Stand structure and light penetration. 3. Sunlit foliage area.
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.