Spatial patterns and climate controls of seasonal variations in carbon fluxes in China's terrestrial ecosystems

Seasonal changes in CO2 exchange determine the strength and dynamics of the carbon budget in terrestrial ecosystems. Knowledge about seasonal and spatial variations in carbon fluxes (net ecosystem productivity (NEP), gross primary productivity (GPP), and ecosystem respiration (RE)) is thus essential to support accurate modeling of carbon flux dynamics. To the best of our knowledge, this study represents the first comprehensive evaluation of how carbon fluxes vary seasonally in different ecoregions in China. Using eddy covariance measurements from ChinaFLUX, we analyzed the spatial variations and climate controls of the phenological and physiological properties of NEP, GPP, and RE across China. Apart from the beginning and ending day of NEP (net carbon uptake), the phenological properties of the carbon fluxes varied significantly with latitude; however, there were no evident trends in the physiological properties of the carbon fluxes with latitude (apart from the annual values of GPP and RE, known as AGPP and ARE). The spatial variation in the physiological properties of GPP and RE was influenced by the precipitation patterns, while the spatial distribution of the phenological properties of GPP and RE was related to air temperature. Moreover, the spatial variation for the end of the growing season changed with the autumn mean soil temperatures. The factors that contributed to the spatial distribution in the annual NEP (ANEP) and annual GPP and RE (AGPP and ARE) were quite different. The mean daily GPP and RE were the main contributors to the spatial variations in AGPP and ARE, while the net carbon uptake period was the main contributor to the spatial variation in ANEP. In this study, we identified a series of ecological parameters and reference values for seasonal patterns that can be used to validate models that simulate changes in regional carbon fluxes