林内の照度 (I)

Abstract

森林内での照度の変化と状態を林内の相対照度を中心に調べた。林内の相対照度の分布は晴天時にはL型分布し, その平均値は対数正規化して求める必要がある。林内の照度は林外の照度が高まるにつれて, その増加率は鈍る。これは, 林外の照度中の散光成分の占める割合の違いによるものであろう。晴天の場合, 一日のうちでは, 林内の相対照度の平均値は12時頃が最も低くなり, L型分布する。季節変化をミズナラ林でみると, 林内の相対照度の分布型は夏の生長期が最も強いL型となり開葉期, 落葉期へ向うにつれくずれ, 分散が大きくなる。平均値は開葉期より次第に減少して9月頃迄つづくが, これは9月迄葉量増加していることによると思われる。林内の相対照度の垂直分布を調べた結果, 林内の相対照度は葉面積に対して指数函数的に必ずしも減少せず, 葉面積の少ないところ (上層) で, 高目の値を示した。これは上層の葉の分布は集中分布しているため, 上層では他層にくらべて遮光率が低いことによると思われる。

The investigation was intended to study for the changes and conditions of light intensities in the forest. The values of relative light intensities under the canopy were almost distributed L type, so that the mean values of these should be obtained by the transformation into log-normal distribution. The rate of increase of the light intensities under the canopy decreases with increase of full light intensities. This is a result of the rate of diffused light in solar illumination. In sunny conditions, the mean values of the relative light intensities under the canopy was the lowest at noon in a day, and these distribution was L type. In summer, the distribution of the relative light intensities under the canopy was L type, and in spring and autumn the variances of the relative light intensities under the canopy became large. The mean values of the relative light intensities under the canopy gradually decreased to September. This result may be depended upon the increase of leaf biomass. The results of the vertical distribution of the relative light intensities in the forest are as follows: The relative light intensities in the forest not always decreased exponentially with leaf area indices. At the upper layer of plant communities, the values of the relative light intensities were higher than the values which were obtained by Beer-Lambert's law. This is a result of dispertion type of leaves. Leaves of upper layer may be clumped, so the rate of light interception at the upper layer will be smaller than the other layers.