be formulated as the balance of the assimilated products by leaves and the consumption such as respiration, death of plants, leaf- and branch-fall etc.. The annual net productions of the dry matter have been estimated to amount to 5-20 ton per ha. in coniferous stands and 3-10 ton per ha in deciduous broad-leaved stands (Table 2). The allotment of produced matter to the stem is affected by stand density. As the total amount of branches per unit area tends to decrease and that of stems tends to increase with higher density (Fig. 1), the high stand density may be profitable for the purpose of stem production. But when the stand density is higher, the number of dead trees caused by natural thinning increases and the mean stem volume becomes smaller. So, the optimum stand density must be determined on the point of which it is expected to produce as much stem volume as possible per unit area, single stems having the favorable average size. The authors estimated the full-density curves and the equivalent-height curves in several species in relation to the stem volume and the stand density. These are very important for us to discuss the stem volume yield (Fig. 2 and Table 3). Further, the authors tried to draw up a diagram of stem volume and stand density, using the case of Cryptomeria japonica as an example, combining the curves of full-density and of equivalent-height (Fig. 3). The latter curves were employed in place of the usual C-D curves (competition-density curves)32)77). And using this diagram, or the rules of competition-density effect, total yields were calculated and compared with each other for numerous models of thinning, which were designed on various grades, intervals, beginning times of thinning and initial planting densities (Table 5). The results obtained were as follows: (Fig. 4-7) The yield in the final cutting increased with the higher final stand density. While the total thinning yields showed the tendency to increase with the initial planting density, the relative density (the ratio of actual density to the full-density estimated by the same equivalent-height curve), and the frequency of thinning. The earlier the beginning of thinning, the more the total yields by thinning, above the 25 percent relative density, and the reverse, below 25 percent. The total yield, i. e. the sum of final and intermediate yields, tended to increase similarly with higher initial planting density, with higher relative density, with shorter interval, and with earlier beginning of thinning over 25 percent level of the relative density or reverse for under 25 percent level. The above mentioned tendencies were very remarkable when the relative density was under 25 percent, but were far less remarkable when the relative density was over 25 percent. Therefore, it may be approximately stated the sum of stem yields keeps the constant value over 25 percent relative density, even if the factors, such as initial planting density, grade, interval and beginning of thinning were changed. The density, of which the relative value is 25 percent, seems to be one in which the crowns nearly come to their closure, so the statment may be allowed that the total yield of stems is little affected by various kinds of thinning management, if the crown closure of the stand is kept unbroken. However, the above mentioned relationship cannot be realized, when the initial planting density of the stand is not so high that, the stand cannot reach the stage of full-density even at its final cutting under unthinned conditions. The tentative thinning schedules presented for Cryptomeria japonica, Chamaecyparis obtusa and Pinus densiflora, which were calculated by the authors according to the relation between stem production and stand density described above. Here, the mean height of the stands is the single variable, the site and age being considered secondarily (Table 7). Concerning the balance of the production and consumption in a stand, respiration is one of the most important fractions of the consumption. Then the optimum thinning grade must be found, in which the most efficient production is expected in the means of reducing the amount of consumption in respiration. The studies on the respiration in the stand and the applications of the results to the thinning problem are the important subjects left for the future. The "L-shaped" distribution of stem volume in a stand could not be corrected with the thinning of removing suppressed or smaller individuals. But the green-pruning on dominant or larger individuals seems to be possible to correct the L-shaped distribution, so that it may be effective for obtaining uniform individual stem volume to practice the green-pruning on dominant individuals connecting with thinning.