Foliage randomness and light interception in 3-D digitized trees: an analysis from multiscale discretization of the canopy

Light models for vegetation canopies based on the turbid medium analogy are usually limited by the basic assumption of random foliage dispersion in the canopy space. The objective of this paper was to assess the effect of three possible sources of non-randomness in tree canopies on light interception properties. For this purpose, four threedimensional (3-D) digitized trees and four theoretical canopies – one random and three built from fractal rules – were used to compute canopy structure parameters and light interception, namely the sky-vault averaged STAR (Silhouette to Total Area Ratio). STAR values were computed from (1) images of the 3-D plants, and (2) from a 3-D turbid medium model using space discretization at different scales. For all trees, departure from randomness was mainly due to the spatial variations in leaf area density within the canopy volume. Indeed STAR estimations, based on turbid medium assumption, using the finest space discretization were very close to STAR values computed from the plant images. At this finest scale, foliage dispersion was slightly clumped, except one theoretical fractal canopy, which showed a marked regular dispersion. Taking into account a non-infinitely small leaf size, whose effect is theoretically to shorten self-shading, had a minor effect on STAR computations. STAR values computed from the 3-D turbid medium were very sensitive to plant lacunarity, a parameter introduced in the context of fractal studies to characterize the distribution of gaps in porous media at different scales. This study shows that 3-D turbid medium models based on space discretization are able to give correct estimation of light interception by 3-D isolated trees, provided that the 3-D grid is properly defined, that is, discretization maximizes plant lacunarity. Key-words : 3-D digitizing; clumping; fractals; leaf dispersion; light interception; tree canopy; virtual plants. INTRODUCTION Simulation models of light interception by vegetation canopies have been developed for many years for purposes ranging from plant production and ecophysiology to remote sensing (Ross 1981; Myneni, Ross & Asrar 1989; Varlet-Grancher, Bonhomme & Sinoquet 1993). Most light models are based on the turbid medium analogy, namely Beer’s law which takes into account the amount of leaf area, and the leaf angle distribution with regard to the direction of incident radiation. The most common application of Beer’s law is the computation of gap fraction P 0 of a horizontally homogeneous canopy in a given direction W :