Fig. 2: Controls on the elevation-dependent pattern of NEP.

a Indirect and direct impacts of geographical factors, climate factors and soil processes on ecosystem CO₂ fluxes. GPP, gross primary productivity; RE, ecosystem respiration; NEP, net ecosystem productivity; PAR, photosynthetically active radiation; TAS, atmospheric temperature; Nr, reactive nitrogen; PREC, precipitation; ECO, Ecosytem type; CO₂, atmospheric CO₂ concentration. Given NEP represents the net balance between GPP and RE, we built its indirect relationship with environmental factors via GPP and RE. The structural equation model analysis was calculated based upon the annual average CO₂ fluxes (multi-year average used if there was more than one year of observations). The blue lines indicate negative impacts, while the red lines indicate positive impacts. The numbers adjacent to the arrows are the standardized path coefficients. The solid lines represent a significant correlation (*P < 0.05, **P < 0.01, ***P < 0.001), whereas the dashed lines indicate lack of significance. The R² values in the boxes indicate how much the dependent variables are explained by the independent variable(s). b Variation in TAS, PREC and Nr with elevation. The dashed lines indicate the linear or nonlinear fit for the environmental factors (the central estimate), whereas the gray shading represents the 95% confidence band of the fits. Source data are provided as a Source Data file.