I. Introduction
Forest canopy structure can be defined as the shape, size, orientation, and positional distributions of all the foliage elements aboveground [1]. It dictates the solar radiation regime within and below canopies through the interaction between sunlight and leaves of the plants. The spatial location and orientation of leaves have considerable effects on the fraction of photosynthetically active radiation [2] and the amount of light intercepted by leaves in a canopy [3]. Among all of the factors affecting the radiation interception, leaf area index (LAI) is one of the most important biophysical parameters. Considerable progress has been achieved during the past several decades with respect to the retrieval of true LAI from field-based optical instrument measurements [4]–[8]. These efforts have previously been reviewed by Weiss et al. [9], Jonckheere et al. [10], and Zheng and Moskal [11]. However, most of these methods were developed from 2-D perspective without considering the foliage height distribution. Moreover, most of the LAI inversion methods only address the angular distribution of foliage elements for any given individual tree or forest stand while assuming the azimuthal or spatial distribution of foliage elements are random [12]–[14].