混交複層林の構造と造成法 (2) : 植栽スギと天然生スギ, ヒノキ, アカマツ, 広葉樹の階層混交について

Abstract

高槻市北部に位置する樫田地区付近には, ヒノキ, スギ, 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。本報告は混交複層林に関する一連の研究として, その中の3つの特徴ある林型をもつ林分の構造を解析し, その造成法について論議したものである。人工スギ・天然生ヒノキ・アカマツ・広葉樹混交複層林 (Stand I) : 本林分は人工植栽された約80年生のスギが天然生のヒノキ, アカマツの閉鎖林冠上に突出し, 中, 下層I乙広葉樹が階層混交する複層林である。林分材積は540m_3/ha, 断面積合計は72m_3/haを超えるので林床は暗く, 針葉樹の後継樹はほとんどみられない。本林分はあまり手入れをしなかったスギの造林地であったが, 約40年前, 生長のおくれたスギと天然生のヒノキを残して伐採した後, アカマツや広葉樹が天然に更新し混交した林分のようで, 典型的な"立て木"による施業といえよう。したがって本複層林の作業法はスギの疎植と粗放な保育, 収穫が基本である。天然生ヒノキ・アカマツ・広葉樹混交複層林 (Stand II) : 本林分はアカマツを上層木とし, ヒノキは全層にわたり, 中, 下層に広葉樹の異種混交する択伐林型を示し, 近畿地方に多くみられる混交複層林である。針葉樹の林分材積は470m_3/ha, 断面積合計は約66m_3/haで林分量が多く密状態にあるが, アカマツの混交率が著しく大きく, 広葉樹が少ないので林内は比較的明るく, ヒノキ稚樹がよく更新している。本林分は約55年前, 現状のような林分が皆伐された後, 林内に成立していたヒノキと, 新たに更新したアカマツ, 広葉樹が階層混交して生長したものと思われるが, この成立過程と同様な粗放施業により本林型の混交複層林を造成できよう。天然生スギ・ヒノキ・アカマツ・広葉樹混交複層林 (Stand III) : 本林分は天然生のスギが最上層に突出し, 上層下部にヒノキとアカマツが, 中, 下層に広葉樹が, さらに下層にスギ, ヒノキの幼樹, 稚樹が異種混交する複層林で特にスギ, ヒノキについては二段林型を示している。現在の林分材積は少なく, 断面積合計も約36m_3/haで閉鎖状態になく林内は明るい。最上層のスギは約60年生で, 本林分は約30年前, 当時径級の大きかったスギ, ヒノキと大部分のアカマツ, 広葉樹が伐採収穫された後, 伐り残された小径級のスギ, ヒノキと, 新たに更新した各樹種が生長した比較的若い天然生林である。本林分の成林過程と同様, 強度の"なすび伐り"を行なうことによってこのような混交複層林を保続させるととはできるが, 本林分のスギはその更新状態からみて高い耐陰性をもっていると思われる。本地域の混交複層林はA_o層, A層の状態から判断して, いずれも地力の維持増進に役立っているようである。

There are various multi-storied stands in which mixed the dissimilar species of Hinoki (Chamaecyparis obtusa) with Sugi(Cryptomeria japonica), Akamatsu(Pinus densiflora)or broad leaved trees at Kasida region located in the northern part of Takatsuki City. In a series of studies related to multi-storied mixture forests, the growing structure of three characteristic forests (Stand I, II, III) in these stands was analyzed, and the silvicultural system of the storied mixture forest was discussed herein. Stand I(Multi-storied stand mixed artificial Sugi with natural Hinoki, Akamatsu and broad leaved trees): This stand is the multi-storied forest of stratified mixture type with Sugi planted about 80 years ago jut out above a closed crown of natural Hinoki and Akamatsu occupying the upper stratum and broad leaved trees composing middle or lower stratum (Fig.l, Table 2). The existing stem volume of the research stand is about 540m3/ha and the basal area about 72m2/ha respectively. The relative light intensity on ground vegitation indicates low value such as 2%(Table 1). Therefore, there are few established conifer seedlings. It seems that the stand is a secondary forest which had grown in the mixed state of reserved artificial Sugi with Akamatsu and broad leaved trees naturally regenerated after intense cutting of the trees except for the small Sugi and natural Hinoki in ill-tended artificial Sugi stand about 40 years ago. This management is known as "Tategi"(reserved tree) method in Kinki district. Therefore, on the silvicultural system of this multi-storied mixture forest, it. is basic to have thin planted Sugi and to practice extensive tending and yield system. Stand II (Multi-storied stand of mixed natural Hinoki, Akamatsu and broad leaved trees): This stand is formed a stratification of selection system type with Hinoki constituting the whole stratum, Akamatsu occupying the upper stratum and broad leaved trees composing the middle or lower stratum (Fig.2, Table 4). Similar multi-storied stands are numerous in the Kinki district. The stem volume of conifer is about 470m3/ha and the basel area about 66 m2/ha(Table 3). Also the stand density is high. However, the mixture rate of Akamatsu is remarkably large and broad leaved trees are few. Therefore, the relative light intensity indicates a high value and Hinoki seedlings are numerously regenerated there. It is presumed that this stand is a natural regenerated forest which had grown in the mixed state of Hinoki seedlings already established in the former stand with Akamatsu and broad leaved trees naturally regenerated after clear cutting about 55 years ago. Therefore, it is possible to cultivate similar multi-storied mixture forest by the extensive silvicultural system as established in the process of Stand II. Stand III (Multi-storied stand of mixed natural Sugi, Hinoki, Akamatsu and broad leaved trees): This stand is formed by a characteristic multi-storied forest with natural Sugi (about 60-year-old) projecting above the upper stratum with Hinoki and Akamatsu occupied at upper stratum, with broad leaved trees composing the middle or lower stratum and in addition seedlings or young trees of Sugi and Hinoki established at lower layer. Especially, stand form (stratification of crown)of Sugi and Hinoki looks like a two-storied forest (Fig.3, Table 6). The existing stem volume and basal area is remarkably smaller than Stand I or II. Therefore, the crown density is very low, and the relative light intensity on ground vegeitation indicates high value such as 4% (Table 5). It seems that the stand is a young secondary forest which had grown in the mixed state of reserved trees of Sugi and Hinoki with Akamatsu and broad leaved trees newly regenerated after intense felling of the large-sized Sugi and Hinoki or other tree species about 30 years ago. Therefore, it is possible to continuously cultivate a similar multi-storied mixture forest by the intense selection cutting system as established in the process of Stand III. However, it is presumed that the natural Sugi growing in this stand have high tolerance. Judging from the accumulated state of Ao layer and A layer (Table 7), it is understood that the multi-storied mixture forests in this district are effective for preservation of soil productivity.