Evaluation of a ray-tracing canopy light model based on terrestrial laser scans

The local light regime within the tree canopy is crucial information for modeling water, carbon and nutrient cycling, and vegetation–atmosphere interactions. We tested the performance of a new model to simulate the light environment in the canopy of a juvenile beech stand under controlled light conditions. The canopy architecture was determined using a terrestrial laser scanner to derive a three-dimensional voxel representation. Depending on whether a voxel represents stem biomass, leaf biomass, or air, different attributes of light are assigned to the voxel. The model combines a representation of the canopy as three-dimensional cells (voxels) with a fast ray tracing algorithm that calculates the absorbed fraction of incoming photosynthetic active radiation (PAR). The simulated light regime of the stand was compared with measurements of the PAR regime inside the canopy (model efficiency Nash–Sutcliffe efficiency (NSE) = 0.88, root mean square error (RMSE) = 124 µmol m−2 s−1) and at the soil surface (NSE = 0.65, RMSE = 22 µmol m−2 s−1). The model needs two input parameters, the edge length of the voxels and the light attenuation coefficient of the voxels. The best simulation results were achieved at a voxel size of 0.03 m. For model calibration, only measurements of the light fraction that reaches the soil surface are needed. The good agreement of the simulated and the measured light regime together with the fast computation by the ray tracing algorithm suggest that the model is also applicable to simulate the light regime of natural forests under variable light conditions.