Ecological Studies on the Coastal Pasania edulis Forests in Northern Kyushu

Coastal coppices dominated by Pasaniu edulis Makino are found along the Genkai Sea in northern Kyushu. In this paper these forests were studied from the view points of floristic composition, growing environments, and biomass production for the purpose to provide fundamental data which are important in managing these forests as environmental or commercial forests. The results may be summarized as follows: A. Floristic composition and environmental factors. (1) Pasania forests have been recognized as coppices in the past, and characteristics of floristic composition have not been clarified due to the lack of character species. (2) According to cluster analysis of floristic compositions, Pasania forests were separated from other communities with only the domination of Pasania edulis, and with no specific follower plants. (3) There was a slight tendency that Pasania forests developed even on more poor sites in available water. (4) Principal component analysis was used for the purpose to know the effect of each environmental factors in the general adaptability. Analysis showed that Pasania forests developed on slightly xeric sites in all communities analized. But no effective factor which restricts distribution of communities was found. According to these results, it was considered that distribution and expansion of Pasania forests depend on man-made factors rather than natural environment. B. Biomass production. (1) Aboveground biomass of 34-year-old coppice dominated by Pasania edulis were 214 ton/ha, and leaf biomass occupied 9.7 ton/ha. These values were larger than those of coppice in same tree height dominated by Castanopsis cupidata var. Sieboldii. (2) The forest canopy of sampled stand was thin, but showed very high density. Relative light intensity declined abruptly around the top of the forest canopy, and was restricted at the low level of 1.6 or 2.0% on forest floor. (3) By stem analysis, stem volume of stand sampled was estimated as 320 m^3/ha. And remarkable elongation was observed in 1 or 2 years after clearcut. This seemed to indicate a possibility of vigorous sprouting. (4) Two different patterns of stem volume development were observed. Sample trees which had DBH less than 10 cm increased their stem volume gradually. On the other hand, samples with DBH more than 10 cm showed a peak in 23 or 25 years old. (5) Ages of stems gained from the same stump were different. It was, therefore, considered that earlier sprouts can not always remain. (6) L. A. I. was 4.8 ha/ha, which was smaller than those of Castanopsis forests. The value of light extinction coefficient (K) was very large as 0.82. But, strict linearity between log (Relative light intensity) and cumulative L. A. I. were not observed. At the upper part of the forest canopy the value of K was 0.21. They were considered to have resulted from change of leaf angles through the leaf development of Pasania edulis. (7) A tendency that vegetation of forest floor become poorer with the development of tree layer, was recognized. It was expected that this tendency continues until1 light condition under the forest canopy will be improved by the self-thinning through the competition between individuals forming the forest canopy.本研究は玄海灘沿岸に広く分布するマテバシイ林を,経済林,環境保全林として取り扱っていくうえでの基礎的資料を提供するため,植物社会,立地環境,一次生産特性という見地から解析した.本研究の結果の概要は以下のとおりである.1.マテバシイ林の種組成と成立環境(1)既存の文献によるマテバシイ林の社会は,一括して二次林として扱われており,表徴種群の欠落のためにその性格は明瞭でない.(2)常緑広葉樹林低部域に出現する種の量的出現度合いをもとに,クラスター分析により解析した結果,マテバシイは特定の随伴種を持たず,マテバシイ林は高木層優占種のみで他の林種から区分された.また,林床に特異的な要素が見られる林分から,特異性を持たない典型マテバシイ群落と言える下位群落の区分が可能であった.(3)地形,温度,土壌,海岸比距等の要因をとりあげ,それぞれについてマテバシイ林の分布規制要因に成り得るかを検討した結果,土壌の保水性が他の林種に比較して乏しい傾向がみられた.さらに,主成分分析により総合的な環境の中での各因子の有効性を解析した結果,若干の乾性立地の傾向がみられたが,分布を規制する要因は抽出できなかった.これらの結果から,マテバシイ林の成立には人為的な要因が大きく影響していることが予想された.2.マテバシイ林の一次生産特性(1)長崎県北松浦郡の34年生マテバシイ萌芽林の現存量と生長量の標本木伐倒により調査した結果,地上部現存量は214ton/haで,このうち葉は9.70ton/haであり,ほぼ同樹高のスダジイ林と比較して大きな値を示した.(2)生産構造をみると,葉層は非常に薄いが密度は高く,ほとんど幅1mの層に集中していた.このため相対照度は葉層に入るとすぐに激減し,林冠下部以上では,1.6～2.0%で推移した.(3)樹幹解析の結果,調査林分の材積は320m^3/haと推定された.また直径生長は10年でピークを示し,その後漸減する傾向にあった.また,1～2年目に著しい伸長生長がみられ,萌芽生長の旺盛さを示した.材積生長では,DBH<10cmの萌芽幹とDBH>10cmの萌芽幹との間に違いがみられ,前者は特に生長速度にピークを示さず,後者は23～25年目にピークを示した.さらに,標本とした各萌芽幹齢が一定でなく,必ずしも初期の萌芽が残存するわけではないことが予想された.(4)L.A.I.は4.8(ha/ha)とシイ林に比較して小さい値を示した.また,吸光係数は0.82と広葉草木群落並みに大きかった.ただし,相対照度と積算L.A.I.は必ずしも比例せず,林冠上部では0.21という小さい値を示した.これはマテバシイの着葉形態によるものと考えられた.(5)マテバシイ林では,生長にともない下層植生が貧化する傾向が認められた.この傾向は,高木層で更に競争が起こり,林床の光環境が改善されるまで続くものと予想された