## 4 Results and Discussion
### Spatial Distribution Characteristics  
#### 4.3.1 Spatial Distribution of Annual-Averaged GPP  
Figure 10 shows the annual maximum (Figure 10a) and annual-averaged GPP (Figure 10b), derived from MOD17A2H, in the Arctic on a pixel basis. The two metrics generally present similar spatial distribution patterns. GPP is relatively low in the northeast of Canada and the regions surrounding Greenland, with an annual-maximum range of 0 to 1.200 g C m\({}^{-2}\) d\({}^{-1}\) and an annual-averaged range of 0 to 0.900 g C m\({}^{-2}\) d\({}^{-1}\). The low GPP over these areas can be explained by the fact that these areas are almost completely covered by grassland and barren land, which have lower GPP (Figure 11c). Therefore, it can be inferred that the spatial distribution of GPP is related to land cover types. Furthermore, the GPP shows a decreasing trend as the latitude increases. This is especially true over the eastern hemisphere of the Arctic, where the annual maximum of GPP drops from 3.300 to 0.300 g C m\({}^{-2}\) d\({}^{-1}\) and the annual-averaged of GPP decreases from 2.400 to 0.300 g C m\({}^{-2}\) d\({}^{-1}\).  
Figure 8: The overall scatterplots of in situ based GPP against MOD17A2H. Flux tower GPP means in situ GPP. Gray belt refers to the confidence interval of 95%.  
Figure 11a,b display the distribution of GPP with latitude and elevation, respectively. It can be seen that GPP generally shows a decreasing trend with latitude, which is in line with the results of [PERSON] et al. (2018). Nevertheless, the sensitivity of GPP to latitude depends on the situation. The results define three groups of latitudes: (1) latitudes less than 62\({}^{\circ}\) N and more than 80\({}^{\circ}\) N; (2) latitudes higher than 62\({}^{\circ}\) N but lower than 66\({}^{\circ}\) N; and (3) latitudes between 66\({}^{\circ}\) N and 80\({}^{\circ}\) N. In the first group, the GPP distribution is sparse because the region located within this scope is quite limited. Thus, GPP shows irregular variation patterns with latitude. In the second group, GPP is relatively stable, indicating the insensitivity of GPP to latitude within this scope. Nevertheless, GPP presents a clear decreasing trend in the third group, demonstrating that GPP is most sensitive to latitude from 66\({}^{\circ}\) N to 80\({}^{\circ}\) N. Similar to latitude, GPP generally shows a decreasing trend with increasing elevation (Figure 11b). However, the decreasing rates are different depending  
Figure 9: The DOY comparison of the timing of the maximum GPP between the sites and MOD17A2H. (**a–e**) Represent permanent wetlands, forests, grasslands, shrublands, and all sites combined, respectively. The gray belt refers to the confidence interval of 95%.  
on the elevation, which is smaller for elevations lower than 700 m, but larger for elevations higher than 700 m.  
Figure 11: The annual-averaged GPP distribution with latitude (**a**) and elevation (m) (**b**) for the whole Arctic. The gray belt refers to the confidence interval of 95%, the blue line refers to the fit line, and the boxplots denote the distribution of the annual-averaged GPP with the latitude. (**c**) The annual-averaged GPP distribution for the entire study period 2001–2019 over different land cover types. FOR, SHR, SAV, GRA, WET, SNO, BAR, and WAT denote the forests, shrulands, savannas, grasslands, permanent wetlands, permanent snow and ice, barren, and water bodies, respectively.  
Figure 10: The spatial distribution characteristics of annual-maximum (**a**) and annual-averaged GPP (**b**) over the Arctic. Units of GPP are g C m\({}^{-2}\) d\({}^{-1}\).  
Figure 11c presents the annual-averaged GPP distribution over different land cover types for the entire study period. It can be seen that all land cover types show considerable interannual variation except for permanent snow and ice as well as barren land (as indicated by the wide distribution of boxplots).