| 概要 |
九州における自然植生の分布は,群系からみると,低山地帯の照葉樹林と山岳地帯の夏緑広葉樹林,その中間にモミ,ツガを主とする常緑針葉樹林で構成されている.本研究は九州中部山岳地帯に位置する三方岳西斜面に分布するモミ(Abies firma)･ツガ(Tsuga sieboldii)天然林を対象として,その天然林の成立過程を明らかにすることにより,保続,拡大を目的とした天然更新の技術開発あるいは林分施業に...対する基礎資料を得ることを目的としておこなったものである.モミ,ツガの更新をおこなう場合,その成否を左右するのは結実量である.そこで結実の周期と結実量をしらぺることにより,結実の豊年は2～3年おきにあらわれることを明らかにした.また,この隔年結果は樹種それ自体の内部的要因により,結実量には個体をとりまく外環境が関係するのではないかと推定した.また,モミ･ツガ天然林におけるモミ,ツガの生長は稚樹の発生段階から生育途中での林分の構造と密接に関連しており,とくに初期生長の遅さとその後の急速な生長増大によって特徴づけられる.したがって,この初期生長を促進し,健全な後継樹の育成をはかるために初期生長に関与する植生環境をしらべる必要がある.そこで,まず稚樹の発生と初期生長に対する明るさの関係をしらべ,雅樹の発生には明るさはほとんど関係ないが,その後の地上部,地下部の発達には,100%の明るさよりも40～80%の明るさが適当であること,また,ツガはモミにくらべて種子が極端に小さく,発生直後の生長は,光,温度,土壌水分等の外的要因に大きく影響を受けることを明らかにした.つぎに,スズタケ群落内のモミ,ツガは,スズタケの高さをこえると,急速に生長が好転しているが,スズタケの高さとほぼ等しい2mの樹高に達するまでに,モミ20年前後,ツガ25年前後の長年月を要している.そこで,当調査地の下層植生の優占種であるスズタケが林床の光環境を悪化させ,稚樹の生長にとって大きな阻害要因となっていることを明らかにし,スズタケの除去は椎樹の発生,生長促進のために有効な作業であることを認めた.一方,モミ,ツガが高木層を占め,うっ閉がすすんだ純林状の林分では,スズタケの発生はほとんどないが,後継樹もほとんど存在せず,モミ,ツガ稚樹は発生,消失をくり返すだけであることを明らかにした.さらに,除伐による光環境の改善試験をおこなって,稚樹はもちろん,高齢の60年生前後の被圧木の生長も好転するが,再び上層樹冠がうっ関すると生長がまた急速に衰退することを明らかにし,稚樹,被圧木の生長促進には単木的な除伐では十分ではなく,かなりの程度の林冠の疎開が必要であることを認めた.以上のような結果をもとに,モミ･ツガ天然林の保続,拡大のためには,人為的干渉を導入した取り扱いが必要であることを明らかにした.
The purpose of this paper is to elucidate the process of the formation of natural forest of Abies (Abies firma) and Tsuga (Tsuga sieboldii), which distributed widely in mountainous district in Kyushu, and provide the fundamental data on their natural regeneration. This investigation was carried out in natural forests of Abies and Tsuga distributed in the Kyushu University Forest of MIYAZAKI. The results obtained are summarized as follows: 1) Topographically, a natural forests of Abies and Tsuga are distributed from the middle part of slope to the ridge of mountain. 2) Pure forest of Abies or Tsuga are rare, usually being mixed with each other. 3) Their age was estimated as about 90～110 years old. In general, it is said that Abies and Tsuga are about 200～300 years old in matured stands and about 400～500 years old in over-matured stands. This indicates the forests investigated in the present study to be young and developing. 4) The distributional patterns of Abies and Tsuga populations were analyzed by the Iδ index. Seedling population was distributed contagiously. Tree populations showed contagious distribution, but that of dominant trees was random. 5) Rich production of seeds of Abies and Tsuga occured at intervals of 2-3 years. 6) The seeds of Abies and Tsuga germinate mostly from May to July, and the seedlings die in the winter. Half of the seedlings that germinated died within two years following germination. After that, number of seedlings decreased slowly. 7) When the seedlings were very small, their root systems were similar but after becoming higher than 50 cm, Abies showed a deep rooted system and Tsuga, a shallow rooted system. Judging from the root form, Abies and Tsuga were assumed to segregate well from each other underground. 8) By the lattice shelter experiment conducted on seedlings, the share of dry matter and T-R ratio were found to vary with the degree of shade. That is, due to inadequate sunlight, growth was greatly obstructed in light less than 10% full sunlight. Overshading retarded not only top growth but also root development. In the full sunlight, the growth of seedlings was also obstructed. It is considered that the factors such as high temperature at the soil surface and soil drought cause the results. Tsuga seedlings were influenced particularly by these external factors since the Tsuga seeds were much smaller than those of Abies. The optimum light intensity for the initial growth of seedlings ranged from 40～80% full daylight. 9) Dominant species of undergrowth in the stands examined was Suzutake (Sasa borealis). Suzutake grows close to the forest floor and consequently receives very little sunlight. Light intensity at the ground level in Suzutake groups was 5～25% the light intensity just above the foliage of Suzutake. When the Suzutake group developed in the stand, it was almost impossible to find Abies and Tsuga seedlings there. 10) The height growth of Abies and Tsuga up to 2.0 m was considered obstructed due to inadequate light on the Suzutake group. Abies and Tsuga elongated rapidly when height growth exceeded at about 2.0 m, equal that of Suzutake. It might be a reason of growth obstruction that Suzutake roots distribute densely within the soil to a depth of about 20 cm. 11) The lower branches of Abies under suppressed condition died, resulting the tree form to be taken on an umbrella shape. The new leaves of such trees were few, but remained on branches for a long period of time. 12) When Abies and Tsuga developed sufficiently, light penetrating into the forest became extremely slight, causing poor growth on the part of succeeding trees and seedlings. The author thus carried out experiments to determine how to improve the light environment for Abies and Tsuga in the shade by the removal of broad-leaved trees. Tree growth was found to improve. But when the removal of such trees was slight, growth suppression occurred again. Thus, to hasten the growth of suppressed trees and seedlings, it is necessary to control or remove the underfloor vegetation effectively.続きを見る
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