**5. Conclusions**

Our study is one of very few to investigate the utility of VHR satellite imagery for monitoring post-fire forest structural attributes (specifically tree sapling regeneration) in terms of LAI and TSA. We found that WorldView-2 VHR imagery outperforms Landsat imagery but still has some limitations. In particular, the high price of VHR imagery as well as the cost of associated computing resources may limit its practical usage. However, this may change in the future as costs are reducing for both UAV Photogrammetry and the launch of VHR satellites, potentially increasing accessibility by those managing forest resources [91,92], including the monitoring of post-fire forest recovery [93]. Our study shows that spectral variations can provide more useful information than image textures for the retrieval of parameters of tree saplings. Tree saplings provide very similar spectral signatures to surviving mature trees in our ecosystem, but this limitation may be less significant in other ecosystems exhibiting vegetation phenology differences or other unique characteristics that can be reflected in VHR imagery. Recognizing surviving mature trees in post-fire landscapes is of critical significance for quantifying post-fire landscape resilience. Our work suggests that shadow analysis can provide useful information for identifying surviving trees and this method merits further study as quick post-fire assessment of surviving trees through VHR imagery may provide useful information for forest managers, such as identifying locations where artificial regeneration may be needed.

Given that seed banks of coniferous tree species are easily damaged, and their seed dispersal distances are relatively short, our results suggested that combined natural forest restoration and proper human help (i.e., aerial seeding) for coniferous forests is necessary in our study area, especially following wildfires that burned severely across large geographic areas. In these areas, northern slopes and valley bottoms where solar radiation is low may require extra attentions from forest managers. While our work also suggests that understory plants can inhibit tree sapling recovery, a more thorough assessment of the effects of understory plants (e.g., soil and water retention) is required for ecological sound design and planning to assist land managers in making decisions about where across the landscape to prioritize reforestation efforts. Decreasing severity and size of wildfires through fuel managemen<sup>t</sup> is known to improve the resilience of forest ecosystems [94–96]. Although edge effects did not exhibit a strong effect in our study due to continuous distribution of this high severity fire [26], islands of unburned patch may play important roles in accelerating natural forest recovery post-fire.

**Author Contributions:** L.F. and J.Y. conceived the research design; L.F. and Y.Y. (Yuanzheng Yang) conducted the field data collection; L.F., E.C., J.Y. and Z.L. conducted analysis and wrote the manuscript. L.F. and Y.Y. (Yan Yan) conducted the analysis of satellite data.

**Funding:** This study is financially funded by the National Key R&D Program of China (2017YFA0604403), the National Natural Science Foundation of China (Project No.31500387, 31470517 and 31800395), and CAS Pioneer Hundred Talents Program.

**Acknowledgments:** The authors appreciate Yongzhi Liu, Hongxin Zhang, Hailong Zhang and Changhe Hu, who are working for the administrative agencies of Huzhong Nutural Reserve and Huzhong Forestry Bureau, for their assistance during field data collection. We appreciate two anonymous reviewers for comments that improved this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; the collection, analyses, or interpretation of data; the writing of the manuscript; or the decision to publish the results.
