Multi-angle measurements of solar-induced chlorophyll fluorescence in tropical forest canopy

Solar-induced chlorophyll fluorescence (SIF) is an electromagnetic signal emitted by photosynthetically active plants, serving as a proxy for photosynthesis. However, SIF measurements in tropical forests, which are vital carbon sinks, remain underexplored. This study conducted continuous multi-angle far-red SIF measurements in a tropical forest in Xishuangbanna, China, from July 2023 to August 2024, using a Multi-Fluo system. We retrieved SIF with four widely used algorithms, including three-band Fraunhofer Line Discrimination (3FLD), Band Shape Fitting (BSF), Spectral Fitting Method (SFM), and Singular Vector Decomposition (SVD). Results showed that BSF outperformed the others, with the strongest correlations with near-infrared radiance of vegetation (NIRvR) (R² = 0.89), absorbed photosynthetically active radiation (APAR) (R² = 0.86), and gross primary production (GPP) (R² = 0.66) at half-hourly scale. Furthermore, diurnal patterns of SIF, SIF yield and NIRvR-derived fluorescence yield (ΦF) were analyzed on cloudy and clear sky conditions. Interestingly, a hysteresis was observed in SIF_BSF yield on sunny days. In addition, averaging data from 17 viewing azimuth angles (VAAs) could explain over 10% improvement for SIF-related relationships compared with single-angle results such as VAA of 0°. This study demonstrates the applicability of BSF for SIF retrieval in tropical forests and highlights the value of multi-angle measurements, providing foundational insights into understanding SIF dynamics in complex tropical ecosystems.