## 5 Conclusions  
In this study, we analyzed the spatio-temporal variation in the annual GPP, and quantified the effects of the GSL, GPPmax, and climate variables on China's deciduous forest ecosystems. We found that the annual GPP of China's deciduous forest ecosystems increased significantly during 2001-2020. Planted forests showed a higher increase rate than natural forests. Though there were no significant differences in the trend in the GSL between natural and planted forests, planted forests showed a larger enhancement in the GPPmax than natural forests. Both the GPPmax and GSL correlated with the annual GPP significantly. The GPPmax trend dominated the trend in the GPP, compared with the GSL. Moreover, the GSL and GPPmax are also highly correlated with the annual mean temperature. The increase in the summer temperature increased the GPP and GPPmax, while the increase in the spring temperature extended the GSL. Our study indicated that it is critical to accurately simulate the roles of phenological and physiological indicators in controlling the trends in the GPP, and to better predict the temporal variation in the terrestrial GPP in the future. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/f14091880/s1](https://www.mdpi.com/article/10.3390/f14091880/s1)]([https://www.mdpi.com/article/10.3390/f14091880/s1](https://www.mdpi.com/article/10.3390/f14091880/s1)), Figure S1: Spatial patterns of the leaf unfolding and leaf falling date trends during 2001-2020 in China's deciduous forest ecosystems; Figure S2: The relationships between the trend in the annual gross primary productivity (GPP) and the leaf unfolding and leaf falling dates across natural forests (a,b), and planted forests (c,d), during 2001-2020. Table S1: The sensitivity of the annual gross primary productivity (GPP), growing season length (GSL), and maximum gross primary productivity (GPPmax) to the annual mean temperature (TEM), annual sum precipitation (PRE), and annual sum global radiation (RAD) across natural and planted forests during 2001-2020; Table S2: The sensitivity of the annual gross primary productivity (GPP), growing season length (GSL), and maximum gross primary productivity (GPPmax) to the annual annual gross primary productivity (GPPmax) to the annual mean temperature (TEM), annual sum precipitation (PRE), and annual sum global radiation (RAD) in different seasons across natural and planted forests during 2001-2020. **Author Contributions:** Conceptualization, Y.L. and W.C.; methodology, Y.L. and X.L. ; validation, Y.L. and X.L. ; data curation, Y.L. and X.L. ; writing--original draft preparation, Y.L. and X.L. ; writing--review and editing, X.L. and W.C.; supervision, W.C. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the National Natural Science Foundation of China Research on the Greening Process and its Mechanism of Regional Ecosystem Services in Kubuqi Desert (42061069); The Science and Technology Innovation Project of Inner Mongolia Autonomous Region Department of Natural Resources \"Application Research on Land Spatial Resilience Pattern and Implementation Path in the Yellow River Basin of Inner Mongolia\"; Key Technology and Application of Ecological Quality Diagnosis and Integrated Management of \"Beautiful Inner Mongolia\" (2019 GG010); Autonomous Region Talent Development Fund Project in 2021; and 2022 Inner Mongolia Autonomous Region \"Grassland Talents\" Project Young Innovation and Entrepreneurship Talents (First Level). **Data Availability Statement:** Data will be made available on request. **Acknowledgments:** The authors would like to thank the anonymous reviewers for their crucial comments, which improved the quality of this paper. **Conflicts of Interest:** The authors declare no conflict of interest.