スギ・ヒノキ高齢林の経営論的研究 : 東京大学千葉演習林における人工林経営に関する実験
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本研究は人工林808haの内約50％を占める400haの高齢林を対象として実践的立場から今後の経営方針を検討した。まず，第1章では千葉演習林の森林経営の歴史を分析し，今後の課題を明らかにした。とくに，これまでの経営の主体は一斉皆伐再造林であったが，今後の林業経営の在り方として環境保全，林業従事者の減少等を考慮し非皆伐施業の導入を検討した。第2章ではこれまで千葉演習林で行ってきた複層林造成の経緯をまとめ，複層林の利点，問題点を明らかにした。また，複層林の基本方針を作成し今後の長伐期複層林造成のための間伐方法を検討した。第3章は1928年に造成された水源函養林としての複層林と1966年に上層木の密度を異にした複層林及び1977年から実施した高齢林の利用間伐にともなう複層林造成試験林の調査結果を解析した。第4章では千葉演習林の75年間にわたる成長測定試験地の資料を基にその成長経過を林分密度と関連させながら解析した。さらに，上木の林齢が400年生以上と推定される超高齢複層林の林分解析を行い今後の長伐期複層林施業を遂行する上での資料とした。第5章ではこれらの間伐方針と実験結果を基に二段林造成等を実施した場合の林分成長，林内の明るさ等の変化を予測した。とくに，100年生スギ林分をモデルとして，無間伐を含め環境保全型間伐法（本数間伐率40％），長伐期施業型間伐法（本数間伐率60％），複層林施業型間伐法（本数間伐率70％）を実施したと仮定し120年生までの林分成長，林分変化を予測した。これらの結果をもとに第11次試験研究計画では次のような4種類の施業法を高林作業級において実施することとした。（1）皆伐施業，（2）長伐期施業，（3）二段林施業，（4）複層林施業上記施業法に該当しない高齢林分に対しては，随時環境保全型間伐を行う。皆伐施業を行う林分は80年生前後とし，対象面積は学生実習，研究利用に供する最小限の2ha/年前後とする。また，最近の測定機器の多様化等から林道に比較的近い林分を対象とする。長伐期施業林分は林道から遠く集約的な施業の不可能な地域を選定し，強度の間伐を行い最大限の収益を得，残存立木の大径化を図る。対象面積は搬出コストの軽減を図るため3ha以上の林分とする。二段林施業林分は林道から比較的近い林分を対象とし面積は2ha/年前後とする。林齢は80年生以上の高齢林を対象とし，二段林造成時に強度の間伐（本数間伐率70％）を行い林床にスギ，ヒノキ苗木を植栽する。下木の植栽密度は2,000本/haの疎植とし，植栽木の競争を避ける。複層林施業地は林道に最も近い林分または搬出路を開設し車両系による搬出可能な林分とし最も集約的な施業を行う。第11次試験研究計画にこれらの施業法を取り入れるに当たり，人工林の植伐に関する労務計画を検討した。This paper is concerned with the future management policy of 400 ha of old plantation forest which occupies about 50% of the total plantation forest in the Chiba Forest. Chapter 1 analyzed the history of forest management in the Chiba Forest and clarified future problems in forest management. Although coniferous plantations have generally been clear-cut, trying non-clear cut management was examined protect in order to the environment and cope with the decrease of forestry workers. Chapter 2 clarified the advantages and problems of multi-storied forests on the basis of the history of establishing multi-storied forests in the Chiba Forest. Moreover, it prescribed the treatments for multi-storied forests and examined the thinning method needed to establish long-rotation multi-storied forests in the future. Chapter 3 analyzed data on a multi-storied forest established in 1928 to conserve water resources, multi-storied forests with different overstory densities established in 1966, and experimental multi-storied forests established after thinning from old plantation forests since 1977. Chapter 4 analyzed the growth process of experimental plots measured over 75 years in the Chiba Forest. Furthermore, it analyzed very old multi-storied forests where overstory trees were estimated to be older than 400 years to consider future management of long-rotation multi-storied forests. Chapter 5 predicted the growth of two-storied forests and the change of light intensity in the forests on the basis of the thinning prescription and the results of experiments. A thinning simulation showed the effects of thinning a 100-year-old sugi stand for protection of the environment (thinning rate of 40% of stocking), for long rotations (thinning rate of 60% of stocking), and for establishment of a multi-storied forest (thinning rate of 70% of stocking). The growth of the stand was predicted for 20 years, assuming that it would be thinned at these rates, or be unthinned. Based on the results above, it was suggested that the following four silvicultural systems should be carried out in high forests during the eleventh working period; (1) clear cutting system, (2) long rotation system, (3) two-storied forest system and (4) multi-storied forest system; and old forests, to which the methods above will not be applied, should be thinned for protection of the environment if necessary. The clear-cutting age should be around 80 years and the clear-cutting area should be 2 ha per year, which is the minimum area for student practice and research on forest management. Stands near the road will be cut because measurement instruments have been diversified. The long rotation system should be employed in stands far from the road and inapplicable to intensive treatment in order to get the maximum gain by intensive thinning and production of large logs from residual trees. The area for this silvicultural system should be 3 ha or more to reduce hauling costs. The two-storied forest system should be used in stands near the road and area for this silvicultural system should be around 2 ha per year. In the system, stands older than 80 years should be thinned intensively (by thinning rate of 70% of the stocking) and sugi and hinoki seedlings should be planted under the stands by low density of 2,000 trees/ha to prevent them from competing. The multi-storied forest system should be done in the stands next to the road or stands suitable for mechanized hauling with tower-yardes etc so on after constructing the forest roads because this silvicultural system is the most intensive one. Finally, labor plans for planting and cutting were made to carry out these silvicultural systems in the eleventh working period.
- 東京大学大学院農学生命科学研究科附属演習林,The Tokyo University Forests,東京大学農学部附属演習林千葉演習林,University Forest in Chiba, Faculty of Agriculture, The University of Tokyoの論文