Studies on the Stand Structure and Light Climate (II) : Methods of Investigating the Sunfleck on the Forest Floor (1)

Abstract

林内光の直射成分, すなわち陽斑点 (Sunfleck, SF) について調べた。林内の相対照度が4. 7％の日野ヒノキ林 (滋賀県蒲生郡日野町綿向山) で陽斑点を調べた結果, 全測定数中陽斑点は23％であるが, 量的にみると総受光量の63％が陽斑点により占められていることがわかった。陽斑点の空間分布はほとんど機会分布をしており, 太陽高度が低くなるにつれ機会分布から一様分布に変化する傾向がみられた。陽斑点出現率 (SFn rate) は1日のうちで太陽高度が最も高い時必ずしも高くならず, その前後にピークを示していた。全天空写真により角高度について林冠空隙率 (G, ％) を調べた結果, 調査林分ではGのピークは40 - 70度の間にあり季節変化しSFn rateのピークを示す角高度と一致した。陽斑点の大きさ (l) とその中心の相対照度 (r_u), 陽斑点の中心からの距離 (d) とその点の明るさ (r) の関係を求めることにより陽斑点の出現率と量の推定が可能となった。陽斑点の必要測定点数 (N) は陽斑点の時期的移動, 測定所要時間を考慮するとこの調査林分では25点 (100m_2) 以上必要である。

This paper deals with the direct light in a stand, defined as sunfleck. Light climate in plant communities has been measured under diffuse light conditions. In surveying the relationship between production of plant communities and incident light, a direct light factor should be added to the relationship. Of special importance is its effect on the growth of understorey plants. The results of this investigation were as follows: 1) In a Chamaecyparis obtusa stand, with a mean relative light intensity of 4.7 per cent, the proportion of the area occupied by sunflecks (SFn rate) was 23 per cent. However sunflecks were attributed with as much as 63 per cent of the total light intensity on the forest floor. 2) The distribution patterns of sunflecks on the floor were mostly random, but had a tendency to change from random to uniform with declining angular height of the sun. 3) The proportion of the area occupied by sunflecks was not the largest when solar angular height was at its diurnal maximum. The propotion of gaps in the canopy at an angular height (G, %), investigated by the hemispherical photographic method showed the maximum value (Gmax) between 40 and 70 degrees by seasonal crown conditions in this stand. The angular height of Gmax varied with species, tree density and so on. In a sapling stand of Alnus hirsuta var. sibirica the angular height of Gmax showed a value close to the zenieth. 4) Total light intensity and the proportion of total sunfleck light to the total light intensity over the forest per unit area (SF rate) could be calculated by the estmation of the relationships between diameter (l) and light intensity at the center (r0) of the sunfleck, distance from the center of the sunfleck (d) and relative light intensity at any point (r). The relationship between r0 or r and l or d gave a close approximation as follows: [Figure omitted] 5) The necessary number of measured sunflecks (N) was 25 (100m2 in area) in the stand, considering the sunflecks varied with time, and the time necessary for measuring sunflecks.