クロマツ人工林に天然生ヒノキ, アカマツ, 広葉樹の階層混交した複層林の構造

Abstract

ヒノキの人工林に天然生のアカマツ等が混交した複層林は西日本の各地に多くみられるが, その反対の事例はきわめて珍しい。本研究は, 昭和62年11月, 広島県西部の西条営林署蛯ケ原国有林に成立している20年生の植栽クロマツに, 天然生のヒノキ, アカマツ, 広葉樹が階層混交した複層林の構造を解析したものである。本林分は生長の著しく不良であった50年生のヒノキ前生林分を皆伐した後, クロマツを約4, 000本/ha植栽し, 通常の下刈りを行なって20年を経過し, 成林したものである。階層構造や直径分布をみると, 斜面中腹の林分 (Plot 2) は階層の分離が明らかで, 主として植栽木のクロマツを上層木とし, 天然生のヒノキ, 広葉樹を中層木とする二段林型の混交複層林であり, 調査対象林分の代表的な林型である。斜面上部は (Plot 1), アカマツの混交率が高く, 一部の箇所については3年前に除伐が行なわれたが, 樹高分布がほぼ正規型で連続的に階層混交した択伐林型となっている。斜面の下部については (Plot 3) 天然生のヒノキの生長がきわめて良好であったので, 10年前, ヒノキの優勢木以外はすべて除伐された結果, 樹高は7 - 13mに達し, 林冠はほぼ閉鎖した単層構造となっている。アカマツや広葉樹のほとんどは皆伐後天然更新したものであるが, ヒノキはその生長経過から判断して, 前生林分内に天然更新していた稚樹が生長したものである。そして大きい個体の生長は著しく大きく, 年平均樹高生長量は40cm以上に達している。本林分は若齢であるため多くはまだ充分閉鎖せず, 胸高断面積合計も小さい。しかしかつて瘠悪であった林地も, 中腹以下では徐々に肥沃性を高めつつあるようである。これらのことから, 林種転換によるクロマツの造林は失敗であったといえようが, 今後, 階層混交林作業が粗放的に行なわれるならば, 天然生ヒノキの生産性はむしろ高まると思われる。

In westan Japan, there are numerous multi-storied forests in which artificial Hinoki (Chamaecyparis obtusa) is mixed with Akamatsu (Pinus densiflora) or other species, but the reverse case is very rare. This paper discusses the growing structure of the multi-storied stand of artificial Kuromatsu (Pinus thunbergii) mixed with natural Hinoki, Akamatsu and broad leaved trees at Ubagahara national forest located in the eastern part of Hiroshima Prefecture, investigated in January, 1987. This stand was cultivated by less improvement cutting and ordinary weeding since the Kuromatsu seedling was planted at a density of about 4, 000 per hectare about 20 years ago, after clear cutting a 50-year-old Hinoki stand growing poorly (Table 1). Viewing the stratification and the diameter distribution (Fig. 1-5), the story of the middle-slope stand (Plot 2) is clearly separated as a two-storied stand; artificial Kuromatsu occupied the upper stratum and natural Hinoki, broad leaved tree composed the middle stratum. This forest type is a representative of this research stand. However, the mixture rate of Akamatsu in the upper place (Plot 1) is remarkably large and the distribution of the tree height resembles that of the normal type. Therefore, in spite of improvement cutting 3 years ago, the existing stand has formed a selection forest type which is continuous in story with dissimilar species. As the stand at the lower-slope place (Plot 3) grew more rapidly than the others, many trees except dominant Hinoki were cut for improvement 10 years ago. At present, the tree height is 7-13m, and the forest crown is relatively dense and forms a mono-stratum. Most of the Akamatsu and broad leaved trees naturally regenerated from seeds of neighboring trees after planting. However, judging from the stem analysis of the annual rings of sample trees (Fig. 6, 7), it seems that Hinoki trees grew from the seedlings which had naturally regenerated in the former Hinoki stand. Furthermore, the height growth of large trees was recently recognized to be vigorous, and its mean height increment so far was estimated to be about 40 cm/year. The existing basal area is remarkably small. Therefore, the stand density is relatively low (Table 2), because the ferest age is young. However, viewing from the accumulated state of Ao layer and soil structure, the soil fertility is improving, compared to the former sterile land (Table 3). As mentioned above, the afforestation of Kuromatsu by the conversion of forest type was not successful. However, the productivity of natural Hinoki may be high if the silviculture system of stratified mixture is used extensively.