Seven-year succession of a subtropical laurel forest following strip clear-cutting on Okinawa Island

摘要

本論文は沖縄の亜熱帯照葉樹林地帯において，天然更新を基盤とする各種天然林施業法に関する研究の一環をなすものである。帯状伐採施業は1994年実行され、保残区残存木の報告に引き続き施業7年後の伐採区について調査した。試験林設定時の伐採区は伐採樹種53種、891株で、今回は根株（伐採株）の腐朽状態と萌芽幹、実生幹、新規発生木を樹高1.0m以上の全出現個体について毎未調査を行った。根株の枯死率（腐朽状態－d－）は34.7％で，施業試験地が隣接する皆伐施業林の一年後、5年後の枯死率より高くなり，根株枯死率は伐採経年に伴って増大傾向を示した。根株の生育状況は約1／3の完全腐朽（枯死）に対して，多数の根株が生存あるいは萌芽幹を発生しており，そのうち根株の12.8％が萌芽幹発生・原型保持（腐朽状態－a－）、42.6％が萌芽幹発生・部分腐朽（b）、9.9％が萌芽茎発生・完全腐朽（c）であった。施業7年後の二次林の樹種は74種で、そのうち49種が萌芽由来、69種が実生由来となり、伐採前林分（天然林）の53種より増加していた。萌芽力が強い天然林優占種のイタジイは、二次林途中相でも優占していた。実生由来と萌芽由来の間で、平均樹幹密度は実生由来が萌芽由来より高く有意差がみられたが、基底面積は有意差がなかった。胸高直径4cm以下の樹幹密度と基底面積は実生由来が萌芽由来より高く、胸高直径4血以上の基底面積は萌芽由来が実生由来より高い値を示し、早期の二次林途中相では実生由来の多数個体が下層に出現し、萌芽由来が上層を占めていた。天然林伐採後、先駆樹種が侵入し樹種増加を伴うが、天然林構成種の萌芽由来と実生由来の発生がみられ、帯状伐採施業7年後の再生林は進行遷移に属することが示唆された。%The mortality of stumps and growth of a secondary forest in cut area seven years post strip clear-cutting in a natural subtropical laurel forest on Okinawa Island were studied. The results showed the mortality rate of stumps was 34.7%, which was higher than those in a similar secondary forest at the first and the fifth year after clear-cutting (13.2, 20.2%, respectively) in nearby site, confirming that the mortality rate of stumps increased with years in a certain years. In contrast to dead of one-thirds of stumps nearly, majority of stumps remained alive, of which, 12.8% of the stumps were remained initial figure, 42.6% of them partly decaying and 9.9% of them invisible with living sprout. The secondary forest was consisted of a total of 74 woody species, of which, 49 and 69 species, respectively, were found in sprout- and seedling-origins, greater than that in the primary forest (53 species) before strip clear-cutting. The primary dominant species, Castanopsis sieboldii, still dominated the secondary forest. The mean stem number of seedling origin stems by plot was significant (p < 0.05) higher than that of sprouting origin ones. However, no significant differences found for the mean basal area by plot between two kinds of stem origins. Seedling stem number in DBH group < 4.0cm had higher values than those for sprout's, while sprout origin stems in basal area were roughly seven-fold more than that for seedling's in DBH group > 4.0cm. The results indicated that the seedling stems shared the understorey and sprout-origin stems shared the crown canopy in the early succession stage. The secondary forest was dominated by sprout stems rather than seedling-origin ones. The great of woody species with abundant of sprout- and seedling-origin stems suggested the secondary forest seven years after strip clear-cutting occurred in a progressive succession.