春日山原始林の特定植物群落(コジイ林)における17年間の動態

1.春日山原始林の保全に寄与する生態学的基礎資料を得ることを目的とし,環境庁が1986年に設置した特定植物群落永久コドラート(20m×20m)で2003年に現地調査を実施し,既存資料とあわせて17年間の群落動態を検討した.2.胸高直径10cm以上の木本についての毎木調査から,コジイの相対優占度は1986年,1997年および2003年に,それぞれ91.7%,93.9%,91.4%を示し,大きな変化はみられなかった.しかしサイズ構造においてはコジイの小径木が少なく,逆にクロバイが増加することによる構造変化がみられた.3.植物社会学的調査により,全体の出現種は2種減少にとどまったが,低木層では種の減少(6種減少)および草本層の種の交代が著しかった.4.1986年から2003年までの17年間に台風によるギャップ形成とシカの採食による林床植生の貧化および種の交代が確認され,これら偶発的要因が森林動態に大きく関与していることが示唆された.The structure of Castanopsis cuspidata forest in the Kasugayama Forest Reserve (KFR, a World Heritage Site and a natural special monument) was studied in 1986, 1997 and 2003. This plant community has been listed as a specific plant community since 1956, because "it is a primeval forest or natural forest that is extremely close to the conditions of a primeval forest". A permanent plot (400 m^2) was established by the Environment Agency to survey the natural environment in 1986. The plot included four tree species (Castanopsis cuspidata, Quercus sessilifolia, Symplocos prunifolia, Neolitsea aciculata) with diameters at breast height (DBH)〓10cm in 1986, and the number had not changed by 2003. C. cuspidata was the dominant species comprising 91.7%, 93.9% and 91.4% of the total basal area in 1986, 1997 and 2003. However, the number of C. cuspidata individuals had decreased remarkably in the small size class (DBH < 20cm) by 2003. Conversely, Symplocos prunifolia increased in the small size class. The ratio of closed canopy changed from 90.3% to 79.1% in 1986 and 2003 as some canopy trees were felled by a typhoon. Additionally, trees with stripped bark and browsed by Sika deer were observed in the forest. These results suggest that the forest structure in the KFR is determined by both physical conditions, such as climate and geography, and by chance factors, such as typhoons and deer.論