## 4 Results and Discussion
### Validation MOD17A2H Based on In Situ
Although the two sites feature forests, their locations are different (Table 1), indicating that the accuracy of MOD17A2H is also influenced by other factors. As shown in Figure 5c, although MOD17A2H slightly underestimates GPP over forests (Bias = \(-0.36\)), overall, MOD17A2H was consistent with site measurements over forests, with RMSE and R of 1.51 g C m\({}^{-2}\) d\({}^{-1}\) and 0.79, respectively. Figure 4: Time series comparison between site-based GPP and MOD17A2H over forest sites, (**a**) and (**b**) refers to FI_Sod and US_Prr. Figures 6 and 7 present the comparison results over grasslands, shrulbands, and permanent snow and ice. MOD17A2H generally underestimates GPP over these land cover types but the degree of underestimation varies with sites, as it is relatively weak for RU_Cok (RMSE = 1.02 g C m\({}^{-2}\) d\({}^{-1}\), Bias = \(-\)0.44) but strong for GL_ZaH and SJ_Blv (with the RMSEs of 0.61 and 0.19 g C m\({}^{-2}\) d\({}^{-1}\) and the Bias of \(-\)0.5 and \(-\)0.17, respectively). It is important to remember that the latitudes of GL_ZaH and SJ_Blv are higher than at RU_Cok, which further demonstrates that the accuracy of MOD17A2H is related to the latitude of the sites. When combining the data points of all the sites (Figure 8), MOD17A2H slightly underestimates GPP, with the bias of \(-\)0.32. The overall accuracy of MOD17A2H is reasonable over the Arctic, with RMSE of 1.26 g C m\({}^{-2}\) d\({}^{-1}\) and R of 0.8, respectively. These indicators demonstrate that MOD17A2H is able to capture the spatiotemporal variation characteristics of GPP in the Arctic. From the validation results based on in situ measurements, it is shown that the MOD17A2H generally underestimates GPP over these land cover types. Nevertheless, depending on the location of the sites, it may underestimate or overestimate GPP within each land cover type. This demonstrates that the accuracy of MOD17A2H is also influenced by other factors in addition to land cover types. For instance, the latitude seems to be associated with the accuracy of MOD17A2H given that the accuracy of MOD17A2H tends to be higher over low latitudes but lower over high latitudes (>74\({}^{\circ}\) N), which might be due to the actual maximum radiation conversion efficiency (\(\epsilon_{\text{max}}\)) of vegetations being quite different from the given \(\epsilon_{\text{max}}\) in high latitude. In fact, the misclassification of a pixel is also responsible for the inconsistency of the accuracies derived from different sites within the same land cover type. Because the classification scheme adopted by IGBP is too general, it cannot reveal the detailed categories carrying out photosynthesis. Moreover,  
Figure 5: Scatter plots of site-based GPP against MOD17A2H over different forest sites (**a**,**b**), and the overall scatter plots by combining all forest sites (**c**). Flux tower GPP means in situ GPP. Gray belt refers to the confidence interval of 95%. MOD17A2H is calculated based on the concept of light use efficiency (LUE), which assumes a fixed maximum radiation conversion efficiency (\(\epsilon_{\text{max}}\)) of each land cover type [19]. This treatment also introduced errors when misclassification occurred. Another cause of the inconsistency is the different degrees of surface heterogeneities within the satellite pixel, which cause the sites to be more or less representative of the satellite pixel. Last but not least, since missing data of in situ may occur unequally during each 8-day period, the temporal representativeness varies across these sites. Despite these uncertainties, the validation results still suggest that the performance of MOD17A2H is better over shrublands and wetlands than it is over forests. Figure 6: Time series comparison between site-based GPP and MOD17A2H over grasslands (**a**), shrublands (**b**), and permanent snow and ice (**c**) sites. Figure 7: Scatter plots of site-based GPP against MOD17A2H over grasslands (**a**), shrublands (**b**), permanent snow and ice (**c**) sites. Flux tower GPP means in situ GPP.