Comparison of leaf angle distribution functions: Effects on extinction coefficient and fraction of sunlit foliage

Abstract

Leaf angle distribution is a key parameter to characterize canopy structure and plays a crucial role in controlling energy and mass balance in soil-vegetation-atmosphere-transfer system. Several leaf angle distribution functions found in literature have been proposed to account for the non-random distribution of leaf inclination angle with one or two parameters. In this paper, these leaf angle distribution functions (Beta distribution function, ellipsoidal function, rotated-ellipsoidal function, Verhoef's algorithm and de Wit's functions) were compared with field data collected in the First ISLSCP Field Experiment (FIFE) project and two sites within Ku-ring-gai Chase National Park, Sydney, Australia. All functions performed reasonably well. However, the comparison showed that the two-parameter functions including the Beta distribution function and Verhoef's algorithm commonly were more consistent predictors than one-parameter functions. G-statistics and χ 2 test applying to the estimates of leaf angle distribution demonstrated that Beta function presented more robustness over other functions, even the ellipsoidal leaf distribution function which has been widely used. Furthermore, the predictions of leaf angle distribution by these functions were used to calculate extinction coefficient and to separate foliage into sunlit and shaded parts. The results suggested that, ellipsoidal function may be suitable to be retrieved with remotely-sensed data and to compute extinction coefficient and fraction of sunlit foliage because this function requires only a single parameter, namely the ratio of the horizontal semi-axis length to the vertical semi-axis length of an ellipsoid. Finally, the comparison of three approaches (Nilson's, Fuchs’ and Ross–Goudriaan's algorithms) for computing extinction coefficient indicated that, there was no significant difference between the three approaches.