九州大学宮崎演習林における長期森林動態モニタリングプロット8年間の推移

九州大学農学部附属演習林宮崎演習林では，3ヵ所（合戦原，丸十，広野）に森林動態モニタリングプロット１ha（100ｍ×100 ｍ）が設定され，100年を超える長期的な計画のもと，胸高周囲長が15cm 以上の樹木を対象に継続して毎木調査が行われている。継続調査の初期8年間の森林動態を調べた結果，3プロットともに，立木密度は低下し，胸高断面積合計と総現存量は増加した。総現存量の年平均増加量は合戦原，丸十，広野のそれぞれで3.3, 3.9, 7.1 Mg/ha/yr で，3つのプロットの平均は4.8 Mg/ha/yr であった。立木密度の低下は胸高直径20cm 以下の小径木で多くみられた。また，胸高周囲長15cm 以上への新規加入個体はいずれのプロットでも枯損木本数を下回っていた。調査期間中，3つのプロットとも種構成や多様度指数に大きな変化はなかった。小径木の減少が継続すれば，現存量や生産量の減少が生じる可能性がある。Three 1 ha monitoring plots, which had named as Kasebaru, Maruju and Hirono, were established at different locations in Shiiba Research Forest. At these plots, the forest dynamics have been monitored under the plan lasting more than 100 years. During 8 years from the starting monitoring, stem densities decreased, and total basal area and total biomass stock increased at the all three plots. Average annual increments of total biomass were 3.3, 3.9 and 7.1 Mg/ha/yr, with the mean of 4.8 Mg/ha/yr, at Kasebaru, Maruju and Hirono, respectively. The decline in stem density was mainly due to mortality of the individuals which have less than 20 cm in stem diameter. The number of recruitment individuals with >15cm in girth at breast height was less than that of dead individuals with >15cm in girth at breast height at the all plots. During the 8 years, species composition and diversity index were not changed at the all plots. The long-lasting decline of small trees may result in the decline of forest biomass and productivity.