**5. Conclusions**

Arctic ecosystems have undergone grea<sup>t</sup> changes in the context of climate change. GPP is one of the most crucial indicators of the response of ecosystems to climate change. However, few efforts have been devoted to exploring the spatial variation and phenological characteristics of GPP in the Arctic. In response to this challenge, this study investigated

the spatial distribution as well as the seasonal (phenological) and interannual variations of GPP in the Arctic using MOD17A2H. Furthermore, the GPP variation trends with land cover types, latitude, and elevation were also explored. In order to ensure that the results were reliable, the accuracy of MOD17A2H was first evaluated using in situ measurements from FLUXNET.

This study found that MOD17A2H generally underestimates GPP over the land cover types investigated in this study, and its accuracy tends to be higher over low latitude but lower over high latitude. However, the overall accuracy suggests that MOD17A2H is consistent with the FLUXNET 2015 dataset (RMSE = 1.26gCm−<sup>2</sup> d−1, R = 0.8, Bias = −0.32), and MOD17A2H can represent the phenological characteristics of GPP (RMSE = 8.9 days, R = 0.62). Based on MOD17A2H, it was demonstrated that the maximum GPP occurred in July. In addition, the spatial distribution of GPP is related to land cover types; for example, forests and savannas have relatively high carbon storage capacity from April to August. By comparing the GPP variation with latitude and elevation, it was shown that GPP generally decreases as the latitude and elevation increase. However, the phenomenon is not evident for latitudes in the range (62◦ N, 66◦ N) and elevation lower than 700 m. The overall trend of GPP in the Arctic is greater than zero and is dominated by the variation of vegetation productivity in June. Furthermore, the response to climate change is different across these land cover types; for example, forests are most sensitive to climate warming. The distribution of the interannual trend in GPP across latitudes and elevations is consistent with the changes in GPP as a function of latitude and elevation.

This study is helpful for understanding the spatiotemporal distribution characteristics of GPP over the Arctic as well as the response of ecosystems to climate change. Nevertheless, the results need to be validated with different satellite products. The number of sites used for validation is limited. Therefore, the presented conclusion about the accuracy of MOD17A2H may not be transferable to other regions. Another limitation is that only individual factors such as land cover type, latitude, and elevation were considered in this paper. Other factors, such as air temperature, precipitation, and snow, that are related to vegetation growth status need to be explored further. This will also be our focus in the future.

**Author Contributions:** Conceptualization, D.M. and J.W.; methodology, D.M. and X.W.; validation, X.L., C.M. and X.W.; formal analysis, D.M. and X.W; investigation, J.W.; resources, X.W.; data curation, D.M.; writing—original draft preparation, D.M. and X.W.; writing—review and editing, D.M., X.W. and X.M.; visualization, J.W.; supervision, C.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was jointly supported by the National Key Research and Development Program of China (2019YFA0607003), the National Natural Science Foundation of China (Grant No. 42071296 and 41801226), and the Fundamental Research Funds for the Central Universities (lzujbky-2020-72).

**Data Availability Statement:** Publicly available datasets were analyzed in this study. This data can be found in Google earth engine platform (https://earthengine.google.com/, accessed on 7 February 2021).

**Acknowledgments:** We gratefully acknowledge the anonymous reviewers and editor of the paper in providing critical and constructive comments, which has helped us to significantly improve the manuscript. We also appreciate the assistance of the all authors of this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
