## 4 Discussion  
Here, we quantitatively analysed how land cover change, climate change, CO\({}_{2}\) fertilization, and photosynthetic capacity change contributed to European GPP dynamics in Europe from 2001 to 2016. Despite some fluctuations, the annual total GPP exhibited  
Figure 8: Dominant factors driving the interannual dynamics in GPP from 2001 to 2016. (**a**) Combined effects of LAI change, CO\({}_{2}\) fertilization, climate change, and interannual V\({}_{\text{cmax}}\) change on annual GPP trends constrained by simulation scenarios of the “All”, “LULC3”, “CO\({}_{2}\)”, “CLM1”, and “PC1” simulation scenarios. (**b**) Trends in total GPP simulated by the FGM in ten different simulation scenarios. Model simulations are shown in Table 1. The labels ** and * in (**b**) represent the GPP interannual trend at significance levels of 0.01 and 0.05, respectively. The R\({}^{2}\) in (**b**) represents the regression coefficient for the interannual GPP trend from 2001 to 2016.  
a significant increasing trend (51.5 Tg C yr\({}^{-2}\), \(p\) < 0.01) across Europe from 2001 to 2016 (Figure 1b). This increasing GPP trend suggests a considerable role of the European vegetation in the terrestrial carbon cycle, which is in agreement with current expectations for the northern middle-high latitude ecosystem. Many studies have provided evidence of increasing vegetation productivity across the Northern Hemisphere, though these studies have attributed the annual increasing GPP trend to different reasons, such as warming temperatures, elevated atmospheric CO\({}_{2}\) concentrations, and land surface greening. However, the role of land cover change and V\({}_{\text{cmax}}\) change in dominating terrestrial carbon assimilation is still not well understood. Thus, we discuss four aspects of the primary findings: (a) the enhancing VPD offset the GPP enhancement caused by warming; (b) the greening effect and CO\({}_{2}\) fertilization dominated the GPP increase across Europe in the last decade; (c) consistent increases in GPP based on different GPP models; and (d) the V\({}_{\text{cmax}}\) change effect on GPP. The annual mean temperature in the study area increased from 2.7 to 3.3 \({}^{\circ}\)C from 2001 to 2016 at an annual rate of +0.029 \({}^{\circ}\)C yr\({}^{-1}\) (\(p\) < 0.01) (Figure 6b). Temperature warming is considered a dominant factor affecting GPP increases in the Northern Hemisphere [41]. Although a strong correlation existed between TEMP and GPP since they both increased during the study period, we could not conclude that the GPP increase in Europe was caused by the warming effect. In this study, we found only a small positive warming effect (+3.7 Tg C yr\({}^{-2}\), \(p\) < 0.05) (Figures 7d and 8b), accounting for 7% of the GPP annual increasing trend, which was much smaller than the reported value (-40%) based on regression methods [41]. The warming effect could enhance GPP by prolonging growing seasons and increasing photosynthetic capacity. However, the warming effect could also cause a reduction in GPP by strengthening water stress and causing stomatal closure [42; 43]. Extreme droughts could profoundly impact ecosystem functions and result in a reduction in GPP [44]. During the study period, there were three extreme droughts in the south-eastern Europe in 2003, 2007, and 2012. The GPP reductions in 2003 and 2012 were largely linked to extreme droughts in Europe. The annual mean temperature was relatively low in 2006 (Figure 6b), and the warming effect offset the GPP decline caused by the enhanced VPD in 2007 (Figure 6c,d). The global average CO\({}_{2}\) concentration has increased, on average, by 1.8 ppm per year since 1992 [45; 46]. Another frequent explanation for the increase in GPP is an elevated atmospheric CO\({}_{2}\) concentration [47; 48]. Assuming spatially uniform and annually increasing CO\({}_{2}\) for large-scale GPP estimation has been proven to be reasonable [49].