**5. Conclusions**

This study has effectively estimated and mapped the distribution of FSV in the Helan Mountains, with a resolution of 30 m. Utilizing the RF algorithm in conjunction with data from Sentinel-2, the study has affirmed the potential of *NDVIRE* in FSV estimation. Among all modeled variables, the novel vegetation index *NDVIRE*, constructed based on the three red-edge bands of Sentinel-2, contributed the most to predicting FSV. Furthermore, the BVBM performed the best among the three models based on the two variables of the band and vegetation index. Finally, this study would assist policymakers in designing forest conservation and management paradigms that could potentially support the sustainability and carbon sequestration dynamics in the Helan Mountains and other montane forest ecosystems.

**Author Contributions:** Study design: Y.H. and J.W.; Data curation: T.M.; Investigation, D.P., L.C. and X.N.; Methodology: Y.H. and X.L.; Software, T.M.; Supervision, Y.H.; Writing: T.M.; Writing: review & editing: Y.H. and M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Key Project of Research and Development of Ningxia, China (2021BEB04061, 2022BEG03050), the National Natural Science Foundation of China (32101524), and the National Natural Science Foundation of Ningxia, China (2021AAC03017).

**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to funder regulations.

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