Beyond Fapar: Modeling Light Utilization in Tropical Forests

The complex three-dimensional (3D) structure of tropical forests generates a diversity of light environments for canopy and understory trees. Light availability in tropical forests is dynamic on diurnal and seasonal timescales based on variability in solar illumination and the fraction of diffuse radiation. The distribution of light availability ultimately controls light utilization—the amount of absorbed photosynthetically active radiation (PAR) available for photosynthesis. Understanding the dynamics of light utilization is critical for interpreting measurements of net ecosystem exchange and improving ecosystem models. Here, we used the Discrete Anisotropic Radiative Transfer (DART) model to simulate 3D light profiles for a range of Amazon forest scenes. DART was initialized using small footprint airborne lidar data, and characteristics of each 3D simulation were constrained using local measurements of leaf properties, incident PAR, and atmospheric conditions. For each simulation, we separated the fraction of absorbed PAR (FAPAR) into light interactions with leaves, branches, and the ground surface. Leaf-absorbed PAR in each 1 m3 voxel was further constrained to account for light saturation effects. Under midday illumination conditions, most canopy leaves were saturated, leading to a reduction in light utilization (0.65 - 0.7) even as leaf-absorbed PAR remained nearly constant (0.82-0.84). Light utilization also varied seasonally, with lowest fractional utilization in the dry season. Clear sky conditions and low solar zenith angles accentuated shadowing and saturation effects during dry season months, decreasing light utilization by 20-25% compared to total leaf-absorbed PAR. These findings suggest that a potential response of Amazon forests to increasing PAR in the dry season, as measured at the top of canopy, is moderated by seasonal variability in light utilization from changes in shadowing and saturation effects at the leaf level. Our results point to the need for additional spatial information on forest structure to improve the representation of light availability in models of tropical forest productivity.