Constraining global terrestrial gross primary productivity in a global carbon assimilation system with OCO-2 chlorophyll fluorescence data

The gross primary productivity (GPP) is the largest carbon flux in the terrestrial carbon cycle. Constraining GPP is critical for understanding the terrestrial carbon sources and sinks. In this study, we attempted to constrain the terrestrial GPP at regional to global scales through optimizing the key photosynthetic parameter (the carboxylation capacity at 25 °C, V c m a x 25 ) using solar-induced chlorophyll fluorescence (SIF) observations from the Orbiting Carbon Observatory-2 (OCO-2). The optimization was made within the Global Carbon Assimilation System (GCAS), in which the Boreal Ecosystem Productivity Simulator (BEPS) model was used to simultaneously simulate the global GPP and SIF in the process-based manner. Optimized V c m a x 25 shows a distinguishable spatial pattern, with the largest values over the crop regions. After optimization, V c m a x 25 of crop is significantly increased. Importantly, the optimized V c m a x 25 of different plant functional types (PFTs) show unambiguous seasonal variations. With these optimized V c m a x 25 , the simulated global GPP in 2015–2016 amounts to 119.1 PgC yr−1, close to the median value (121.3 PgC yr−1) of the observation-based estimates. Global GPP decreases by 8.3% relative to the value simulated using prior V c m a x 25 . In detail, GPP of crops increases by 16.4%, but it decreases over the other PFT regions, ranging from −4.4% over grasses to −34.0% over deciduous needleleaf forests. The spatiotemporal variations in the optimized PFT-dependent V c m a x 25 also reshape the seasonal cycle in GPP. We regard that it is an effective pathway to constrain GPP based on the satellite SIF and the process-based assimilation system, which can provide us an opportunity to better understand the terrestrial and global carbon cycle.