小雪地帯における階層混交した不成績 人工林の構造と取り扱い万について

Abstract

林種転換にもとづく多雪地帯の不成績造林地の調査に引き続き, 本報告は1989年10月に調査した少雪地帯の新宮営林署管内大又, 大越両国有林に存在する階層混交した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。両林分とも落葉広葉樹を主とする天然林を皆伐した後, 大又調査地はスギ, ヒノキを約2 : 3の割合で3, 000本/ha植栽し, 下刈りを5回行なったほかは無手入れのまま56年を経過し, 大越調査地はスギを3, 000本/ha植栽した後下刈りを2回のみ行なって30年を経過した林分である。大又調査地では造林木中の優勢木は集中的に分布するが, 劣勢木や広葉樹はランダムに分布している。大越調査地ではいずれもランダムに分布する。両調査地とも広葉樹の樹高分布はL型であるが, 造林木は樹高10m付近で分離する二山型の分布を示し, 上層木と下層木をもつ複層構造となっている。大又調査地における成林の見込みのある胸高直径8cm以上の造林木の幹材積は約144m_3/haで, 平均樹高も低いが, 広葉樹と合せると330m_3/haを超える。一方大越調査地の造林木の幹材積は175m_3/haで, 高林齢の前者より多いが, 広葉樹の混交は比較的少なく, 針広合せて230m_3/ha程度である。胸高直径の成長経過から判断すると, 両調査地とも造林木中の優勢木は植栽直後から良好な成長を持続しているに対し, 劣勢木は大又調査地では植栽後20年頃より, 大越調査地では6年頃より成長を減退している。広葉樹はいずれも成長が旺盛である。両調査地とも土壌層は深く, 団粒構造は発達し. B_D型土壌と判断されるので, スギやヒノキの不適地ではない。以上のことから, 造林の不成績の原因は, 自生樹種の旺盛な成長に対抗した十分な下刈り, 除伐をしなかったことにあると判断された。即ち, 自然の再生力が勝っていたといえよう。しかしとのままの状態で推移させても, 大又調査地では造林木が約40％, 大越調査地では70％以上の材積率を持つ混交複層林に育つ可能性は高い。また有用木以外の広葉樹を適宜除伐することも林業上有効な方法と思われた。

This paper is part of a series on the poor plantation coming from conversion of forest type in a snowy region and discusses the growing structure and the management of the unproductive stand stratified-mixed with native trees in the Oomata and Ookoshi National Forest of Shingu District Forest Office located in the eastern part of Wakayama Prefecture, investigated in October 1989. The Oomata Research Stand was cultivated by ordinary weeding and less improvement cutting since the Sugi (Cryptomeria japonica) and the Hinoki (Chamaecyparis obtusa ) seedlings were planted at a density of about 3, 000 per hectare 56 years ago. The Ookoshi Research Stand was cultivated by weeding only twice after the Sugi seedlings were planted at a density of about 3, 000 per hectare 30 years ago, after the natural forest of most deciduous broadleaved tree was clear cut. Both research stands are multi-storied forests of stratified mixture type with broadleaved tree of native species (Fig.1, 2). However, the dominant trees of Sugi and Hinoki at. Oomata Research Stand are established aggregately, even though the oppressed tree and broadleaved tree naturally regenerated after planting are distributed at random. On the other hand, all trees at the Ookoshi Research Stand are in random distribution. The distribution of the tree height of the broadleaved tree in both research stands resembles that of the L type. However, the distribution of the planted tree is clearly separated at about a 10 m height as a two peak type, and that stratification forms a two-storied structure with upper and lower stratum trees (Fig.4, 6). The existing stem volume of the planted tree over 8 cm in diameter at the Oomata Research Stand is about 144 m3/ha, and the mean height is remarkably low, but the total volume of the planted tree and broadleaved tree so far is estimated to be about 330 m3/ha (Table 1). On the other hand, the volume of the planted tree at Ookoshi Research Stand is about 175 m3/ha more than that of the older Oomata stand. However, as the mixture ratio of broad leaved tree is small, the total volume of all trees is small being about 230 m3/ha (Table 2). Judging from the process of diameter growth of sample trees (Fig.7, 8), the growth of the dominant planted trees in both research stands is continuously vigorous after planting, but the growth of the inferior planted trees has been suppressed from about the 20th year after at Oomata and about 6th year at Ookoshi. However, the growth of the broadleaved tree is generally vigorous. Judging from the character of the soil profile such as deep soil layer and developed grumb structure (Table 3), the soil of both research stands is fertile. Therefore, the stand soil can not be said to be unfit for planting of Sugi and Hinoki. As mentioned above, the poor growth of the planted trees may be caused by insufficient weeding and improvement cutting which act against the vigorous growth of native trees. Namely, it seems that the natural reproductivity was greater than the establishment of plantation. However, if the existing stand is succeeded on the natural status, a multi-storied forest consisting of native trees mixed with about 40% volume rate of planted trees at Oomata Research Stand and over 70% volume rate at Ookoshi should be formed. Furthermore, adequate improvement cutting of some broadleaved trees excluding useful trees may be an effective means of forest management.