天然更新に関する研究 (III) : 尾鷲地方におけるヒノキ林の更新

Abstract

尾鷲地方は一般に急峻な地形, 多雨という自然的条件のほか, 密植短伐期皆伐とひんぱんな間伐にともなう下刈りなどの施業的条件のため, 地力の低下が著しく, またヒノキの天然更新もきわめて困難な地域である。しかし林分によってはヒノキ稚樹の発生, 成立がみられるので, 比較的更新の良好な7林分を対象として, 種子の散布から稚樹の成立までの過程を1972年8月から3ヵ年間調査, 解析し, 特にこの地方の更新の困難性について検討してみた。種子の結実量には豊凶の差があり, またその散布量も地形や林分の状態によって不均一になるが, 並作以上の結実量があれば散布種子数と稚樹の発生, 成立本数との間にはほとんど相関がない。したがって天然更新の困難な理由は種子の生産, 散布状態ではなく, 種子の発芽後に問題があるように思われた。播種による追跡調査の結果, 多雨なこの地方では樹冠から大粒の雨滴によって種子がはねとばされ, 発芽直後の稚苗が掘り起されやすいことがわかった。しかし落葉枝や下層植生の適当な被覆はこれらの障害を防ぐ効果があった。一方, 立枯病害や虫害はそれほど多くなかった。また急斜地においては種子や稚苗の流失が多く, ほとんど定着できないようであった。稚樹の枯死は一般に10数cm以下の小さいものに多い。成立稚樹数は毎年の稚樹の発生と枯死, 消失の収支によってきまるが, 下層植生が比較的少ないところでは増加し, 繁茂するところでは減少する傾向がみられる。なお当地方では数年ごとに繰り返される下刈りによってほとんどの稚樹が刈り払われてしまうが, これが更新を困難にしている一つの理由である。一般に光条件にめぐまれた林縁付近ほど林内より稚樹の成立数が多く平均高も大きいが, その差は年とともにますます広がるようであった。また林縁付近では大きい稚樹ほど生長率は高く, 年に50 - 80％ほど生長するものも少なくないが, 林内では全般に生長率は低く平均20％前後で, 特に下層植生の存在するところの生長は悪いようであった。したがって尾鷲地方でも急斜地以外では林冠と下層植生を適当にコントロールすればヒノキ稚樹の更新は可能であろう。

On the whole, the climate of the Owase district in Mie Prefecture is rainy and the dominant feature of the terrain is a steep slope. The Owase forestry which consists mostly of Hinoki (Chamaecyparis obtusa) plantaion is chiefly characterized by the felling of short rotation and the habitual weeding repeated at intervals of several years. Therefore, the Hinoki stands of this district tend to gradually decrease in soil productivity, on the other hand, it seems that the natural regeneration of Hinoki seedlings is not easily accomplished. However, it was not so difficult to find the survival and establishment seedlings in the stand where the environment was more suitable for regeneration. The purpose of this study is to follow the regeneration process from seed dispersal to the establishment of seedlings. Emphasis on the factors of obstruction for the natural regeneration in the seven stands (Table 1) which have been investigated for the three years since August 1972 were discussed in this paper. Rich or poor crop years of Hinoki seed do not occur at regular intervals. The seed fall is mostly disseminated at random, but in the stand with a varied topography or near the forest border an uneven dispersal was found (Fig. 1, 2). However, there was no correlation found between the number of seeds dispersed and the number of seedlings established, as long as there is an abundant crop of seeds which is necessary for satisfactory regeneration so common seed year. Therefore, it apeared that the successful regeneration of seedlings in Owase district is intensely influenced by conditions for germination of seed and initial establishment of seedlings after seed dispersal. As a result of the experiment by seeding in peat-pot and direct seeding on the surface of forest floor, it was observed that the seeds are splashed out and germinated seedlings are grubbed up by a big raindrop through a crown so often. However, adequate existence of an undergrowth and covering of a litter fall sheltered frequently from the losses of seeds and seedlings (Fig. 3). Although, infant seedlings are most vulnerable during the first few weeks after germination, the attack of fungi and insects was not particularly active during this period. In general, it was observed that the fallen seeds and the germinated seedlings on the steep slope are washed away by the runoff from the rainfall, and there is a very dificult for the fixing of seedlings. Generally, the mortality of the little seedlings under about 10cm high indicated a high rate (Fig. 4), but the mortality diminished rapidly with an increase in age and size. The numbers of existing seedlings are determined by an input which is the new seedlings and an output which is the death of seedlings for one year. It was recognized that the number of seedlings established where lacking or little growing in undergrowth tends to increase, on the contrary, where abounding in its to decrease (Fig. 5). In addition, although most of the seedlings in the stand of Obarano area were cut out by weeding previous thinning in December 1974, this is one reason why the existence of big seedlings is lacking in the Hinoki stand of the Owase district. In general, most seedlings were found near the stand edge, and the number gradually decreased towards the inner part of the plantation. The mean height of the seedlings at the stand edge was larger than that of the inner ones, and the difference tended to be relatively remarked in process of years (Fig. 6, 7, 8). The annual growth rate of the bigger seedlings established at stand edge was higher by about 50~80% than little ones, but on the whole the annual growth rate of seedlings in the stand showed about 20% (Fig. 9). Therefore, it is presumed that the seedlings existing beneath a thick undergrowth in a plantation are obstructed growth due to insufficient sunlight. Judging from the above regeneration process, excepting a steep slope stand, it seemed that a successful natural regeneration of Hinoki in Owase district can occur only if the appropriate space for the establishment and subsequent growth of new seedlings is created in the crown and ground floor vegetation by thinning and weeding or other means.