タブを主とする天然生照葉樹林のリターフォールによる養分の還元について

Abstract

タブを主とする天然生の照葉樹林のリターフォール量を3年間にわたって調べた。またこのリターフォールに含まれて, それぞれの林床へ還元されたチッ素, リン, カリウム, カルシウム, マグネシウムの5養分元素の量を測定した。結果は次のようである。1) 各区のリターフォール量は5. 78 - 7. 79ton/ha・yrで, うち葉リターは4. 39 - 5. 65ton/ha・yrであった。2) このリターに含まれてそれぞれの林床へ還元された養分元素の量はチッ素は64. 2 - 83. 6kg/ha・yr, リンは4. 9 - 8. 9kg/ha・yr, カリウムは26. 6 - 49. 8kg/ha・yr, カルシウムは58. 3 - 92. 5kg/ha・yr, マグネシウムは8. 3 - 11. 3kg/ha・yrであった。この量は隣接した壮齢のヒノキ人工林よりもかなり多く, またわが国の針葉樹林, 落葉広葉樹林, 常緑広葉樹林等で調べられた結果と比べても多い方に属していた。3) リターフォール量は5 - 6月と11 - 12月に比較的多かった。したがって養分の還元もこの時期に多かったことになる。しかしわが国の他のタイプの森林に比べれば落葉期の集中性は少なく, ほぽ年間を通じてリターフォールがあった。4) 葉リター中の養分元素の含有率は季節的に変化した。チッ素, カリウムの含有率は夏に低く, 冬に高かった。カルシウム, マグネシウムの含有率は初夏に低く, 初冬に高かった。5) 還元された養分元素量をリターの組成別にみると, どの養分元素も葉による還元がもっとも多く, 次いで枝, その他の植物器官 (芽鱗, 花, 等), タネ (種子, 球果, 果実), 虫のふん, 虫の遺体, 樹皮の順で少なくなっていた。

The amount of dry matter and nutrient elements such as N, P, K, Ca and Mg of litterfall in the natural evergreen broad leaved forest dominated by Machilus thunbergii Sieb. et Zucc. was measured for three consecutive years at Tokuyama Experimental Forest Station of Kyoto University, located in Tokuyama city, Yamaguchi pref. Climatic conditions in this site were 15.2℃ for mean annual temperature and 2, 076 mm for annual precipitation. Two plots were set up in the studied forest on Feb. 11, 1975. One of them was settled at the upper part of the slope (plot T-1) and the other was at the lower part of the slope (plot T-2). Machilus thunbergii was the dominant tree in the both plots, however, there was a difference in floristic composition except of M. thunbergii among two plots. The mean hight of tree in T-2 was slightly higher than in T-1. On the other hand, the density in T-1 was comparatively higher and the total basal area was larger than those in T-2. The measurement of litterfall with five traps (50×50cm) in each plot was carried out for every month. Litterfall was separated into leaves, branches and twigs, barks, cone and seeds or fruits, other plant organs (scales, inflorescences, etc.), insect residues and insect feces, and dry weight for each component was measured. Subsequently, the concentrations of N, P, K, Ca and Mg were analyzed for each component. The results obtained in this study were as follows. 1) In three years measurement the amount of litterfall in the two plots was ranged from 5.78 to 7.79 ton･ha-1･yr-1 for total litters and from 4.39 to 5.65 ton･ha-1･yr-1 for leaf litter (Table 1 and 2). 2) Litterfall varied seasonaly and the monthly amount was high in the late spring (May and June) and the early winter (November and December) (Table 1). While the variation was not remarkable as compared to seasonal variation of litterfall in Chamaecyparis obtusa forest and deciduous broad leaved forest. 3) The annual amounts of nutrient elements contained in total litters were 54.2 ~ 83.6kg･ha-1･yr1 for N, 4.9 ~ 8.9kg･ha-1･yr-1 for P, 26.6 ~ 49.8kg･ha-1･yr-1 for K, 58.3 ~ 92.5kg･ha-1･yr-1 for Ca and 8.3 ~ 11.3kg･ha-1･yr1 for Mg, respectively (Table 4). They were very high as compared to the results obtained in the adjoined stands of Chamaecyparis obtusa. It attained about 2 times for N, 1.7 times for P, 3.5 times for K, 2.6 times for Ca and 2 times for Mg. 4) The concentration of nutrient element in leaf litter varied with season. The concentrations of N and K were low in summer and high in winter. Those of Ca and Mg were low in the late spring and increased gradually, then reached to the maximum in December or January (Fig. 1, 2, 3, 4 and 5). 5) The proportion of leaf litter was highest in the quantity of nutrient element contained in each composition of litterfall, and it became low in the following order: branches and twigs, other plant organs (scales, inflorescences, etc.), cone and seeds or fruits, insect feces, insect residues and barks (Table 4).