Validation of simulated SIF and GPP by the 3D radiative transfer model FLiES-SIF: A case study in a cool temperate deciduous forest

Global terrestrial ecosystems control the atmospheric CO₂ concentration through gross primary production (GPP) and ecosystem respiration processes. The estimation of GPP is essential for various applications ranging from yield prediction to evaluation and prediction of the impact of regional and global environmental changes. The chlorophyll fluorescence is one of the energy release pathways of excess absorbed radiation in the photosynthetic process, and Sun-Induced Fluorescence (SIF) is used to infer the photosynthetic capacity of plant canopies. The satellite-derived SIF potentially provides a direct pathway of leaf-level photosynthesis to global GPP, however, it is not clear how the leaf-level SIF emission contributes to the top of canopy directional SIF. Because of the non-linear light interactions within plant canopies, SIF radiances emitted at top of plant canopies are not simply the sum of individual leaf contributions. The developed model, Forest Light Environmental Simulator-SIF (FLiES-SIF), was based on FLiES, which is the 3D Monte Carlo plant canopy radiative transfer model. Original FLiES was extended to compute the radiance of SIF emitted to the observation view angles. Additionally, the availability of SIF on GPP estimation was indicated by using FLiES-SIF. The model can also estimate absorbed photosynthetically active radiation (APAR) per unit voxel in a canopy and the quantum yield of SIF as SIF/APAR. The precision of GPP estimation was improved by the two models (Farquhar's photosynthesis model and FLiES-SIF). In this study, we validated the model using remotely sensed vegetation information in a cool temperate deciduous forest in Takayama, central Japan (AsiaFlux: JP-TKY). The top canopy layer is dominated by birch (Betula ermanii) and oak (Quercus crispula), the sub-canopy layer and shrub layers are dominated by maples (Acer rufinerve and Acer distylum), panicled hydrangea (Hydrangea paniculate) and forked viburnum (Viburnum furcatum), and the understory is dominated by an evergreen dwarf bamboo (Sasa senanensis) and seedlings of Q. crispula, A. distylum, A. rufinerve, H. paniculata and V. furcatum.