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Key messageMonthly variation in gross primary productivity (GPP) between on-years and off-years were different. Main drivers of GPP in on-years were abiotic. In off-years drivers were biotic and abiotic.AbstractUnderstanding biotic (living or once-living organisms) and abiotic (non-living physical and chemical elements) influences on seasonal variation in carbon fluxes in Moso bamboo forest is important for predicting future carbon sequestration under climate change. Although differing physiological and ecological characteristics of Moso bamboo forest between on-years and off-years have been observed, the drivers of annual differences in carbon fluxes remain unknown. In this study, drivers of variation in carbon fluxes were analyzed based on gross primary productivity (GPP) and biotic factors (leaf area and chlorophyll content here, represented by vegetation indicesVIs) and abiotic factors. Results showed that average monthly GPP between on-years and off-years was significantly different from January to June, mainly due to natural variation in biotic factors. The monthly variation in GPP during on-years was mainly influenced by abiotic factors, whereas that in off-years was determined by the combination of biotic and abiotic factors. Monthly variation and differences in GPP between on-years and off-years were well represented by VIs. The GPP was more strongly correlated with VIs in off-years than in on-years, owing to large seasonal variation in canopy chlorophyll content. Hence, GPP estimated from both air temperature and simple ratio was more accurate than that estimated from air temperature alone. Overall, the difference in GPP between on-years and off-years and its underlying mechanisms can be used to accurately estimate carbon fluxes in Moso forest and predict carbon fluxes under future climate warming.