**5. Conclusions**

Quantifying the variations of interannual CUE among ecosystems has proved to be a useful tool when calculating interannual carbon budgets. However, the intraannual change of CUE may present di fferent characteristics. The assessment of temporal and spatial variations of CUE at shorter time scales and the impact on them of phenological and climatic factors are still poorly understood. This study attempted to reveal spatial patterns of CUE of natural ecosystems at di fferent temporal scales in the SNP, China. The di fferences of CUE between months and seasons were significant. Monthly average CUE showed the highest in October and lowest in May. Average CUE was the highest in autumn, followed by summer. The variability of NPP accumulation in di fferent seasons was significant. The highest CUE values were observed in MF in the growing season, indicating better ecological protection e ffects. The spatial variations of CUE were di fferent. The pixels with rising CUE were mainly concentrated in southwest GRA and eastern DBF. The SOS was generally observed in March and April, while EOS dates were found in October and November. The earlier SOS and later EOS exerted a positive influence on CUE in the SNP, especially in 80% of the areas covered by broad-leaved forest. Longer LOS might cause the increase in CUE. In addition, CUE was positively correlated with precipitation and temperature in most areas of the SNP. Increasing trend of CUE in the SNP suggested a protective barrier function of natural ecosystems in the protected farmland region.

**Author Contributions:** B.L. and F.H. had the original idea for the study and designed the research. B.L. analyzed the data and wrote the paper. F.H. supervised the research and provided significant suggestions. L.Q., H.Q. and N.S. was involved in the data processing and the manuscript reviewing.

**Funding:** This research was supported by the National Natural Science Foundation of China (Grant No. 41571405 and 41571115) and the key project of the National Natural Science Foundation of China (Grant No. 41630749).

**Acknowledgments:** The authors would like to thank the reviewers and editors for their valuable comments and suggestions.

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