Analyzing the effect of structural variability and canopy gaps on forest BRDF using a geometric-optical model

Abstract

A geometric-optical reflectance model is presented which estimates the bidirectional reflectance distribution function (BRDF) of forest canopies by modeling four shadowing pattern components (i.e., illuminated crown, illuminated ground, shadowed crown, and shadowed ground. The model represents the forest canopy as a group of discrete tree crowns and uses a deterministic ray tracing procedure to generate a two-dinwnsional scene of shadow pattern components. It provides the type of shadow fraction information used to calculate the reflectance of forests when it is treated as an area-weighted sum of the shadow component reflectances. The model is designed to test the effect of canopy structure parameters on the bidirectional reflectance of forests and can easily be adapted to accommodate different crown shapes, crown sizes, stem densities, and distributions. The model was used to isolate and quantify the effects of crown shape, canopy cover, stein distribution pattern, and canopy gaps on the shadowing pattern and the canopy reflectance, using a set of statistically generated forest scenes and data gathered for a natural tropical lowland forest scene. The illuminated crown component and the illuminated ground component have the greatest impact on the reflectance values in the red and near-infrared region respectively. The red BRDF, mainly influenced by the illuminated ground component, proves to be sensitive to canopy cover, tree pattern distribution, and canopy gaps. The near-infrared BRDF, affected by the illuminated crown component, is sensitive to canopy cover, crown-center height distribution, and crown shape. The sensitivity of the BRDF to canopy gaps and crown clumping was investigated in more detail. Reflectance-measurements close to the hotspot are found to be insensitive to clumping while retaining sensitivity to crown cover. Forward scattering measurements are fcntnd to be more sensitive to gap size and frequency. The spatial effects of plantations, logging, or road features on the BRDF is further analyzed.