Evaluating the impact of canopy spatial heterogeneity on Solar-Induced chlorophyll fluorescence and Model-Based quantification

Solar-induced chlorophyll fluorescence (SIF) is a powerful proxy for vegetation photosynthesis. However, its broader application remains constrained by the insufficient understanding and quantification of canopy spatial heterogeneity. We used the 3D discrete anisotropic radiative transfer (DART) model to simulate SIF. Heterogeneous and homogeneous canopies were compared to quantify the impact of canopy spatial configuration on total leaf-emitted SIF (SIFtotal) and hemispherically integrated top-of-canopy SIF (SIFhemi). Angular effects were examined by comparing top-of-canopy SIF (SIFtoc) with SIFtotal estimated at different viewing angles. We also compared the relationships of simulated SIFtotal and SIFtoc with fraction of absorbed photosynthetically active radiation (fAPAR) and vegetation indices (VIs). In addition, we evaluated the relationships between the coefficient of variation (CV) of VIs and SIF, and developed a correction function for estimating heterogeneous-canopy SIF using homogeneous-canopy SIF. The results reveal that (1) both SIFtotal and SIFhemi are significantly higher in homogeneous than in heterogeneous canopies; (2) estimating SIFtotal from SIFtoc effectively reduces angular dependency; (3) SIFtotal exhibits the strongest correlation with fAPAR, while SIFtoc aligns better with NIRv and EVI; and (4) VI-derived CVs are negatively correlated with SIF, with CVNIRv and CVEVI at 10 m resolution demonstrating robust performance in fitting the ratio of heterogeneous-to-homogeneous canopy SIF, validating the proposed correction function. This study highlights the critical role of canopy structural heterogeneity in shaping SIF and presents an approach to convert homogeneous-canopy SIF to heterogeneous-canopy conditions, offering a novel pathway toward more accurate SIF estimation and application.