芦生天然林におけるスギ当年生稚樹の個体群動態

Abstract

当年生スギ稚樹の個体群動態に関する研究を天然林において行った。林内と林外にL層除去区と無処理区を設定し, スギ種子を人工播種し, 発生した稚樹の発育段階と死亡要因を個体追跡法を用いて調査した。稚樹は全試験区でほぼ同時期に発生を開始した。発生率は, 全試験区で低かったが, 林内より林外のほうが, またL層を除去したほうが高かった。林内においては発生開始直後の死亡率が非常に高く, ほとんどの個体が子葉段階で死亡した。林外においても発生開始直後の死亡率が高く, その後8月の乾燥期にも死亡率が高くなった。死亡要因として, 食害, 乾燥害, 病害・菌害. 消失がみられた。消失の原因として食害と降水による流失が考えられた。消失を含めた食害は, 全試験区で長期にわたって発生し, 林内区の死亡率を著しく高めていた。また食害は, 子葉段階に特異的に発生していると思われ, 地床条件による差は小さかった。乾燥害は林外区に多く, 長期間降雨がない時期に低い発育段階から発生した。L層除去によって, 乾燥害の発生は減少した。

The population dynamics of current seedlings of Cryptomeria japonica D. Don was investigated in a natural forest. The plots were made on different floor conditions. The growing stages and mortality factors of the seedlings planted artificially on the floor were studied by marking method. The time of the emergence of seedlings was almost same among all floor conditions. The emergence rates were low in all plots, and the one under canopy was lower than open area and the rate on the floor not removed L layer was lower than that on the removed floor. Under canopy just after emergence of seedlings, the mortality was very high and almost seedlings were dead at the initial growing stage in the forest. In open area, the mortality was higher just after emergence and in August, or dry season. Mortalities of seedlings were caused by animal predation, drought, disease and disappearence. Most of disappearence of seedlings might be caused by animal predation and/or washing out by rain water. Animal predation containing disapperaence occured in all plots for a long periods. It made the mortality of seedlings under canopy very high. Mortality from animal predation was specific to the initial growing stage of seedlings in all plots. The mortality from drought was higher in open area than under canopy, and occured at the first in initial growing stage at the period which had no rain for many days, and then gradually in more advanced stages. Mortality from drought decreased by the remval of L layer.