京都市近郊における天然生ヒノキ・アカマツ混交林の林分構造と風致施業 (続報)

Abstract

京都大学農学部附属演習林上賀茂試験地の二次林は, 京都市街地の風致林として重要な地位にある。本報告はこの風致林に関する一連の研究として5年前に行なった間伐の影響による生産構造の変化と, 更新の経過および保存林の構造について論議したものである。上木の胸高直径の本数分布は間伐直後と同様ほぼ正規型で, 階層構造にもほとんど変化がみられないが, アカマツの本数はマツ枯れによって著しく減少した。ヒノキの平均胸高直径は一部のプロットを除き1. 4cm前後増加したが, 隣接木の側圧を受けていた優勢木が特に成長を促進した。またヒノキの胸高断面積合計は13 - 15％. 林分材積は15 - 18％増加したが, アカマツを合わせると成長量は僅かであった。ヒノキ, アカマツを合わせた現在の林分葉量は8 - 10ton/ha (乾重) で, この5年間に一部のプロットを除き1ton/ha程度回復したが, マツ枯れによる林冠空隙の増加の影響もあって再閉鎖の速度は比較的遅いようであった。それ故林冠下の平均相対照度も1/5 - 1/2ほど低下したのみで, まだ6 - 10％程度の明るさを保っている。間伐前年に除伐された広葉樹はそれほど密な状態ではないが, すべての調査地とも高さ1m以下の下層植生として再生した。プロット25-Aで調査した結果, 今回天然生のヒノキの当年生稚樹は約32, 000本/ha発生し, 間伐後更新した高さ20cmまでの2年生以上の稚樹は約34, 000本/ha成立している。それらの分布様式は小さい集団をもつ集中分布であった。アカマツの当年生稚樹は比較的少ないが, 2年生以上の椎樹は約6, 000本/ha成立し, しかも大きい集団をもつ集中分布を示した。ヒノキおよびアカマツの稚樹と上木のヒノキの分布相関は互にさけ合いの状態にあり, 両方の椎樹はともに林冠の隙き間によく更新するようであった。一方, 保存林の構造は, アカマツ, 広葉樹の多いほかは間伐前の隣接調査地と変わらないが, 後継稚樹はほとんど成立していなかった。以上のことから判断して, 風致的にはヒノキの大径木に多様な樹種が階層的に混交した複層林の方がヒノキ単層林より価値が高いと思われるので, 今後稚樹の更新と成長に適した空間を維持するため, 樹高幅程度の広がりをもつ群状択伐等の施業を取入れることは有効適切な手段であろう。

The secondary forest of Kamigamo Experimental Forest Station affiliated with Kyoto University is situated on an important amenity forest of the urban fringe of Kyoto. This paper, which belongs to a series of studies related to the amenity forest, discusses the transition of the growing structure and the regenerating process effected by thinning earlier than five years ago, in addition to the structure of reserved forest neighbouring research stands. The number distribution of the diameter of upper stratum trees mixed Hinoki (Chamaecyparis obtusa) with Akamatsu (Pinus densiflora) resembled that of the normal type which was identical to form immediately after thinning (Fig. 1) and the stratum structure was not much changed. However, the numbers of Akamatsu extremely decreased from the withering by pine wood nematodes. The average diameter at breast height of Hinoki increased about 1.4cm, except for one plot (Table 1), and especially the diameter growth of dominant trees which had suffered lateral pressure by neighboring trees was promoted than other trees (Fig. 2). Then, although the basal area and the stem volume of Hinoki increased in 13 ~ 15% and 15 ~ 18% respectively, the increment of the stand volume included Akamatsu with Hinoki was a little (Table 1, 2). The existing biomass of foliage included Hinoki and Akamatsu was 8 ~ 10 ton/ha (dry weight) and the foliage has been recovered about 1 ton/ha during five years, except for a plot. However, the re-closure of crown seemed to be comparatively slow due to the increase of the gap by the pine withering (Fig. 3). Therefore, the average relative light intensity under canopy dropped about 1/5 ~ 1/2 only and the mean value is still kept in 6 ~ 10% (Table 3). In every research stand, the broad leaved trees which were removed at the previous year of thinning reproduced sparsely as the undergrowth less than 1 m in their height (Table 4). In the result investigated on Plot 25-A, the 1-year-old seedlings of natural Hinoki regenerated at about 32, 000 per hectare, and the seedlings over two years at less than 20 cm in height which have been regenerated after thinning established at about 34, 000 (Table 5). Their distributional patterns showed a contagious distribution having small clumps (Fig. 4). The 1-year-old seedlings of Akamatsu were few, but the number of seedlings over two years old was about 6, 000 per hectare (Table 5). Then, they were distributed contagiously having large clumps (Fig. 4). As the result of the analysis of the interspecific correlation by MORISITA, Hinoki seedlings or Akamatsu seedlings an upper stratum trees of Hinoki was dispersed in such a state to be repulsive to each other (Fig. 6), thus it seemed that their seedlings were able to regenerate well in the gap of crown. On the other hand, the structure of the reserved forst was similar the neighbouring reserach stand before thinning except that there were much Akamatsu and broad leved tree (Table 6, Fig.7, 8), further, the succeeded seedlings of Hinoki and Akamatsu established in forest floor few. Judging from the above mentioned, it seems that the multi-storied stand stratified mixed large sized Hinoki with various species can be more highly evaluated than the pure stand of Hinoki for amenity forest. Therefore, the silvicultural system such as the group selection system having a width as tree height must be effective and appropriate to sustain a gap of crown for regeneration and growth of various seedlings.