天然更新に関する研究 (IV) : 近畿, 中国地方における各種ヒノキ林の更新

Abstract

本報告は近畿, 中国地方におけるヒノキ林の天然更新の実態と, その機構を明らかにする目的で, 1975年から3ヵ年間, 紀伊半島の南部から北部にかけて存在する笠塔山, 高野山 (以上和歌山県), 登奈井尾 (三重県) 国有林の5林分と, 中国山陽道の西部に位置する野路山 (広島県), 城山 (山口県) 国有林の4林分について調査を行ない, 同時にこれまでの調査報告も合せて検討したものである。各調査地によって更新の経過やその状態は種々異なるが, いずれもヒノキ稚樹は発生, 成立し, 地域的な相違はほとんどみられなかった。また標高との相関もあまり明らかでなく, むしろ局所的な林分環境の影響が大きいようであった。光条件からみて, 稚樹の発生, 成立にもっとも適していた林分は, 林内孔状地 (野路山43林班B) と, ヒノキ・アカマツ混交林 (同39林班) であった。また下層植生が少なく適度な間伐が行なわれた場合 (高野山, 登奈井尾) の林内, 林縁更新は良好であった。一方下層植生が繁茂する林内では光不足になりやすく特に平均相対照度が1％前後以下になると更新が全く困難になるが, 林外ではむしろ植生が適当量存在する方がよく更新する (笠塔山, 登奈井尾, 城山) ようであった。しかし更新の可能な範囲は林縁から樹高の1 - 2倍以内の距離までである。稚樹の定着しやすい条件から判断すると, 表土の移動の少ない緩斜地 (登奈井尾, 野路山) は有利であるが, 急斜地でも下層植生が適度に成立する場合は更新が可能である (笠塔山, 城山)。このように, 上木と下層植生の構成, 成立の状態がヒノキ稚樹の更新にもっとも大きな影響を与えるので, これらをうまくコントロールすることができれば, 天然更新の可能性は著しく高くなるであろう。

Hinoki (Chamaecyparis obtusa) is one of the useful planting species in Japan, but it is regrettable that there remain few natural forests of it and there exist few naturally regenerated forests or stands of it in Japan now. This study is to investigate the present condition of the natural regeneration established of Hinoki in Kinki and Chugoku Districts locating in the western area of Japan and to analyse its mechanisms. Especially, this paper summarizes the result of the investigation which we have made on the 5 stands at Kasatohsan, Kouyasan and Tonaiwo national forest locating the southern part to the northern part of the Kii-hanto Peninsula and on the 4 stands at Norosan and Shiroyama national forest locating the western part of the Chugoku-Sanyodo, taking 3 years beginning at 1975. Although the condition of regeneration is found different between the stands investigated, the Hinoki seedlings has been established in every stand where the difference due to characteristic of altitude or region has scarcely been found between them. They seem to have been affected rather by local environment of each stand. Viewing from the effect of light condition, it could be said that the stands locating in openings of inner forest and mixed stands of Hinoki with Akamatsu (Pinus densiflora) are most suitable to regeneration of Hinoki seedlings (Fig. 10, 11). Then, good regeneration of seedlings has been found within stand as well as stand edge in case of those stands having less undergrowths and repeated to adequate thinning (Fig. 5, 6, 8, 9). On the other hand, it has been observed that more thriving undergrowths in stand disturve the regeneration of seedlings more from good growth due to lack of light and the regeneration of seedlings is hardly to be made in those stands capable of receiving light only with the mean relative light intensity of 1% or less (Fig. 1, 3, 4). It has also been recognized that outside stands (young plantation) growing adequate quantity of vegetation promote regeneration of seedlings more but in the case regeneration of seedlings is possible only within such a range of distance from the edge of stand as 1 to 2 times of the height of trees (Fig. 1, 4, 6, 13). Judging from the conditions helping seedlings fix, gentle-sloped places of which surface soil is less movable are more favorable to fixation of seedlings (Fig. 8, 9, 10, 11). However, natural regeneration of seedlings is possible even in steep-sloped places if undergrowths are moderately established there (Fig. 1, 4, 6, 13). As mentioned above, the composition of upper trees with undergrowths and the condition of establishment affect most regeneration of Hinoki seedlings. Therefore, the possibility of natural regeneration would be highly increased if these factors mentioned above could be well controlled.