混交複層林の構造と造成法 (I) : ヒノキ, アカマツ, 広葉樹の階層混交について

Abstract

京都市北部の京北町付近にはヒノキとアカマツあるいは広葉樹の階層的に異種混交した複層林が広く存在する。本研究はそのうち2つのタイプの林分 (Stand A, B) の特性を解析し, その造成法について論議したものである。Stand A (人工ヒノキ・天然アカマツ混交複層林) : 本林分のヒノキは70年前に人工植栽され, その後アカマツが天然更新し混交して成林したものである。現在のアカマツの材積混交率は緩斜地 (7°) のPlot 1で83％, 急斜地 (25°) のPlot 2で64％で, ヒノキより生長もよい。林内は比較的明るく, そのためヒノキ稚樹と下層植生がよく成立している。D分布や階層構造をみると, 林分の成立初期は単層構造であったが, 樹種の生長差にもとづいて複層構造になったので, Plot 1では上層のアカマツと中層のヒノキが明らかに分離している。これに反し, Plot 2 ではアカマツが優勢木であるが18 - 20mの樹高階で重なり, 階層としての明らかな境は認められない。一方下層植生も含めると著しく不連続な複層林になっている。このような混交複層林を造成するには, まずアカマツを点状か周辺 (60m以内) に残して皆伐し, ヒノキを植栽する。天然生のアカマツが生長しヒノキと競合し始めたら基本的にはヒノキ2 : アカマツ1の比率になるよう除間伐すればよい。Stand B (天然生ヒノキ・アカマツ・広葉樹混交複層林) : 年輪の生長解析の結果から判断すると, 本林分は約65年前ほぼ同様の林分が強度に伐採された後, ヒノキの林内残存稚樹と天然更新したアカマツ, 広葉樹が混交して生長した二次林であると推定される。直径4cm以上の各樹種の混交率は本数ではヒノキが, 材積ではアカマツが多く, 広葉樹は少ない。D (>4cm) 分布や階層構造は明らかに連続し, 混交した摂伐林型を示している。しかしアカマツは下層に成立せず, 広葉樹は上層に存在しない。本林分の造成法はアカマツの母樹をhaあたり数本程度散生させて, 伐期に達した上, 中層木をすべて収穫伐採する。天然生ヒノキ稚樹を10, 000本/ha前後に本数調整しながら林床の相対照度が50％以上になるよう下刈りを伺回か行なう。成林時の本数混交率はヒノキ2 : アカマツ1が望ましい。以上の混交複層林はA_o層の堆積量から判断して, ヒノキの単純林より林地保全に役立つようである。

There are various multi-storied stands in which mixed the dissimilar species of Hinoki (Chamaeciparis obtusa) with Akamatsu (Pinus densiflora) or with broad leaved trees at Keihoku Town located in the northern part of Kyoto City. In these stands, the growing structure of two characteristic forests (Stand A, B) was analyzed, and the silvicultural system of the storied mixture forest was discussed in this paper. Stand A (Multi-storied stand mixed artificial Hinoki with natural Akamatsu): This stand is the mixed forest which was grown with Hinoki afforested 70 years ago and then with Akamatsu naturaly regenerated in its plantation. The existing volume ratio of Akamatsu is 83% in Plot 1 of gentle-sloped place (about 7°) and 64% in Plot 2 of steep-sloped place (about 25°), and Akamatsu grow more rapidly than Hinoki (Table 1). The relative light intensity under the canopy indicates high value such as 8-12% (Table 2), therefore, there are overgrown various undergrowths and natural Hinoki seedlings (Table 4, 5, Fig. 4). Viewing the DBH distribution (Fig. 1, 2) and stratification (Fig. 5, 6), it seems that the vertical structure of the forest has changed from mono-stratum to multi-stratum in proportion to the growth rate of each species. Therefore, the story in Plot 1 is cleary separated in such a way as Akamatsu occupies at upper stratum and Hinoki composes middle stratum. On the other hand, in Plot 2, although Akamatsu is dominant tree and Hinoki dominated, both the trees overlap with each other in 18-20m height class, and it is recognized that the story is continuous state. In addition, including the natural Hinoki seedlings and the undergrowths in lower stratum, it can be said that Stand A is composed remakable discontinuous story. The silvicultural system of this multi-storied mixture forest is as follows: In the first place, the stand is cut clear except the mother trees of Akamatsu which were reserved here and there or around the cutting area (inside distance of about 60m), in succession, Hinoki is planted there. If the naturally regenerated Akamatsu grows so that it will compete with Hinoki, it is neccessary to adjust the numbers of Hinoki and Akamatsu so as to obtain the rate of 2 to 1 respectively by improvement cutting and thinning. Stand B (Multi-storied stand of mixed natural Hinoki, Akamatsu and broad-leaf tree): Judging from the analysis of annual rings of the sample trees, it is presumed that the stand was the secondary forest which had grown in the mixed state of Hinoki seedlings naturally established in the stand with Akamatsu and broad leaved trees naturally regenerated after intensely cutting down about 65 years ago. In the mixture rate of the each species over 4cm DBH, the number of Hinoki is more, on the contrary, the volume of Akamatsu is more than others respectively, and the broad leaved tree is remarkably few (Table 6). In DBH (over 4cm) distribution (Fig. 8) and stratification (all trees) (Fig. 10, 11) of Stand B, the border of each species is overlaped and is not clear, at present, the stand is formed a selection forest type with mixed dissimilar species. However, Akamatsu can not establish at lower layer and broad leaved trees lack from upper layer. The silvicultural system of the similar forest is as follows: All the tree except Akamatsu reserved in scattered for mother tree (about 5-10 trees per ha) in upper and middle stratum is felled for commercial or salvage cutting. The number of natural Hinoki seedlings in lower stratum are controlled to be about 10, 000 per hectare, on the other hand, the undergrowths are repeatedly weeded to make over 50% of relative light intensity on ground floor. When the stand will have developed and matured, it is adequate to adjust the mixture ratio of the number of Hinoki to the one of Akamatsu to be 2 to 1 respectively. Judging from the accumulated amount of litter and raw humus (A0 layer), it seems that the multi-storied mixture forest is more effective than the mono-storied pure forest of Hinoki for soil conservation.