Fig. 2

Monthly variability of \({\text{X}}_{{{\text{CO}}_{{2}} }}\) (a), Qg (b), RH (c), and SIF 757 (d) over the period from January 2015 to December 2019. Where \({\text{X}}_{{{\text{CO}}_{{2}} }}\) column average of carbon dioxide in the atmosphere (ppm), Qg global radiation (MJ m⁻² day⁻¹), RH relative humidity (%), SIF 757 sun-induced chlorophyll fluorescence at 757 nm (Wm⁻² sr⁻¹ μm⁻¹)