Étude du rapport entre l'éclairement relatif sous forêt et l'éclairement lumineux incident en phénophase feuillée

Abstract

The relation between the relative light intensity in a forest stand and the absolute light intensity above the canopy during the growing period - In the research center of Virelles (mixed oak forest) the relation between the relative light intensity and the absolute incident light intensity was studied. Only for the light intensity values measured on the real noon, a mathematical relation was found and this for the growing periods 1965 and 1966. With the decrease of the light intensity in the open, the relative light intensity increases. For an identical change of the incident light intensity, the variation of the relative transmission becomes smaller, as the crown contact becomes more intense. The relative light intensity under the canopy, during an ideal day of the clear weather type, is a function of the sun height and at the same time of the absolute light intensity in the open. A measure for the mean relative light intensity of some day can be given by instantaneous measurements of the relative light intensity, only during the real noon period. The found relation may be explained by the fact that direct incident light is more reflected by the canopy than diffuse light.