Variation of Clumping Index With Zenith Angle for Forest Canopies

Canopy clumping index (CI) characterizes the extent of the nonrandom spatial distribution of foliage elements within a canopy and is critical for determining the radiative transfer, photosynthesis, and transpiration processes in the canopy. It is widely perceived that the CI increases with zenith angle ($\theta$ ), because between-crown gaps decrease in size and number with increasing $\theta $ . In this study, we demonstrate that this is not always true. Analytical equations between CI and $\theta $ are first developed based on widely used forest canopy gap fraction (GF) theories. The results show that the zenith angular variation of CI is closely related to crown-projected area or crown shapes [i.e., the ratio of the crown height to its diameter (RHD)]: CI increases with $\theta $ for canopies with “tower” crowns (RHD > 1), but decreases with $\theta $ for “umbrella” crowns (RHD < 1) and does not vary much with $\theta $ for “sphere” crowns (RHD = 1). These results are validated in a large-scale remote sensing data and image simulation framework (LESS) platform and published datasets, including the measurements in field and radiative transfer model intercomparison (RAMI) forest stands. The findings are essential for the derivation of angular-integrated (hemispherical) CI from in situ measurements and multiangular remote sensing.