Unrevealing site-dependent relationship between solar-induced chlorophyll fluorescence and gross primary productivity using the terrestrial ecosystem carbon cycle simulator

Solar-induced fluorescence (SIF), a small light signal emitted during the photosynthetic process, is a powerful tool for tracking gross primary productivity (GPP) across scales, particularly in evergreen needleleaf forests, which are traditionally challenging to monitor with remote sensing. Terrestrial biosphere models (TBMs) that incorporate a SIF module can help address the spatiotemporal limitations of field and satellite observations and explain variations in the SIF-GPP relationship across different temporal scales and ecosystems. In this study, we developed TECs-SIF, a TBM that integrates the spectral invariant property-based radiative transfer model across leaf and canopy scales to simultaneously simulate canopy SIF emissions and GPP and investigate how the SIF-GPP relationship varies across forest ecosystems. We calibrated and validated TECs-SIF using data from three evergreen needleleaf forest and one deciduous broadleaf forest AmeriFlux sites: Southern Old Black Spruce (CA-Obs), Delta Junction (US-xDJ), Niwot Ridge Forest (US-NR1), and University of Michigan Biological Station AmeriFlux site (US-UMB). Our results show that TECs-SIF as a promising tool that accurately simulates SIF and GPP across various temporal scales (Hourly: SIF: R² = 0.48–0.87, Root Mean Squared Error (RMSE) = 0.03–0.12 W/m²/μm/sr; GPP: R² = 0.60–0.79, RMSE = 1.82–5.31 μmol/m²/s; Daily: SIF: R² = 0.64–0.91, RMSE = 0.02–0.09 W/m²/μm/sr; GPP: R² = 0.89–0.97, RMSE = 0.51–2.05 μmol/m²/s), captures nonlinear relationships at hourly intervals, and linear trends at daily and monthly scales. Meanwhile, SIF-GPP relationship is site-dependent across temporal scales, influenced by canopy structure (e.g., CI) and leaf traits.