2つのタイプの天然生落葉広葉樹林の細根量とその垂直分布について

Abstract

京都大学芦生演習林の天然生落葉広葉樹林で, 斜面上部のB_B型土壌と斜面下部のB_D型土壌の2つのプロットの細根の量と垂直分布をしらベた。B_BのプロットではB_Dのそれに比し, 乾性種が多く小径で立木本数が多い林分である。両プロットで立木間に設けた1m_2の方形枠について, 表層から10cmごとに掘りとって測定した結果, B_Dのプロットの根量は10. 25t/ha, B_Bプロットで15. 65t/haでB_Bプロットの方が根量が多かった。これらの根を径2mm以下, 2 - 5mm, 5 - 20mmに区分すると, B_Dプロットで径5 - 20mmが比較的に多く, B_Bでは逆に径2mm以下が多い傾向があった。根は地表に多く集中し, 下層に向って減少するが, B_Bプロットの方が変化が急であって, 70cm以深にはみられなかった。B_Dプロットでは径5 - 20mmの根が深さにともなってほとんど変化せず, このことが全量の減少速度を小さくしていた。径5mm以下の細根の地表からの減少速度は2つのプロットで明らかな差はなかった。細根は両プロットとも地表に集中し, 20cm深までに全細根量の65-77％を含む。その量はB_Bプロットでとくに多く, F・H層, A層が物質循環の主な場であることを示唆した。

1. The studies on the biomass and the vertical distribution of fine root was carried out in natural deciduous broadleaved forest at Ashiu, Kyoto University Forest, Kyoto prefecture (N35°18′, E135°43′). 2. Two stands developed on different soils along a slope were selected for study plots. B plot is located on the ridge of the slope. There was a rather thick development of F ･ H-layer on the floor and the soil can be classified into BB type. D plot is located on the foot of the slope. A0-layer was thin and A-layer was rather deep, and the soil can be classified into BD type. Stand structure and floristic composition differed between two plots, i.e., tree size was small and density was high for B plot, while, tree size was big, density was low and above ground biomass for D plot was about 3 times larger than B plot. 3. One of 1m2 quadrate was set up between tree individual for each plot, and dig up soil with root for each 10cm interval. 4. The root biomass for B plot was 15.65t/ha ･ 70cm, including the root in F ･ H-layer, while that for D plot was 10.25t/ha ･ 90cm. This result suggest that the stand developed on BB soil likely to develop much more fine root than that on BD soil, particularly in the surface of soil, though the above ground biomass was much smaller than that on BD soil. This might be caused by the difference of soil or site condition between BB and BD. However, there was a remarkable differences in stand structure and floristic composition between two plots. Therefore, it is needed to examine the effect of factors other than edaphic one on the fine root development. 5. There was a difference between two plots in the frequency distribution of root size class. The proportion of finest root (less than 2mm in diameter) was higher for B plot than D plot. This means that D plot showed higher proportion for 5-20mm size class. However, as far as the size distribution in top soil (0-20cm in depth), there was no definite difference between two plots. The size class less than 2mm covered 40-50% of total for both plot. 6. The fine root concentrated into top soil, both in B and D plot. As for fine root less than 5mm in diameter, the proportion of root contained in top soil (0-20cm in depth) to total was 65% for D plot and 77% for B plot, respectively. The root biomass decreased with depth rather sharply in B plot than in D plot, and there was no root in the layer deeper than 70cm in depth for B plot. While, the rate of decrease rather gentle and some of root was still found in the layer deeper than 60cm, for D plot. Though, the fine root concentrated into top soil for both soil, the degree of concentration was high for B plot. Moreover, the density of the fine root in top soil was about 2 times as high for B plot than for D plot. It is predictable that the top soil layer, including F ･ H and A layer, though it is thin, would be the major field for transfering materials between soil and trees, particularly for B plot.