Calculation of the fraction of sunlit and shaded leaf area in heterogeneous canopies with discontinuous plant crowns

Abstract

The fraction of sunlit leaf area, $f_{sun}$, is a critical parameter for simple scaling of plant biophysical models from leaf to canopy, and is a useful bulk parameter describing interactions between canopy structure and function. Because of the highly non-linear response of many plant biophysical processes (e.g., photosynthesis) to light, using canopy-averaged light fluxes as the basis for scaling calculations does not yield correct whole-canopy fluxes. Accurate scaling requires calculation of the fraction of sunlit and shaded leaf area, the equations for which have been available in the literature for many decades for homogeneous canopies. However, when canopies become heterogeneous, such as in the case of many agricultural crops, the assumptions inherent in traditional approaches for estimating $f_{sun}$ are violated. In this work, a simple geometric model for the sunlit leaf area fraction in discontinuous canopies is presented and evaluated against a detailed 3D leaf-resolving model for simplified plant geometries and realistic tree reconstructions derived from LiDAR data. Results illustrated that the proposed model closely matched the 3D model. An alternative definition of $f_{sun}$ is presented and evaluated that can be easily applied within any model that calculates the fraction of canopy absorbed radiation, which was shown to produce $f_{sun}$ values that were nearly identical to the 3D model even in the presence of foliage clumping and woody branch elements.