Simulating emission and scattering of solar-induced chlorophyll fluorescence at far-red band in global vegetation with different canopy structures

Recent satellite retrieval of solar-induced fluorescence (SIF) has provided promising information for constraining carbon flux simulation with terrestrial biosphere models (TBMs). For this purpose, it is necessary to implement a SIF model in TBMs. SIF emerging from leaves generally undergoes multiple scattering processes within a canopy. However, it remains unclear how canopy scattering impacts observed SIF, which is used to constrain regional and global GPP modeling. Here, we developed an efficient scheme to account for the canopy scattering in a SIF model. Then we coupled this mechanistic representation of SIF to Boreal Ecosystem Productivity Simulator (BEPS) and evaluated the new model (BEPS-SIF) simulations with ground and satellite-based SIF measurements. The results showed that BEPS-SIF reproduced global patterns of SIF observed by a satellite sensor and captured the seasonality of SIF reasonably well over different regions. The canopy scattering effects on the relationship between SIF and gross primary productivity (GPP) were also examined at the global scale using the BEPS-SIF model. We confirmed that SIF emitted from leaves was more directly linked to GPP globally than the observed canopy-leaving SIF, with higher improvements of the coefficient of determination (R-2) in dense vegetated regions and lower improvements of R-2 in sparse vegetated regions. This study highlights the importance of implementing the canopy scattering effect in a fluorescence model of TBMs and the need to account for this effect in the use of satellite SIF data for constraining global GPP modeling.