林木の空間分布の研究 (I) : 人工一斉林における優勢木、劣勢木の分布

Abstract

一般に人工一斉林では, 生長過程を通じて個体間に優劣の差が生じ, この差は時間の経過とともにますます大きくなる。従って生長初期の個体重等の頻度分布の正規型は, 生長がすすむにつれてしだいにひずみを生じるようになる。この傾向は空間的には森林を構成する各個体の空間の占有方法の変化となってあらわれる。そこで, 幹又は茎の直径 (D), 樹高又は茎長 (H), D_2Hおよび個体重 (W) 等の頻度分布から, 各々の階級に属する個体の分布型を求め, 森林の分散構造を解析し, これを定量化する必要がある。本報告では, 全体の集団をDによって, グループA (平均値より大きい値をとる個体からなる) とグループB (平均値より小さい値をとる個体からなる) の二つのグループに分けた。各グループに属する個体をそれぞれ, 優勢木, 劣勢木とみなし, これらの分布型を「平均こみあい度 (m) と平均密度 (m) の関係」を用いて, 生長過程の一時点で, 平面上で解析した。ここでは, 人工一斉林の観点から, 30年生のヒノキ (Chamaecyparis obtusa) 人工林, 15年生のクロマツ (Pinus thunbergii) の巣植, 3年生のヤマハンノキ (Alnus hirsuta var. sibirica) および参考資料として, オオアワダチソウ (Solidago serotina) 群落において調査した。mとmの関係を用いて解析すると, 優勢木では, 劣勢木にくらべて, 個体間によりつよく負の集合性と, より一様的な分布傾向がみられたのに対して, 劣勢木では, 個体間によりつよく正の集合性と, より集中的な分布傾向がみられ, 両者の分布形態に相違が見出された。

In even-aged forest, the initial difference between dominant and suppressed trees tends to be exaggerated as time goes on, which results in change of the form of frequency distribution of plant weight from the symmetrical normal curve at early stage to the asymmetric types at later stage. In relation to this, for a full understanding of the process of plant production in a forest, it is important to know how the dominant and the suppressed trees are spatially arranged in the forest community. It can be done by dividing a population into several groups with regard to stem diameter (D), tree height (H), D2H or the plant weight (W), and analyzing the spatial distribution of respective groups of trees. In this paper, the population was divided into two groups according to D, such as group A (over mean value) and group B (under mean value), and the distributional pattern of trees of each group on two-dimensional space were analyzed at a given time by an analytic method based on the relation of 'mean crowding (m*) to mean density (m)' (IWAO, 1968). The experiments were carried out in three kinds of artificial forest communities and one natural herbaceous community: 30-year-old stand of Chamaecyparis obtusa plantation forest, 15-year-old nest planting stand of Pinus thunbergii, 3-year-old stand of Alnus hirsuta var. sibirica and the natural herbaceous community of Solidago serotina. For each case, m and m* were calculated for several different quadrat sizes and plotted on a graph. Since the regression of m* on m was proved to be approximately linear in most cases, the analysis was made in terms of the values of the intercept (α') and the regression coefficient (β'). Here, α' indicates the basic components of distribution (signle individual or colony) and β' suggests the pattern of distributions of such basic components. It is found in all the 4 species that the distribution of the dominant trees was more regular than that of the suppressed trees. The finding may have some general implications in relation to the mechanism of the production structure in even-aged forest community.