階層混交したスギ不成績人工林の構造と取り扱い方について

Abstract

林種転換にもとづく不成績造林地は日本の各地に多くみられるが, その林分の実態はあまり明らかでない。本報告は1988年11月, 兵庫県中央部の山崎営林署赤西国有林に存在するスギ不成績林分の構造を解析し, その取り扱い方について論議したものである。本林分はブナを主体とした広葉樹に一部スギの混交した天然林を約82ha皆伐した後, スギを約3, 000本/ha植栽し通常の下刈り, 除伐を行なって30年を経過した林分である。斜面下部の調査林分は平均樹高が約13. 2mのスギ上層木でほぼ単層状に閉鎖し, 一般の平均的なスギ林分と同様の成長をしている。これに反し斜面上部においては, 植栽木のスギはほとんど生存しているが, 雪圧によって根曲がりしたり傾斜した成長の不良な被圧木が多い。また, 樹高5m以上の上層木は全本数の30％ほどで, 主として群状に成立し, その平均樹高は下部調査地の1/2程度の6. 8mである。さらにこの造林地には下刈り終了後再生した樹高3m以上の広葉樹が10, 000本/ha以上も全層にわたって混交している。上部調査地のスギの樹高および胸高直径の本数分布はL型で, ほぼ正規分布を示す下部とは明らかに異なり連続的な複層構造となっている。広葉樹もほぼL型分布で, 大きい樹高階ほど有用広葉樹の占める割合が大きい。樹幹析解による樹高および胸高直径の成長経過から判断すると, 下部調査地のスギは継続的に良好な成長をしているに反し, 上部調査地においては植栽直後から成長の悪いスギが多いようである。このような成長不良は土壌層が浅く石礫が多い貧弱な土壌に起因するようである。したがってこの不成績造林地は適地の判断の誤りによって発生した造林失敗地であるといえる。しかし上層木のスギや有用広葉樹はこの土壌環境に応じた成長をしているので, このままの状態で推移しても若干の人工スギと天然広葉樹との混交複層林になる可能性は高く, また有用木以外の広葉樹を適宜除伐することは林業上一つの有効な方法と考えられた。

Often in Japan, conversion of forest type has not yielded productive plantations, but the composition of that stand has not been analyzed. This paper discusses the growing structure and the management of the unproductive stand of Sugi (Cryptomeria japonica) plantation at Akasai National Forest located in the middle part of Hyogo Prefecture, investigated in Novenber, 1988. This stand was cultivated by ordinary weeding and improvement cutting since the Sugi seedlings were planted at about 3, 000 per hectare for about 82 hectares 30 years ago, after the natural forest of broad leaved tree mixed a few with natural Sugi was clear cut. In the research stand on the lower-slope, the forest crown closes and forms a monostratum of planted Sugi. As the mean height of dominant tree is about 13.2 m, the stem growth tends to be nearly equal in mean value of general Sugi plantation. On the contrary, the planted trees at the upper research stand are mostly alive, but there are numerous oppressed trees growing poorly such as those with bended stem base or slant stem by snow pressure. The dominant tree is about 30% of the total number and establishes aggregately, and this mean height is about 6.8 m as half that of the lower research stand. In addition, the Sugi poor plantation is mixed in the whole stratum with broad leaved tree naturally regenerated after close of weeding, and the number of the tree over 3 m in height so far is about 10, 000 (Table 1, Fig. 1, 2 ). The distribution of the tree height and the diameter at breast height of Sugi in the upper research stand resembles that of the L type, and the existing stand forms a multistoried structure which is continuous in stratification. On the other hand, that distribution of lower research stand tends to indicate a nearly normal distribution (Fig. 3, 4 ). The distribution of broad leaved tree also resembles that of L type, and the ratio of the useful broad leaved tree as higher height class is larger than the small (Fig. 5 ). Judging from the process of diameter and height growth by the stem analysis of sample trees (Fig. 8, 9 ), the growth of Sugi at the lower research stand is recognized to be vigorous without interruption, but the growth of most of Sugi at the upper research stand is continuously small after planting. It seems that the poor growth of the upper research stand is caused by sterile soil such as shallow soil layer and abundant gravel (Table 2). Therefore, this poor plantation by the conversion of forest type can be said to be a failure stand of afforestation to come from a mistake of judgment for the fitting site of planting. However, as the Sugi dominant tree or the useful broad leaved tree in the upper plantation grows depending on the soil environment, the multi-storied forest of few mixed artificial Sugi and natural broad leaved tree may be formed if the existing stand is succeeded on the natural status. Furthermore, the improvement cutting by adequate control of the number of some broad leaved trees excluding useful trees may .be an effective means of forest management.