ヒノキ人工林における再生産過程の検討 (III) : 大枝ヒノキ人工林における天然生ヒノキ稚樹の動態と分散構造

Abstract

京都府立大学大枝演習林内のヒノキ人工林で, ヒノキ種子の落下から, 発芽, 当年生稚樹の定着までの過程を個体数の変化や分散構造から, 定量的に把握しようと試みた。ヒノキ人工林内に20m×20mの方形区を設置し, その中をさらに1. 25m×1. 25mの小方形区に分けた。そして, 1974年の春, 方形区内のすべての当年生稚樹が, どの小方形区内に入るかを記録し, 個体識別するとともに, 各々の稚樹の発芽床のタイプを3種類に類型化して, 発芽後おおよそ1年間の稚樹の動態や, 分散構造などを調べた。また, 1973年の秋から1974年の春まで, 方形区内に設置した16個の種子トラップを用いて, 種子の落下量の調査も行なった。その結果, 次のことがわかった。1. 林内に診ける種子の落下は, 著しく集中的で, 散布むらがみられた。2. 発芽した稚樹は, 集中分布しており (図4), 林内における稚樹の発生には, むらが大きかった。これは, 種子の落下量の多いところでは, 稚樹の発生も多いという現象 (図5) を反映し, 種子の落下が集中的であったことが強く影響している。そして, 林内での発芽条件には, 大きな片寄りはないと思われた。3. 林内において, 発芽後, すごぶる多数の稚樹が消失するものの, 当年段階でも, ある一定以上の時間がたてば, 死亡率は低下し個体数は比較的安定する (図6)。4. 発芽から翌年の春までの稚樹の分布は, ずっと集中的で, その集中性は, 発芽後2 - 3週間を除き, 時間がたつにつれていっそう高まる (図7)。5, 発芽後2 - 3週間の稚樹の死亡は, 発芽床の種類にあまり関係なく (図10), 平面空間的にも, ランダムにおこっているようであった。6. 発芽後2 - 3週間を過ぎると, 稚樹の生存は発芽床のタイプによって特徴づけられ, 蘇苔地や鉱物質土壌の露われた箇所では, 落葉層で覆われたところよりも, 生存率はきわめて高まる (図10)。その結果, 稚樹の分布は, 林内でパッチ状にみられる蘇苔地や鉱物質土壌の露出地に, 局限される。7. 当初, 種子の落下量の多かった箇所では, 発芽後おおよそ1年たっても, 相対的に多くの稚樹が生存し, 種子散布の集中性が, 1年後の稚樹の分布をも, 大局的には, 決めているようであった。

1. This study was carried out for the purpose of making clear the initial stage of natural regneration of Hinoki stand with particular emphasis on dynamics and distributional structures of Hinoki seeds and current seedings. 2. The investigation was performed at the Hinoki plantation of the Ooe Experimental Forest Station of Kyoto Prefectural University, in Kyoto city. 3. A 20m×20m quadrate was settled in the stand and the quadrate was divided into 1.25m×1.25m subquadrate for elucidating the distributional patterns of Hinoki seedings. 16 seed traps were set up in parallel within a quadrate (Fig 1). Seed crop estimation was carried out from autumn in 1973 to spring in 1974. All Hinoki seedings germinating in 1974 were labeled with number tags. The location and the seedbed conditions of each seedings were also recorded. And then, dynamics of seedings numbers were pursued until next spring. 4. The dispersion of fallen seeds on the forest floor showed contagious distribution estimated from Morisita's index Iδ. 5. The germinant seedings were also contagiously distributed (Fig 4). The relationship between number of localy fallen seeds and number of germinant seedings were found to be closely related (Fig 5). Therefore, the contagious distribution of the seedings was reflected on that of fallen seeds, and, germination condition was not so localy bias on the forest floor. 6. First year's disapperance of Hinoki seedings were enormous, particulary many seedings were losed during the three months after germination (Fig 6). 7. The distributional patterns of current Hinoki seedings were contagiously distributed with small clumps and large clump through the various stages and the degree of contagious distribution became stronger, as times went on, except a few weeks after germination (Fig 7). 8. Survival of the seedings within a few weeks after germination was not so contagious rather random, and seedings died almost regardless of their seed bed conditions (Fig 7 & 10). 9. After the weeks the seedings became better survived on the seed bed such as moss and nineral soil than on the litter (Fig 10). Consequently, the seedling distribution was gradually limited on the such micro sites as moss and mineral soil distributing patchly on the forest floor. 10. Relatively many seedings were found, next spring, on the forest floor, where the dispersion of fallen seeds were contagiously denser.