## 4 Discussion  
### Effects of V\({}_{\rm{cmax}}\) Change on GPP Estimation  
Many studies have demonstrated that V\({}_{\rm{cmax}}\) changes across both space and time. Leaf chlorophyll abundance is closely linked to photosynthesis capacity [56; 59; 123]. The V\({}_{\rm{cmax}}\) derived from the LCC (i.e., LCC-derived V\({}_{\rm{cmax}}\)) showed strong seasonality and significant spatial variation across Europe (Figure S3a) but only slight interannual variation over the limited study period of 16 years (Figure S3b). A comparison with the EC-GPP and other GPP or SIF products supported our hypothesis: the consideration of spatiotemporal changes in V\({}_{\rm{cmax}}\) provided more reliable GPP estimations for Europe. Compared with the GPP estimations obtained by fixing the V\({}_{\rm{cmax}}\) using the TRY database, the inclusion of temporally and spatially explicit V\({}_{\rm{cmax}}\) using the satellite-derived LCC product reduced the bias in the estimated daily GPP (Figure 2) and increased the spatial consistency between the FGM GPP and other GPP products or GOSIF data (Figure 9). The positive impact of the LCC on GPP simulations on the site level in Europe that found in our study is comparable with the results of previous studies on single sites (with R\({}^{2}\) enhanced by 10-12% and RMSE decreased by 24-32%) [124; 125] or across multiple sites with different PFTs (with R\({}^{2}\) enhanced by 9-22% and RMSE decreased by 15-32%) [123]. In this study, we found a 23% increase in R\({}^{2}\) and a 25% decrease in RMSE (Figure 2) between the modeled GPP and EC-GPP for 19 EuroFLUX sites across 9 PFTs using the LCC-derived V\({}_{\rm{cmax}}\) to estimate GPP. On the regional scale, we found a 17% decrease in the annual GPP across the 19 EuroFLUX sites and a 24% decrease in the annual GPP for Europe. The maximum leaf photosynthesis capacity is known to change with the seasons under the influences of multiple factors, such as leaf development [53], changes in climatic variables [126], and drought conditions [127]. In this study, we found a higher V\({}_{\rm{cmax}}\) during summer than in spring and autumn (Figure 7). However, the V\({}_{\rm{cmax}}\) field measurements were generally collected close to the peak growing seasons. Thus, assuming a constant V\({}_{\rm{cmax}}\) based on the TRY database led to the overestimation of V\({}_{\rm{cmax}}\) in spring and autumn, which further resulted in an overestimation of GPP. The overestimation of the TRY-based GPP was in agreement with previous findings observed on a global scale. However, [PERSON] et al. (2019) found only a 7% decrease in the annual GPP across 124 sites and a 7% decrease in the global GPP [123], which is much lower than the regional V\({}_{\rm{cmax}}\) change effect on GPP across Europe (17-24%). While the V\({}_{\rm{cmax}}\) across Europe showed a small (but not significant) increasing trend from 2001 to 2016 (Figure S5), we found that including the interannual changes in V\({}_{\rm{cmax}}\) had only a minor impact on the interannual GPP change trend for the limited study period of 16 years (Figure 10). However, we could not neglect the interannual changes in V\({}_{\rm{cmax}}\), since plants may continue to acclimate their leaf chemistry and photosynthesis capacity in response to climate change, especially in response to continued global warming and elevated CO\({}_{2}\) concentrations [128]. According to optimality theory, rising CO\({}_{2}\) and warming can reduce the global canopy demand for Rubisco and result in reductions in V\({}_{\rm{cmax}}\) in the long term [129]. In contrast, we found that the LCC-derived V\({}_{\rm{cmax}}\) showed an increasing trend (that was not statistically significant) across Europe (Figure S3), which is also revealed by the V\({}_{\rm{cmax}}\) estimated from SIF data [51; 57]. Plants in high arctic regionsare sensitive to changes in temperature [130].