Global photosynthetic capacity of C3 biomes retrieved from solar-induced chlorophyll fluorescence and leaf chlorophyll content

The maximum Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (V cmax ) has been widely used to help determine the photosynthetic capacity for carbon cycle modelling. Field measurements of V cmax are very limited, so fixed V cmax values over growing seasons are often used, causing considerable uncertainties in modelled results. Also, the lack of a global V cmax dataset with seasonal variations limits our understanding of the spatial and temporal patterns of V cmax , and hence photosynthesis. This study proposes a refined data assimilation scheme to retrieve daily V cmax using remotely sensed solar-induced chlorophyll fluorescence (SIF) and leaf chlorophyll content (LCC) data. Using the refined method, we produced a global V cmax dataset with daily 0.1° × 0.1° resolutions. The seasonal variations in V cmax have been successfully retrieved. V cmax retrieved in this study was compared with field-measured V cmax from past literature. Good agreements were achieved in relation to V cmax measured at 201 sites (R2 is 0.78, and relative RMSE is 34%) and the seasonal variation in V cmax measured at 9 sites (R2 ranges from 0.57 to 0.99, and the relative RMSE ranged from 6% to 28%). Our method effectively utilizes the information from both SIF and LCC, showing advantages over previous approaches that estimate V cmax based solely on using SIF or LCC. Our results suggest that the spatial distribution of V cmax depends on the spatial distribution of biomes, along with air temperature and absorbed photosynthetically active radiation (APAR). Higher seasonal mean air temperature and APAR tend to lead to higher seasonal mean V cmax . The seasonal variation in V cmax is driven by the seasonal variations in air temperature and APAR, rather than their seasonal means. We also derived and analyzed V cmax25 , which is V cmax normalized to 25 °C (V cmax25 ) and has been found to be more closely related to the Rubisco content. V cmax25 also has considerable spatiotemporal variations, and the variations differ to that in V cmax . Different biomes have different ratios of V cmax25 to LCC. Air temperature and APAR exert weak but positive effects on the seasonal variation in V cmax25 . V cmax retrieved in this study enhances our understanding of the spatiotemporal pattern of leaf photosynthetic capacity on the global scale and is potentially useful to terrestrial carbon cycle modelling.