## 4 Results and Discussion
### Spatial Distribution Characteristics
#### 4.3.2 Variation of Monthly GPP  
Figure 12 displays the spatial pattern of multiyear (2001-2019) averaged monthly GPP. The phenological cycle of vegetation is clearly shown in the figure. GPP is very low from November to March, with values close to 0. This is because the photosynthesis of vegetation was restricted due to the extremely harsh environment and limited lighting hours. From April to July, with rising temperatures, melting snow and sea ice, and increasing hours of light, the carbon fixation ability of vegetation is stronger, which contributes to the gradual expansion of GPP from the northwest of Canada and the low latitudes of Russia to the entire Arctic. In addition, the mean GPP over the whole Arctic shows a rapid increase during this period, with the values increasing from 0.0859 g C m\({}^{-2}\) d\({}^{-1}\) in April to 3.739 g C m\({}^{-2}\) d\({}^{-1}\) in July. However, the GPP gradually decreases from August to October as the mean values decrease from 2.215 to 0.0453 g C m\({}^{-2}\) d\({}^{-1}\). This may be attributed to the decrease in temperature, the shortening of day-length, and the senescence of vegetation during this period. The northwestern region of Canada, a small low latitude portion of Russia, and the regions surrounding Iceland have the longest growth period since they are the first to begin and the last to stop photosynthesis. Figure 12 demonstrates that spatial heterogeneity is small during the dormant months (from November to March) and the mid and late summer months (July and August) when GPP is consistently small or large, but large during the transition months (April, May, September, and October) and early summer (June). There are two main reasons for this: the first is the spatial difference in land cover types, the second is the temporal difference in climate conditions [47]. The GPP in the Arctic has a distinct seasonality with the greatest values in July (Figure 13). Throughout the year, most land cover types follow the general seasonality of GPP; they are lowest from January to March, begin to increase in April and reach a maximum in July, decrease from then on and fall back to the lowest values in November. Forest and savannas present the largest GPP from April to August, followed by shrublands, water bodies, permanent wetlands, and grasslands (Figure 13). Permanent snow and ice as well as barren land ranks last. Nevertheless, from September to October, water bodies show slightly larger GPP than other land cover types. These results demonstrate that forests and savannas have a rather high carbon storage capacity during the growing season (from April to August) but water bodies are the biggest contributors to carbon fixation from September to October. The results of Figure 13 are certainly not anticipated because land cover types such as permanent snow and ice, barren land, and water bodies, which cannot carry out photosynthesis, show considerable GPP. This can be explained by the definition of land cover type of IGBP, which is determined by the dominant land cover of a pixel. Water bodies refer to those pixels that are at least 60% covered by permanent water bodies; barren denotes that at least 60% of the pixel is non-vegetated barren (sand, rock, and soil) areas with less than 10% vegetation; permanent Snow and ice means that at least 60% of the pixel is covered by snow and ice for at least 10 months of the year. Therefore, the misclassified part of a pixel is the source of GPP for these three land cover types. From the results above, it is inferred that only extracting the vegetated pixels, as classified by the land cover, will lead to errors in calculating carbon storage in the Arctic. Figure 13 also shows that the monthly GPP has the widest distribution in June, indicating that the interannual variation of GPP is the most significant in June. This is understandable since vegetation grew at the fastest speed from May to June (indicated by the largest slope) and thus is more affected by climate change.