## 7 Conclusions  
In this study, firstly, we applied the verified GLO-PEM to estimate the NPP every eight days, from 2001 to 2019, based on remote-sensing data and climate data. Studies showed that an increasing trend occurred with an average of 3.95 gC-m-2-a-1, especially during the period of 2016-2019. In addition, we explored the vegetation phenology, using the estimated NPP data from the time series at the early and ending date of the growth season. We found that there is a significant advance in SOS and a delayed EOS from 2001 to 2019, causing the prolong of the whole growing season of forests in Northeast China. The above studies have provided a basis for further phenology regularity. In future research, we also need to consider the driving mechanism of vegetation phenology and pay more attention to the inversing methods.  
Finally, methods including Sen-Mann-Kendall trend and Hurst index were combined to explore the sustainability analysis and future ecological environment in Northeast China. Generally, more researchers explore the future ecological environment using different climate scenarios based on the older reference year. We proposed a different method to explore the NPP changes from the time series of 2001-2019 in the future. Studies demonstrated that the overall NPP presented a not-significant increasing trend from 2001 to 2019, but 47.42% of the area still exhibited a decreasing trend, mainly in the central part of Zhangguangcai Mountain and the northern part of Changbai Mountain. In the future, the regions with a reverse-direction NPP change will be bigger than the one with a co-directional feature in Northeast China. Moreover, the regions with a continuous degradation will account for 22.22%, which distributed in Jiahe City and Dunhua City in the central part of Northeast China. These conclusions could provide a strong potential template for tropical managers and sustainable management in Asia, even in the global climate change.  
**Author Contributions:** Conceptualization, Q.J. and X.D.; Data curation, C.W. and K.L.; Formal analysis, C.W.; Funding acquisition, Z.Z.; Investigation, Z.Z.; Software, C.W. and K.L.; Supervision, Q.J. and X.D.; Writing--original draft, C.W.; Writing--review & editing, Q.J. and X.D. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This project was funded by the Key Project of National Key Research and Development Plan (No.2017 YFC050400103), National Science and Technology Projects (No.2017 ZX07101004 and No. 2017 ZX07108002), the Major Research Plan of National Natural Science Foundation of China (No. 41901234 and No.51909052), the Fundamental Research Funds for the Central Universities (No. 2015 ZCQSB03) and the Hebei Natural Science Foundation (No. D2015207002).  
**Acknowledgments:** Data provision and support from projects of the National Natural Science Foundation of China (No.71225005) and the Exploratory Forefront Project for the Strategic Science Plan in IGSNRR, CAS are also appreciated. In addition, we gratefully acknowledge the financial support provided by the \"Restoration and Reconstruction Technology and Demonstration project of typical Degraded Wetlands in Northeast China\", as well as the help from all the professors and master's students of Beijing Forestry University.  
**Conflicts of Interest:** The authors declare no conflict of interest.