Stand structure and light penetration. 3. Sunlit foliage area.
Abstract
The foliage areas exposed to direct sunlight, or shaded from it by other foliage, can be calculated for any particular foliage-area index given the inclination of the foliage (a) and of the sunlight (β), provided certain assumptions are made. The area of sunlit foliage increases with β when α> β, but is independent of β when α ≤ β; it tends to increase with a when β exceeds about 30°, but to decrease with rise in a when β is less than 30°. Decrease in foliage-area index increases the proportion of foliage that is sunlit, and reduces thé effects of foliage and sunlight inclinations. The sunlit leaf-area index of closed stands is less than 1 when the solar elevation is below about 30°, is between 1 and 2 for many stands when the sun is higher, and seldom exceeds 3 even at noon in the tropics. Defects in two previous mathematical treatments of sunlit foliage area [H.A. 29: 1715; 35: 1392] are discussed. -R.B.