アカマツ・ヒノキ・広葉樹混交林の構造と二次遷移

Abstract

本報告は京都市北部郊外の京都大学演習林上賀茂試験地に存在する二次林の諸構造を解析, 記録するとともに, 遷移の過程を推定しようと試みたものである。調査は現在若いアカマツが優占するヒノキ・広葉樹混交林 (林分I) と, ヒノキが優占するアカマツ・広葉樹混交林 (林分II) の16×16mの方形プロットで行なった。林分1は閉鎖状態ではなく, 下層にヒノキ稚樹や広葉樹が多い。林分IIは閉鎖林で, ヒノキの枯死木が多く, 下層植生も少ない。上層に混交する広葉樹のうち, 約1/3が常緑広葉樹である。この天然生林におけるヒノキの樹高分布型は, 相対的な立木密度の増加に従って, 更新の継続している状態のL型から競争状態の正規型へ変化していくものと思われる。森下の分散指数 (l_δ) および厳の平均こみあい度 (m) と平均密度 (m) の関係を用いて林木や稚樹の分布様式を解析した結果, ヒノキの優勢木はランダムに分布するが, 劣勢木や稚樹は集団をもつ集中分布を示す。特に林分Iの稚樹の集中性はきわめて高く, 一方集団内も集団間もランダム分布を示す。またアカマツと広葉樹の分布様式は, 大きい集団をもつ集中分布である。林分IIの枯死木は明らかに集中的に分布し, 寄り集まって枯死しているようである。森下の分布相関指数 (R_δ") を用いて解析したところ, 上層を共有するアカマツとヒノキは, 互いにさけ合って分布している。また階層間では, ヒノキの優勢木と劣勢木との間に正の相関が認められ, 劣勢木が被圧されている状態を示している。これに反し, ヒノキ稚樹は相反の状態を示し, 優勢木の樹冠のすき間に更新しているようである。森林の相観は最上層から樹高の1/3下方までの階層に含まれる樹種の本数割合で特徴付けられる。すなわち林分1ではアカマツ約77％, 林分IIではヒノキ72％で, それぞれの樹種が優占している。上層から順次積算した無被陰の樹冠面積に対する被陰樹冠面積の関係は指数関数的で, 切片の値からアカマツがヒノキより陽性種であること, ヒノキでは密度が高くなると被陰樹冠の増加率は小さくなることがわかった。資料木の年齢解析から, 調査林分はヒノキ, アカマツ混交林を, 林分Iは約30年前に皆伐し, 林分IIは約50年前に皆伐, 約30年前に間伐した後, 自然に放置された二次林であり, さらに伐採前すでに林内に更新していたヒノキの稚樹や, 伐採後あるいは聞伐後更新したいろいろな樹木が生長した森林であることが明らかになった。したがって以上のことから, 林分IIは林分Iより森林の発達が人為的に進められた森林であると判断されよう。

This report is to state the experimental results of our study to analyze and record the growing structure of the secondary forests existing in Kamigamo Experiment Station of Kyoto University Forest which locates in the northern suburbs of Kyoto City and besides to presume the process of its succession. Many investigations were performed on the quadrat plots of 16 m×16 m in a mixed forest of Chamaecyparis obtusa and broad leaved tree occupied predominantly by young Pinus densiflora (Stand or Plot I) and in a mixed forest of Pinus densiflora and broad leaved tree occupied predominantly by Chamaecyparis (Stand II). Stand I was insufficient stocking and there were many seedlings of Chamaecyparis and broad leaved trees in its lower stratum. Stand II was a closed forest and there were many dead trees of Chamaecyparis and few undergrowths there. Approximate one thirds of the broad leaved trees mixed in the upper stratum were composed of evergreen broadleaf trees (Table 1~3). It seems that the height distribution of Chamaecyparis obtusa in the natural regenerated forest will be changed from the L type where its seedlings are regenerating to the normal distribution type where the condition of competition is seen, in proportion to the increase of relative stand density (Fig. 2, 5). As the result of the analysis of distributional pattern of seedlings and trees by means of Index of dispersion (Iδ) by MORISITA and relation of Mean crowding (m*) to Mean density (m) by IWAO, it was seen that the dominant trees of Chamaecyparis obtusa were distributed at random, and on the other hand the dominated trees and the seedlings were distributed contagiously, having clumps. Specially the contagious degree of the seedlings in Stand I was very high, and both the distributions of intra-clump and inter-clump were at random (Fig. 7~9). The distributional pattern of Pinus densiflora and the broad leaved trees showed a contagious distribution having large clumps. The dead trees of Stand II were apparently distributed contagiously and seemed to be dead aggregately (Fig. 10). As the result of the analysis of its distributional pattern by means of Index of interspecific correlation (Rδ″) by MORISITA, it was seen that the community of Pinus densiflora and Chamaecyparis obtusa where they have the same upper stratum commonly was dispersed in such a state which two communities were repulsive to each other. In the inter stratum, it was seen that a positive correlation existed between the dominant tree of Chamaecyparis obtusa and the dominated tree of the one, where the dominated tree was oppressed; on the contrary, the seedlings of Chamaecyparis obtusa showed a repulsive state and it seemed that they regenerated in the spaces between the crowns of dominant trees (Fig. 11, 12). The physionomy of forest was so characterized as to be in proportion to the ratio of number of tree species in the stratum positioning up to one thirds of the overall tree height from the highest stratum. In Stand I Pinus densiflora was dominant occupying approximate 77%, and on the other hand in Stand II Chamaecyparis obtusa was dominant occupying approximate 72% respectively (Fig. 13, 14). It was recognized that the correlation of the shaded crown area to the non-shaded crown area which was integrated in consecutive order beginning at the highest stratum was exponential, and that Pinus densiflora was intolerant than Chamaecyparis obtusa from the data of the values of their intercept and in case of Chamaecyparis obtusa the increasing rate of the shaded crown became smaller as the density become higher (Fig. 16). From the age analysis of the sample trees, it was cleared that Stand I was the secondary forest which had left at natural state after a mixed forest of Chamaecyparis obtusa and Pinus densiflora was felled about 30 years ago, and on the other hand Stand II was the secondary forest whose mixed forest was felled about 50 years ago and thinned about 30 years ago. Furthermore, it was understood that both the forest stands grew up of the seedlings of Chamaecyparis which had already regenerated in the stand before each cutting, and of the various tree species which had regenerated naturally in the stand after cutting or thinning (Fig. 17~19). Judging from the result of analysis mentioned above, therefore, it can be said that the development of the forest of Stand II was more progressed artificially than Stand I.