## 4 Results and Discussion
### Trend Estimates of GPP  
The interannual variation trend of GPP is shown on a pixel basis (Figure 14); almost half of the Arctic presents significant positive trends, but the magnitude of trends shows distinct spatial variation. In the northwest of Canada and the latitude lower than 70\({}^{\circ}\) N of Russia, the trend of GPP varies significantly, ranging from 0.005 to 0.08 g C m\({}^{-2}\) year\({}^{-1}\). By contrast, in the northeast of Canada and the regions surrounding Greenland, the GPP trend varies slightly, ranging from 0 to 0.01 g C m\({}^{-2}\) year\({}^{-1}\). The small range of the GPP trend is likely to be associated with grassland and barren land. The former has a relatively small spatial variation, as indicated by the centralized distribution of the interannual trends in GPP. The latter has a very small GPP, which is almost constant over time (Figure 11c). Figure 14: The spatial distribution of interannual trend of GPP (g C m\({}^{-2}\) year\({}^{-1}\)) over the Arctic (at 97.5% confidence level based on MK test). Figure 13: Multiyear averaged monthly GPP distribution over the entire study period for different land cover types (denoted by colored lines) over the Arctic. The black boxplots denote the multiyear monthly GPP distribution of all land cover types. As seen from Figure 12, there is almost no vegetation productivity in the Arctic from November to March. Thus, Figure 15 only presents the interannual trends from April to October. It is clear that the interannual trends show significant differences between different months. From April to June, the interannual trends in northwestern Canada gradually increase and reach a maximum in June, with an overall trend of 0.068 g C m\({}^{-2}\) year\({}^{-1}\) (Figure 15c). From then on, the interannual trend decreases gradually until October (Figure 15d-g). Furthermore, we also find that the interannual trends in July and August are more spatially heterogeneous than in other months. These results demonstrate that the response of vegetation to climate change is not consistent between different months and over different areas. It is interesting to find that the interannual trend is the most significant in June (Figure 15c) and shows a similar spatial pattern to the overall interannual trend (Figure 14). This demonstrates that the interannual variations of GPP in the Arctic may be dominated by the change of vegetation productivity in June. The distributions of the interannual trends in GPP with latitude and elevation are shown in Figure 16a,b. The interannual trends first increase at the latitude of 51\({}^{\circ}\) N and reach a maximum of 0.018 g C m\({}^{-2}\) year\({}^{-1}\) at 57\({}^{\circ}\) N. From then on, the interannual trend decreases significantly until 62\({}^{\circ}\) N. However, the interannual trend seems to be independent of latitude from 62\({}^{\circ}\) N to 66\({}^{\circ}\) N. Then a clear decreasing trend can be observed from 66\({}^{\circ}\) N until 80\({}^{\circ}\) N. Therefore, we can conclude that the interannual trend of GPP is sensitive to the latitude, except in the regions located between 62\({}^{\circ}\) N and 66\({}^{\circ}\) N, which is similar to the distribution of annual-averaged GPP with the variation of latitude. This is mainly  
Figure 15: Spatial distribution of the interannual variation trend (g C m\({}^{-2}\) year\({}^{-1}\)) of monthly GPP over the Arctic, (**a**–**g**) refers to April to October. because the regions located between (62\({}^{\circ}\) N, 66\({}^{\circ}\) N) are in the same climatic zone, namely in the north temperate zone. Figure 16b demonstrates that the interannual trend of GPP generally shows a decreasing trend with increasing elevations. However, the sensitivity is relatively weak at low elevations (<700 m) and significant at larger elevations (>700 m). The low GPP and interannual trend in the regions with relatively higher altitude and latitude Figure 11a,b and Figure 16a,b mainly due to the low precipitation and temperature, which are not conducive to plant growth [48; 49].