森林立地学会誌 森林立地44(2), 2002, 31～36
Jpn. J. For.
environment44(2), 31-36 2002
土壌孔隙組成を用いた森林の保水容量の推定とその要因解析
諫本信義＊
＊大分県林業試験場
森林の水源涵養機能を土壌孔隙組成による保水容量特性より推定した。大分県下246林分を対象に計905個の400cc円筒試料を採取し，粗孔隙保水容量，（pF 0～2.7相当），中・小孔隙保水容量（pF 0.6～2.7 相当）及び大孔隙保水容量（pF 0～0.6相当）を深さ1mまでの土層深と孔隙率の積により算出した。粗孔隙保水容量は雨水の貯留と排水のポテンシャルを，中・小孔隙保水容量は水資源として有効な雨水の貯留を，そして大孔隙保水容量は土中への雨水の誘導と土壌中での排水のポテンシャルをそれぞれ指標する機能と位置づけた。
粗孔隙保水容量，中・小孔隙保水容量及び大孔隙保水容量の全林分平均（土標準偏差）は，それぞれ339.6(±75.9)mm, 243.6(±56.9)mm, 96.9(±40.5)mmであり，全体的に広葉樹林で保水容量が大きく，原野では小さかった。各保水容量を目的変数として，林種，林齢，土壌母材などの５外部環境要因及び容積重，土性などの３土壌特性要因を用いて数量化Ⅰ類による要因解析を行った。この結果，土壌特性要因の効果が大きく，保水容量に対し主体的に影響を与えていることがうかがわれた。一方，外部環境要因では，副次的であったが，広葉樹林が針葉樹林に比べ保水容量が多い傾向にあること，加齢により保水容量が増加することなどの結果が得られた。これは，森林に対する人偽的な営為，すなわち土壌孔隙に配慮した森林の取り扱いによって，水源涵養機能の増進は可能であることを示唆している。
Nobuyoshi Isamoto: Estimation of the water holding capacity of forest stands using soil pore distribution and an analysis of the related factors
The present report describes an estimation of the potential of the water holding capacity (WHC) on the basis of soil pore distribution, and an analysis of related factors. Each sample was collected from 905 spots of 246 forests and plains in Oita Prefecture using a soil sampling cylinder (400cc).
WHC was calculated by the product of the soil depth (up to 1m) and soil porosity as to soil classes which were divided into 3 classes according to pore size. The water holding capacity of these three classes WHC was correlated to the pF value, and was expressed as follows. WHC of coarse size pores (pF0～2.7; for short: Sc) indicates a reserve of rain water and potential of deliverability. WHC of small and medium size pores (pF0.6～2.7; for short: Sms) indicates available a reserve of rain water as a water resource. WHC of large size pores (pF0～0.6; for short: Sl) indicates conduction of rain water into soils and the potential of water delivery in the soils.
The average (±standard
deviation) WHC (Sc, Sms, and Sl) within the whole forest stands and plains was
339.6 (±75.9) mm, 243.6 (±56.9) mm, and 96.9 (±40.5) mm respectively. Every WHC was greater
in broad-leaved forests and poor in plains. As for each item of the water
holding capacity, factor analysis was executed as an objective variable using
Hayasi’s quantification method (I), for five environmental factors and three
soil characteristic factors. The result was that soil characteristic factors
have a dominant influence upon the WHC. Though environmental factors effect the
WHC unsignificantly, the category score was greater in the soil of broad-leaved
forests than in the soil of conifer forests and plains. Therefor, it was
elucidated that the water holding capacity was promotive, and its capacity was
increased by aging. This suggests that an increase in water holding capacity is
possible through human efforts, namely careful treatment to increase soil
porosity.