冷温帯下部天然生林の吏新技術 VI : 光環境の異なるミズナラ天然生林内に樹下植栽したスギ, ヒノキの成長

Abstract

芦生演習林のミズナラが優占する天然生落葉広葉樹林に皆伐, 強度間伐, 弱度間伐, 無間伐の上木処理を行った4試験地を設定した。樹下にスギ, ヒノキを植栽し, 上木, 下木の10年間の成長過程を調べた。1) 設定時における試験地の立木本数は2500 - 3500本/ha, 材積は98 - 127m_3/ha, 平均直径は10cm前後, 樹高は7. 5m前後であった。本林分はミズナラ, リョウブ, コハウチワカエデ, アカシデ, ミズメなどによって構成されていた。ミズナラは本数の54 - 71％, 材積の68 - 95％を占め, 直径階分布の上位で正規分布型を示し, 下位にミズナラより耐陰性が高い樹種がL型を示す複合型の分布型を示した。2) 上木の年成長率は強度間伐区 (52m_3/ha) では上木処理後向上し, 10年間に徐々に低下した。弱度間伐区 (77m_3/ha) では処理の影響が現れにくく, ミズナラ劣勢木に枯死するものが多く, 年成長率の低下が著しかった。無間伐区 (127m_3/ha) でも期間中に枯れたものはミズナラだけで, ミズナラ以外の樹種の年成長率が優っていた。3) 上木処理後1年目の開葉期の林床の相対照度は, 強度間伐区で45. 4％, 弱度間伐区で22. 9％, 無間伐区で10. 6％であった。3年目に照度はそれぞれ1/4, 1/3, 1/2に低下した。9年目の測定値は3年目と大きな違いはみられなかった。4) 植栽した下木のスギ, ヒノキの生存率はそれぞれ96％, 71％となり, ヒノキで低い値を示した。伐採率が高い試験区ほど枯死率が高く, 枯死が発現する時期は早まった。5) 下木は試験期間中に多くの被害が観察された。スギは虫による食害を受けるものが多かったが, 直接成長に影響するような被害は比較的少なかった。ヒノキは樹高成長を抑制された個体に, 乾燥あるいは被圧に起因すると思われる葉の脱落や, 主軸が損傷されたものが多かった。スギ, ヒノキともに直接雪に起因すると思われる被害は比較的少なかった。6) 皆伐区ではスギは植栽後2年目, ヒノキは3年目より他の試験区より樹高, 直径成長が優るようになった。しかし, スギは4年目以降, 樹高成長が衰えるようになり, 間伐区との差が小さくなった。強度, 弱度間伐区では両試験区間で大きな成長差はみられず, スギ, ヒノキともに3年目以降無間伐区との成長差がみられるようになった。無間伐区では他の試験区に比べ成長低下は明らかであったが, 両樹種ともに年間の樹高成長は10cm前後となり, 比較的伸長量は大きかった。全ての試験区でヒノキはスギに比べて樹高成長が優っていた。7) 下木の個体重量の年成長率は皆伐区, 間伐区では植栽後上昇し, ピークが訪れ, その後急激に低下して安定した。皆伐区ではスギは2年目, ヒノキは3年目に成長率が70％前後となり, その後低下して, 5年目以降はスギは20％, ヒノキは30％前後となり, スギの成長低下が著しかった。間伐区ではスギ, ヒノキともに弱度, 強度区で大きな差はみられなかった。両樹種ともに4年目にピークが現れ, その値は皆伐区より低かった。無間伐区では試験期間を通じて両樹種ともに20％前後の安定した値を示した。8) 形状比はスギ, ヒノキともに試験期間前半にあっては, 各試験区で値は上昇し, 3年から4年目より安定するか, その後は徐々に低下する傾向を示した。伐採率が高い試験区ほど形状比の低下が始まる時期は早く, その程度も大きい傾向がみられ, 試験期間後半には低い値を示した。各試験区ともにスギに比べてヒノキの形状比は高かった。

Study plots were set in a natural Mizunara (Quercus mongolica var. grosseserrata) stand in Kyoto University Forest in Ashiu located at northern east part of Kyoto Prefecture to investigate the growth of upper-story trees and underplanted Sugi(Cryptomeria japonica) and Hinoki (Chamaecyparis obtusa) seedlings for 10 years. The plots were composed of clear cutting, heavy thinning, light thinning and non cutting stands. The results were as follows : 1) Standing tree number was 2, 500-3, 500/ha, standing tree volume was 98-127 m3/ha, average breast height diameter was about 10 cm and average height was about 7.5 m in each plot at the setting plots before the treatment of upper-story. This stand was composed of Mizunara, Ryobu (Clethra barbinervis), Kohauchiwakaede (Acer Sieboldianum), Akaside (Carpinus laxiflora), Mizume (Betula grossa) and other species. Mizunara was 54-71% in number (DBH≧5 cm) and 68-95% in volume in each plot. The frequency distribution of DBH class of plots was complex type which showed normal type by Mizunara in higher DBH class and L type by the other species in lower class. 2) Annual growth rate of Mizunara stem volume of upper-story in heavy thinning plot (52 m3/ha) became the highest after thinning and made a slow descent for 10 years. In light thinning plot (77 m3/ha) it was low during the period of experiment, because many oppressed trees of Mizunara have been dead. In non cutting plot the rate of other species was higher than that of Mizunara. 3) The relative light intensities in summer measured immediately after cutting were 45.4% in heavy thinning plot, 22.9% in light thinning plot and 10.6% in non cutting plot. The measured values of the 3rd year after treatment became one fourth, one third and one a half of these values in the same order, but those of the 9th year became little lower than those of the 3rd year, respectively. 4) The survival rates of Sugi and Hinoki seedlings were 96% and 71% in all plots during the period of experiment respectively. The higher the rate of cutting was, the higher the mortality rate became and the sooner the time of mortality was. 5) Sugi seedlings were heavily browsing by insect, but the growth was not so damaged. The growth of Hinoki whose spindle and leaves were damaged by drought and shade, or frost and animal was inhibited in clear cutting plot and thinning plots, remarkably. Sugi and Hinoki seedlings were damaged little by snow in all plots. 6) The growth of Sugi seedlings in clear cutting plot became the best in all plots from the 2nd year after planting, but the height growth went down from the 4th year, and became to differ little from that in thinning plots in the later half of the period of experiment. The growth of Hinoki seedlings in clear cutting plot became better from the 3rd year and after that was the best in all plots during the period of experiment. The growth of Sugi and Hinoki seedlings in thinning plots was better than that in non cutting plot from the 4th year, and differed little between the two. Annual height growth of Sugi and Hinoki seedlings in non cutting plot was about 10 cm. The height growth of Hinoki seedlings was better than that of Sugi in all plots, slightly. 7) Annual growth of seedling weight in clear cutting plot was the highest from the 2nd year after planting in Sugi and from the 3rd year in Hinoki, and went down rapidly and became stable from the 5th year. The value of Sugi in clear cutting plot became lower than that in thinning plots in the later half of the period of experiment, but that of Hinoki continued to be the highest in all plots. The values of thinning plots were the highest in the 4th year, but the values of maximum were lower than that in clear cutting plot and differed little between the two. In non cutting plot the values of Sugi and Hinoki was the lowest and the most stable. 8) The rate of H/Do of Hinoki seedlings was higher than that of Sugi in all plots. The values became higher after planting and went down from the 3rd or the 4th year slowly. The higher the rate of cutting was, the sooner the time when it went down became, and the lower the rate of H/Do became from the middle of the period of experiment.