クスサンが発生したモミジバフウ林のリターフォールについて

Abstract

本報告には, 京都市北区にある京都大学農学部附属演習林上賀茂試験地で育てられているモミジバフウ (Liquidambar styraciflua Linn.) 林において, 1979年から1990年までの12年間のリターフオールを調査し, その季節変化および年変動を求め, 虫糞量からクスサン (Caligula japonica Moore) などの食葉性昆虫類による被食量 (加害葉量) を推定した結果がまとめられている。モミジバフウの平均年落葉量は4. 71ton/haで, その92 - 95%が10月下旬から11月上旬に集中している。クスサンに全葉が食害されたモミジバフウは約1カ月後には新しい葉を展開するが, この新しい再展開葉量は約2. 0ton/ha・yであった。落枝量は0. 26 - 1. 63ton/ha・yで年々増加の傾向にある。芽鱗は32. 0 - 43. 4kg/ha・yで, 6月上旬まで落下はつづくが, 5月中旬までにその大部分は落下した。花・果などの生殖器官の落下は1985年から見られ, モミジバフウの成長にともなって次第に多くなっている。虫糞量は23. 0 - 2. 366. 3kg/ha・yで, クスサンの糞が, 年変動の大きな原因であった。クスサンの大発生年であった1979年には, その虫糞が2, 344. 2kg/ha・yも回収され, その量は年総リターフォールの42. 6%で, 落葉量に次いで多かった。クスサン以外の食葉性昆虫類の虫糞は12. 6 - 46. 2kg/ha・yで大きな年変動は見られなかった。昆虫類は1. 7 - 47. 4kg/ha・yで, クスサンの幼虫の死体, 蛹, 繭などが多く見られた。総リターフォール量は5. 43ton/ha・y (4. 14 - 6. 52ton/ha・y) であった。虫糞量からモミジバフウ林における食葉性昆虫類の摂食量は40 - 3, 371kg/ha, y, その加害葉量は54 - 4, 546kg/ha・yと推定された。クスサンによる全葉食害域に設置されたトラップに捕捉された虫糞量からモミジバフウ林の被害葉量を推定すると, 5. 8 - 6. 2ton/ha・yとなり, 着葉量に相当する値が得られた。

The annual and seasonal fluctuations of litter fall and grazing loss of leaves by herbivorous insect specially the giant silk moth, Caligula japonica MOORE, were investigated in a sweet gum, Liquidambar styraciflua LINN., stand at Kamigamo Experiment Station of Kyoto University Forest, Kyoto City. The study site is an artificial experimental stand of sweet gum which was planted in 1968. Fifteen traps, each having surface of one square meter ( 1 m × 1 m), were set up eighty centimeter above the ground of the stand. Fallen litters in the traps were collected one or several days interval from late April or May to July and ten or fifteen days inteval from August to December in 1979~1990. Collected litters were divided into the seven components by hand sorting, i. e. sweet gum leaves, branches, bud scales, sexual organs, feces of herbivorous insects (giant silk moth and others) , dead bodies of insects and the others. In winter, fallen branches on the ground were collected at the setting site of each trap. Each component was oven-dried and weighed in the laboratory. The grazing loss of leaves by herbivorous insects was estimated by the regular steps from the feces captured by the traps. Annual leaf falls were rather stable with range from 4.36ton to 5.00ton/ha in 1982~1990. Annual leaf fall at ordinary level was 4.71±0.27ton/ha and leaf falls from late October to early November were accounted for 92~95% to its total annual fall. In case all leaves were eaten by the giant silk moth, new laeves developed soon a month after defoliation, its amounts were estimated at about 2.0ton/ha. Bud scales litter were at 32.0~43.4kg/ha･y from bud break (late April) to early June. Annual branch-falls ranged from 0.26 to 1.63ton/ha･y. Sexual organs were at 4.0~73.7kg/ha･y after 1985 and tend to increase with the growth of sweet gum. The feces collected varied much from 23.0 to 2, 366.3kg/ha･y and its amount was mainly composed by the feces of giant silk moth. The feces of herbivorous insect except the giant silk moth were weighed at 12.6~46.2kg/ha･y in 1979~1990. Those values are seem to be ordinary level when compared with the values obtained in various healthy forest stand. Remarkable amounts of dead larvae and pupae of giant silk moth among the insect bodies were collected. The annual falls of insect body residues were weighed at 1.7~47.2kg/ha･y. Total litter falls were summed up 4.14~6.52ton/ha･y with average of 5.43ton/ha･y. The amount of annual feeding leaves by herbivorous insect was estimated at 40~3, 371kg/ha･y and that of damaged leaves was estimated at 54~4, 546kg/ha･y in a sweet gum stand. Amount of damaged leaves were estimated at 5.8~6.2ton/ha from feces which were fallen in traps at all-leaves defoliated area. These values were equal to leaf biomass of this sweet gum stand investigated in 1968 and 1991.