秩父山地イヌブナ－ブナ林の齢構造と更新特性

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秩父山地の標高1,160m地点にあるイヌブナ－ブナ林の更新様式を探る目的で，1990 年に毎木調査と年輪解析を行い，林分構造と齢構造を明らかにした。また現在の面積が48 ㎡と252 ㎡の林冠ギャップ2ヶ所について発生年代と修復状況を明らかにした。さらに現在林冠を構成している樹種により区分した樹冠群ごとに，樹冠を構成している樹木の更新状況を林分構造，齢構造，成長経過から明らかにした。これらのことからイヌブナ－ブナ林の更新特性について検討した。調査区で1990 年に出現していた樹種数は26 種で，本数密度は2,800 本/ha，胸高断面積合計は33.2 ㎡ /ha であった。林冠は主にイヌブナとブナで構成され，イヌブナが本数の72.0%，相対優占度(RD) の78.8％を占め優占していた。ブナは本数の1.0%，RDの15.8％を占めていた。立枯木や幹折木等の枯死木は樹種数が６種で，枯死木の本数密度は189 本/ha，胸高断面積合計は9.3㎡ /ha であった。そのうちイヌブナが本数の89.4％，RD の60.9％を，ブナが本数の3.0％，RDの37.2％をそれぞれ占めていた。調査区に占める林冠木の樹冠投影面積の割合は87.2%で，樹種別割合はイヌブナが65.7%，ブナが17.2%，他の広葉樹類が4.3%であった。ギャップの占める割合は12.8%で，最大面積は252 ㎡，二番目が48 ㎡，三番目が37 ㎡，その他は15 ㎡未満であった。イヌブナが優占する樹冠群では直径30cm以上，樹高12m以上の幅広いサイズや樹齢120 年以上の幹を持ったイヌブナ株が樹冠群全体に広く分布していたことから，イヌブナ樹冠群は全体が一斉に更新したのではなく，イヌブナが株内の幹を交代させながら徐々に更新してきたものと推察された。ブナが優占する樹冠群は現在の面積が118 ㎡～ 328 ㎡で，1740 ～ 1770 年に発生した個体により林冠が構成されていることから，ほぼ同年代に更新したものと考えられた。しかし，その後のブナの直径成長経過は個体により異なっていた。その理由のひとつとして，成長の良好な時期のある個体は成長量を増大させた年に，そのブナと樹冠が接していた場所に林冠ギャップが生じ，光条件が好転したことがあげられた。1955 年に胸高直径65cm のブナが倒れた際に生じた林冠ギャップは，現在の面積が223 ㎡で，林冠ギャップ形成から35 年経過した1990 年の時点でも大きく疎開したままであった。同林冠ギャップではハウチワカエデ，コハウチワカエデ，ヒナウチワカエデなどのカエデ属が主体となって更新していた。一方，本調査区に隣接して1934 年に50.17ha が伐採された林分や，1990年に0.35ha が伐採された本調査区の14 年後の林分では，伐根からの萌芽により更新したイヌブナと，ウダイカンバ，ミズメ，ウリハダカエデ，ミズキなどの先駆性の高い落葉広葉樹類が優占していた。これらのことから，林冠ギャップの更新や修復に関わる樹種は，攪乱の種類や林冠ギャップの大きさによって異なることが示唆された。In order to explore the regeneration pattern in a natural beech forest (Fagus japonica Maxim. and F. crenata Blume), a research plot with an area of 0.35 ha was established at an altitude of 1,160 m in the Chichibu mountains, central Japan. The diameter at breast height (DBH) and height (H) of all trees taller than 1.3 ｍ were measured．Stem analysis had been carried out in the study plot in 1990 on 45% of the trees in order to clarify and categorize the age structure. Using the tree species that constituted the measured canopy in 1990, crown cluster and gaps were measured and classified into separate groups. The regeneration characteristics in the natural beech forest were analyzed on the basis of the relationship between annual ring analysis and crown dynamics. Twenty-six species were recorded in the study plot. The total number of trees and basal area (BA) were 2,800/ha and 33.2 ㎡ /ha, respectively. The study stand was dominated by F. japonica , which occupied 72.0% in terms of number of trees and 78.8% in relative dominance (RD). The number of trees and RD of F. crenata were 1.0% and 15.8%, respectively. Six species were recorded as dead trees (DBH ≧ 5cm) in the study plot. The total number and basal area of dead trees were 189/ha and 9.3 ㎡ /ha, respectively. The ratio of the number and RD of dead trees of F. japonica against all dead trees were 89.4% and 60.9% and those of. F. crenata were 3.0% and 37.2%, respectively. Total crown cover was 87.2% of which 65.7% was occupied by F. japonica, 17.2% by F. crenata and the remaining 4.3% was occupied by other deciduous tree species. The total gap area in the canopy was 12.8%. The largest gap was 252 ㎡ , followed by 48 ㎡ and 37 ㎡ . Other gaps were less than 15 ㎡ . In the crown cluster group dominated by F. japonica, specimens of F. japonica that had stools from which stems with a DBH greater than 30cm, height larger than 12m and age more than 120 years were widely distributed. Therefore, the whole crown cluster dominated by F. japonica did not seem to have regenerated simultaneously but was regenerated gradually by stem replacement from stools. The area of the crown cluster dominated by F. crenata in 1990 was 118 - 328㎡. F. crenata trees were considered to have regenerated at almost the same time between 1740 and 1770. However, the diameter growth progression differed individually in the case of F. crenata. One reason for the increase in diameter growth of the crown trees of F. crenata was improved light conditions caused by the formation of small-scale gaps adjacent to the crown of dominant F. crenata trees. A large-scale gap with an area of 223 ㎡ still existed in 1990 even after it was formed 35 years ago when a specimen of F. crenata of 65cm DBH fell in 1955. In this gap, species such as Acerjaponicum, A. sieboldianum, and A. tenuifolium, have mainly regenerated. To estimate the recovery process of disturbed stands, we observed two secondary stands, one was cut by 50.17 ha in 1934 near the study plot and the other was this study stand which was cut by 0.35ha accidentally in 1990, 14 years after its establishment. Both of these clear-cut areas were dominated by pioneer deciduous broad-leaved trees such as Betula maximowicziana, B. grossa, Acer rufinerve, and Swida controversa. From these results, it was suggested that the species that regenerate in gaps or fill gaps differs greatly according to the magnitude of disturbance and size of gap.

秩父山地イヌブナ－ブナ林の齢構造と更新特性

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