和歌山演習林におけるモミ, ツガ林の生産力調査 : 第3報 リター量の季節変化および食葉性昆虫による被食量について

Abstract

和歌山演習林の天然生モミ・ツガ林の生産力調査の一環として行っている, リター量に関する調査について, 1968年6月から1973年12月までの資料をとりまとめたものである。調査は, 和歌山演習林内4林班の中腹 (標高約700m) に生立している, モミ・ツガ天然林で行われ, リター量の季節変化を求めるとともに, 食葉性昆虫類による被食量を推定した。リターは, モミ・ツガの落葉, モミ・ツガの落枝 (樹皮を含む), モミ・ツガの球果・種子, 虫糞, 寄生植物 (マツグミ), 広葉樹落葉, 昆虫の死体, その他に選り分けられた。モミ・ツガの落葉は10月, 11月に, 広葉樹の落葉は11月にとくに多く, 明らかな季節変化がみられた (Fig. 3, 8)。モミ・ツガの落枝には, 台風の接近のための強風冬期の降雪のような, 物理的な力が影響していると思われ, とくに季節変化はみられなかった (Fig. 5)。モミ・ツガの球果・種子には, 年変化がみられ, 結実の豊凶が明らかにあらわれた (Fig. 6)。虫糞量は夏期 (7 - 9月) に多く, 寄生植物も同様に夏期に多い傾向がみられた (Fig. 8)。総リター量は, 3. 6 - 7. 4ton/ha・yearで, 平均すると5ton/ha・yearとなち。総リターのうち, モミ・ツガの落葉・落枝が90％を占めていた (Fig. 3)。シャクガ類幼虫の摂食量と脱糞量の関係は, log F=0. 9875 log D+0. 0877 F : 摂食葉乾重量mg, D : 糞乾重量mg となった (Fig. 10)。リター中の虫糞量から食葉性昆虫類による被食量は, 114 - 188kg/ha・year, 平均139kg/ha・yearと推定された。

The seasonal variation of litter fall and primary consumption by herbivorous insects was studied in a mixed stand consisting of natural Japanese fir (Abies firma Sieb. et Zucc.) and Japanese hemlock (Tsuga sieboldii Carr.) in Kyoto University Forest in Wakayama Prefecture. During sixty-seven months from June, 1968 to December, 1973, twenty traps of a square meter (1m×1m) were set up one meter above the ground in investigated stand and litter fall was collected monthly. These traps were made of Tetoron-cloth of fine mesh and were made in the shape of a funnel. Additionally, ten traps of four square meter (2m×2m) for branch litter were set up on the ground in the same stand in August, 1970 (Fig. 2). The litter was divided into the following eight fractions by hand sorting: Leaves of fir and hemlock. Branches of fir and hemlock including bark. Cones including seeds. Insect faeces. Parasitic plant (Taxillus kaempferi). Broad leaves. Dead bodies of insects. Others. Each fraction was oven-dried and weighed in the laboratory. The primary consumption by herbivorous insects in canopies of fir and hemlock was estimated from insects faeces captured by traps. Seasonal variations of the amount of leaf litter was shown to be similar each year. Natural defoliation was observed from October to November in Japanese fir and hemlock, while the defoliation of broad leaves only occurred in November (Fig. 3 and 8). The branch fall of fir and hemlock seemed to be caused by the strong wind (e. g. the typhoon) and the snowfall in winter, therefore, the seasonal variation was not observed (Fig. 5). The cone and seed production of fir and hemlock was rich in one year (1969 and 1972) and poor in the other years (Fig. 6). The amount of faeces of herbivorous insects in summer (from July to September) occupied 49~55% of the total faeces egested for one year, while it was only 1.7~3.0% in winter (from December to February) (Fig. 7). The dry weight of annual litter fall was calculated 3.6~7.4 ton/ha with an average of 5 ton/ha. The leaves, branches and bark of fir and hemlock occupied about 90% of the total annual litter fall (Table 3). The correlation between food consumption and faeces of spanworm which fed on leaves of fir and hemlock, are shown through the straight line on the logarithms (Fig. 10), that is; log F = 0.9875 log D + 0.0877 F: dry weight of feeding leaf in mg D: dry weight of faeces in mg The annual loss of leaves by primary consumption of herbivorous insects in fir-hemlock stand was estimated at 114~188 kg/ha with an average of 139 kg/ha in dry weight.