Exploring the Relationship between Light Use Efficiency and Fluorescence Yield under Optimal Conditions across Different Biomes

Understanding the relationship between light use efficiency (LUE) and yield of remotely-sensed solar-induced chlorophyll fluorescence (SIFy) is of primary importance to explain the strong linear relationship between SIF and gross primary production (GPP) at ecosystem level across different biomes. We explored the relationship between LUE and SIFy under optimal conditions to avoid potential impacts from environmental stress. Three fraction of absorbed photosynthetically active radiation (fAPAR) proxies were initially considered, including the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI) and MODIS fPAR product. NDVI was finally selected as a robust fAPAR proxy for all biomes. Analyses were made at flux tower scale using GPP from FLUXNET 2015 and SIF from two reproduced SIF-like products (CSIF and SIF*). Results showed that LUE and SIFy are positively correlated regardless of biome type (with R² = 0.58 and 0.47 for CSIF and SIF*, respectively), which can be explained by their common sensitivity to canopy chlorophyll. On the other hand, croplands and evergreen needle leaf forests stood out for higher LUE-SIFy ratio, due to higher electron use efficiency (EUE) for C4 crops and lower escaping efficiency (f_esc) of SIF for coniferous canopies, respectively. Results shown by this study indicate that i) most of inter-site LUE variability under optimal conditions can be tracked by SIFy, ii) once EUE and f_esc are well modeled, a relatively universal relationship between GPP and SIF can be expected for all biomes and iii) SIFy based on reproduced SIF-like products has the potential to upscale optimum LUE from tower to global scale.