天然生タブ林の生長経過

Abstract

京都大学の徳山試験地 (徳山市鉢窪) に成立しているタブを主とする二次林に2つの調査区を設け, 以後10年間, 2 - 3年おきに毎木調査を行なって, 種組成の変化, 種ごとの生長経過を追跡した。2つのプロットはともにタブを主とする林分で, 立木密度で40％以上, 断面積合計では50％以上を占める。高木層にはタブに混って陽性の落葉広葉樹とアカマツがみられるが個体数は少ない。これらのうちアカマツ, ヤマザクラは10年聞に枯死するものが多く, 二次遷移の初期に生長した陽樹群が次第に減少してタブの純林に近づきつつあるようにみえる。プロット1は斜面上部にあって, タブについでクロキ, ヤマハゼが多いのに対し, プロット2ではこの両種が極めて少ない。低木層では, プロット1でヒサカキ, ネズミモチが多く, タブ, クロキ, ヤマコウバシ, イヌビワがつづく。プロット2ではネズミモチが圧倒的に多く, つづいてヒサカキ, タブ, イヌビワであり, ハナイカダ, クロモジ, アオキなどがみられる。これらの点からプロット1はプロット2よりやや乾性であるといえる。立木密度はプロット1が大でプロット2は約60％の密度であるが, 平均単木のサイズが大で, 断面積合計では83％となっている。10年間にアカマツはほぼ全滅し, ヤマザクラも枯死率が高かった。その他, ヤマハゼも枯死して個体数が減少している。これに対し, タブ, クロキは枯死と新加入がバランスしていて著しい個体数の変化はみられなかった。断面積合計はプロット1で30. 6m_2・ha_-1から43. 6m_2・ha_-1に, プロット2では25. 5m_2・ha_-1から39. 0m_2・ha_-1に増加し, 現在の生長は大きい。その50 - 60％をタブが占めている。高木屑にみられるタブ以外の種は亜高木, 低木層にほとんどみられないのに対し, タブは高木から低木まで連続して存在している。このことからタブの優占度が次第に増大し, タブの純林へと発達していくようにみえる。プロット1ではこれに亜高木層を中心としてクロキが混生し, 林床にはヒサカキ, ネズミモチの優占する群落が, プロット2ではタブを主とし, クロキを欠き, 林床にネズミモチ・ヒサカキの多い群落が成立するものと期待される。

Studies on the change of the floristic composition and rate of growth for each species were conducted in a secondary forest stand, dominated by Machilus thunbergii, developed on a slope in the Forest Experimental Station of Kyoto University in Tokuyama, Yamaguchi Prefecture (E 131°48', N 34°03'). The mean annual temperature is 15.2℃, and the annual precipitation is 2090 mm. An evergreen broad-leaved forest or laurel forest is a climatic climax in this area. Two plots (P-1 and P-2), each 20 x 20 m in size, were set up on a slope in Oct. 1972. P-1 was located on the upper part of the slope, and P-2 was on the middle part. The DBH and height of every tree larger than 4.5 cm in the DBH was measured at a 2 or 3 year interval over a period of 10 years, from Oct. 1972 to Feb. 1983. M. thunbergii is the dominant species in both plots, and it covered more than 40% in density and more than 50% in basal area. Several species of deciduous broad-leaved trees, such as Cornus brachypoda, Prunus jamasakura and Rhus sylvestris, and Pinus densiflora mixed in the tree layer with M.thunbergii, though the density was low. These species are intolerant or exploitive species, and it is assumed that they established and grew quickly during the early stage of secondary succession. This implies that the stage of the stand studied at present is that changing from intolerant to tolerant during succession. The density of Rhus sylvestris and Symplocos lucida was rather high in P-1 while they were very low in P-2. These two species were rather small in height and DBH as compared with the dominant individuals in the tree layer. Many of them were found in the lower part of the tree layer. Eurya japonica and Ligustrum japonicum were most abundant in the shrub layer (DBH <4.5 cm, H> 1.3m), and M.thunbergii, S.lucida, Lindera glauca and Ficus erecta followed, in P-1. L.japonicum was the most dominant species in the shrub layer of P-2, and E.japonica, M.thunbergii and F.erecta followed. Helwingia japonica, Lindera umbellata and Aucuba japonica were recorded though the density was very low in P-2. The lack of R.sylvestris and S.lucida, and the presence of H.japonica and A.japonica suggests that the moisture condition in P-2 would be better than in P-1. The density of tree (D≧4.5cm) of P-2 was about 60% of P-1, though the basal area of P-2 was 83% of P-1, and the average size of an individual tree was larger in P-2 than in P-1. This indicates that the soil fertility in P-2 might be higher than that in P-2. Almost all Pinus densiflora individuals died during a period of 10 years, and the mortality of Prunus jamasakura was high: 60%. Some of R.sylvestris died and decreased in density. On the other hand, there was on change in density for M.thunbergii, since the dead and newcomer were nearly equal in number. S.lucida, L.japonicum and F.erecta increased in number. The basal area increased from 30.6 to 43.6m2/ha for P-1, and from 25.5 to 39.0m2/ha for P-2, respectively. The rate of increase was very high. M.thunbergii covered more than half of the increment of the stand. Deciduous broad leaved tree species in the tree layer had no or very few individuals in the shrub layer, while M.thunbergii and S.lucida were abundant not only in the tree layer but also in the shrub layer. This implies that M.thunbergii and S.lucida are tolerant or conservative species. Therefore, it is presumed that the dominance of M.thunbergii increases in density and biomass with the development of the stand, accompanied by a decrease in intolerant species. In this stand, there is no individual of Castanopsis spp. nor Cyclobalanopsis spp. which are the most common and dominant species in a laurel forest, and the diversity of species of evergreen broad-leaved trees was very low. The following two assumptions are presented here as the reason of this fact: (1) M.thunbergii establisched so densely that the other species were eliminated. (2) As the result of human disturbance, many tree species had been elimiinated from around this station, and there is an insufficient seed source to develop a mixed laurel forest.