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 (3 is less than 30°. Decrease in foliage-area index increases the proportion of foliage that is sunlit, and reduces the effects of foliage and sunlight inclinations. The sunlit-leaf-area index of closed stands is <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 [F.C.A. 12: 1940, 18: 2126] are discussed.-R.B.