Preliminary Report of a Method for Estimating Sunlit Leaf Area within a Corn Canopy

Abstract

This paper describes a method for measuring a horizontal sunlit area within plant communities as a preliminary to further studies of the photosynthesis in plant communities. This method was applied to a corn canopy, giving successful results on the sunlit leaf area.1) For the determination of the horizontal sunlit area within plant canopies, a measuring bar on which points are marked at a interval of a cm is moved at the constant interval of a cm, forming an apparent squere grid at a given height z (see Fig. 1). In this case the horizontal sunlit area (Sd) is given bySd(z)=na2, where n is the number of sunlit points at a height z. In this experiment, 6cm was chosen as an adequate value of a (see Table 1).2) As can be seen in Figs. 2 and 3, the height distribution of the horizontal sunlit area (Sd) and that of radiation intensity (It) were found to be approximated bySd(z)=SdHexp{-Ksh(1-z/H)}andIt(z)=ItHexp{-Kth(1-z/H)}, where z and H are the height above the ground and the height of the plant canopy, SdH and ItH are the values of the respective quantities at the height of H, and Ksh and Kth the extinction coefficients of the sunlit area and of the radiation intensity, respectively. The values of Ksh were 1.35, 1.83 and 2.18, and those of Kth were 0.70, 1.25 and 1.40 at the sun altitudes of 61°, 50° and 33°, respectively. Sd and It were also approximated by an exponential function of LAI (see Fig. 4 and eqs. (6) and (7)).3) As a first approximation, following relation was used to evaluate the sunlit LAI (Fd).Fd=∫H0Sd(z)/SdH⋅f(z)dz, where f(z) is the leaf area density at the height of z. The results obtained are presented in Fig. 5 and Table 2. The magnitudes of the sunlit leaf area were 54, 44 and 40 per cent of the total leaf area at the sun altitudes of 61°, 50° and 33°, respectively.