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

Abstract

京都大学農学部附属演習林上賀茂試験地の森林は京都市街地周辺の風致林としてきわめて重要であり, 風致地区に指定され皆伐を制限されている。本研究は, 1984年3月, この林分の成立過程や現在の林分構造を解析し, 今後の風致的施業について検討したものである。資料木の年輸解析の結果から, 現存林分は前の林分内にすでに更新していた天然生ヒノキ稚樹と, 約60年前に皆伐された後天然に更新したアカマツが混交して生長した二次林であり, さらに下層のヒノキ劣勢木は皆伐後か, 約40年前の間伐後に天然更新し生長したものからなりたっていることがわかった。劣勢木のH/Dは優勢木より著しく大きい。また一般に, アカマツの単木あたりの葉量は少ない傾向がみられた。調査林分におけるアカマツの材積混交率は, 北斜面のプロット24, 25では17 - 29％, 南東斜面のプロット27では約34％であった。なお下層植生は調査前年の1983年に除伐されていた。林分の幹量および葉量は林分構造の同様な他の調査地の値と類似していたが, 樹高生長は全般的に悪かった。森下のI_δ法によって各林木やヒノキ稚樹の分布様式を解析した結果, ヒノキについては平均直径以上の優勢木もそれ以下の劣勢木もほぼランダム分布を示すことがわかった。しかしアカマツや枯死木の分布様式はランダムか集中性がみられた。これに反しヒノキ稚樹は明らかに集中分布で, 集団的に更新しているようであった。直径分布および階層構造からみて, ヒノキとアカマツの種境界は重なりあって不明瞭であり, また優勢木と劣勢木の階層も連続した状態であるといえる。さらにヒノキ稚樹や下層植生も含めると, 本林分は異種混交の択伐林型であるといえよう。しかしアカマツは下層に成立できなし、。本調査の終了後残存木の生長とヒノキ稚樹を更新させるため, 地床上の相対照度が10 - 15％となるよう間伐が行なわれた。休養的評価から判断して, 天然性アカマツを混交した天然生ヒノキの複層林は, ヒノキ純林の単層林より都市近郊の風致林としてより価値が高いと思われるので, 各調査林分が成熟するであろう10 - 20年後, ヒノキとともにアカマツも天然更新させるため, 群状もしくは帯状択伐法により樹高幅ほどの広さの伐採を行うことは適切であろう。

The forest of Kamigamo Experimental Forest Station affiliated with Kyoto University is a very important amenity forest situated at the urban fringe of Kyoto where the extent of clear cutting is limited by municipal ordinance. In this forest, the regeneration process and the growing structure of several stands were analyzed in March, 1984 and the management system of amenity forests was discussed herein. Judging from the analysis of annual rings of the sample trees, the existing stands were secondary forests, which had grown in the mixed state of Hinoki (Chamaecyparis obtusa) natural seedlings already established in the former stand with Akamatsu (Pinus densiflora) naturally regenerated after clear cutting about 60 years ago. Furthermore, the dominated trees of the lower stratum consisted of Himoki which had regenerated naturally on the stand floor after clear cutting and after thinning about 40 years ago (Fig. 1). The ratio of height to diameter (H/D) of the dominated trees was larger than that of the dominant trees (Fig. 2). In general, it was recognized that amount of leaf per individual of Akamatsu was scanty (Fig. 5). The existing volume ratio of Akamatsu was about 17-29% in Plot 24 and 25 on north slope and about 34% in Plot 27 on the southeast slope. Moreover, the undergrowth was weeded in 1983. The biomass of stems and foliage tended to be nearly equal in value to those other mixed forests having similar stand composition. However, they seemed to have generally slow height growth (Table 1). The analysis of the distributional pattern of Hinoki seedlings and trees by means of Morisita's Iδ (Index of dispersion) (Table 2) and the distribution of tree crown (Fig. 8, 9), showed that both the dominat trees (D>D¯) and dominated trees (D<D¯) of Hinoki were distributed generally at random. However, the distributional pattern of Akamatsu and dead trees showed a contagious or random distribution in each plot. On the other hand, the seedlings of Hinoki (H<2 m) were apparently distributed contagiously and seemed to be regenerated aggregatively. In the DBH distribution (Fig. 7) and stratification (Fig. 8, 9, 10), the border of two species of Hinoki and Akamatsu overlapped and was not clear, and the story of dominant and dominated trees was continuous. In addition, including the natural Hinoki seedlings and the undergrowth, the stand formed a selection forest type with mixed dissimilar species. However, Akamatsu can not grow at a lower layer. After this investigation, the forest was thinned to make about 10-15% of relative light intensity on ground flora (Fig. 7, Table 3), to regenerate Hinoki seedlings naturally and to promote remaining tree growth. Judging from the recreational evaluation, it seems that the multi-storied stand mixed natural Hinoki with natural Akamatsu is more effective than the mono-storied pure stand of Hinoki for amenity forest of urban fringe. Therefore, when each stand has developed and matured 10-20 years later, they should be cut at the width as tree height by strip or group selection system, to regenerate Hinoki with Akamatsu naturally.