Quantitative Evaluation of Leaf Inclination Angle Distribution on Leaf Area Index Retrieval of Coniferous Canopies

Guangjian Yan,Xihan Mu,Ronghai Hu,Guoqing Zhou,Fan Li,Jinghui Luo,Hailan Jiang,Jianbo Qi,Donghui Xie

doi:10.34133/2021/2708904

Abstract

Both leaf inclination angle distribution (LAD) and leaf area index (LAI) dominate optical remote sensing signals. The G-function, which is a function of LAD and remote sensing geometry, is often set to 0.5 in the LAI retrieval of coniferous canopies even though this assumption is only valid for spherical LAD. Large uncertainties are thus introduced. However, because numerous tiny leaves grow on conifers, it is nearly impossible to quantitatively evaluate such uncertainties in LAI retrieval. In this study, we proposed a method to characterize the possible change of G-function of coniferous canopies as well as its effect on LAI retrieval. Specifically, a Multi-Directional Imager (MDI) was developed to capture stereo images of the branches, and the needles were reconstructed. The accuracy of the inclination angles calculated from the reconstructed needles was high. Moreover, we analyzed whether a spherical distribution is a valid assumption for coniferous canopies by calculating the possible range of the G-function from the measured LADs of branches of Larch and Spruce and the true G-functions of other species from some existing inventory data and three-dimensional (3D) tree models. Results show that the constant G assumption introduces large errors in LAI retrieval, which could be as large as 53% in the zenithal viewing direction used by spaceborne LiDAR. As a result, accurate LAD estimation is recommended. In the absence of such data, our results show that a viewing zenith angle between 45 and 65 degrees is a good choice, at which the errors of LAI retrieval caused by the spherical assumption will be less than 10% for coniferous canopies.

Highlights

Leaf area index (LAI) is an important parameter of vegetation canopy structure and is critical in the quantitative calculation of global mass and energy exchange between the biosphere and the atmosphere [1, 2]
Despite the high variability of leaf inclination angle distribution (LAD) in reality, a spherical distribution assumption is often used for coniferous canopies due to the difficulties in measuring it
We estimated the possible range of the G-function of two species by rotating the reconstructed needles of the branches measured by our developed equipment—Multi-Directional Imager (MDI), which was an alternative to the manual measurement of the leaf inclination angle

Summary

Introduction

Leaf area index (LAI) is an important parameter of vegetation canopy structure and is critical in the quantitative calculation of global mass and energy exchange between the biosphere and the atmosphere [1, 2]. Beer’s law [3] is widely used to retrieve LAI from gap fraction. An important factor that highly affects the retrieval accuracy of LAI based on Beer’s law is the leaf projection coefficient (G-function). The Gfunction is the effective projection proportion of leaves along the observing directions [4] and is dependent on the leaf inclination angle distribution (LAD) (assuming a uniform azimuth angle). Some groundbased measurements utilize the gap fraction data close to a view zenith angle of 57.5°, where the values of the G-function with various LADs are close to 0.5 [14,15,16,17]. Assuming G to be 0.5 may introduce large errors to the retrieval of LAI in other view directions

Objectives

Results

Conclusion