*2.2. Field Botanical Surveys*

Compared with other forest parameters, forest diversity is related to spatial variability. Prior to field excursion, one would need to determine what size plots can achieve a stable range of spatial variability. In this study, we used the semi-variogram to determine the investigated plot size, which quantifies the spatial variability due to distance change [2]. Specifically, we calculated the square deviation between adjacent pixel values to test spatial variability with the Sentinel-based NDVI band. Semi-variance gradually increases with the distance between pixels until it starts to level off. Our findings indicated that lag distances of 50 m correspond to the scale for tree species variability in the study area (Appendix A, Figure A1). Thus, the plot size of 50 m × 50 m was identified as optimal in terms of capturing spatial variation in tree species diversity. From June to July 2019, field surveys were conducted. Based on spatial distribution randomness and road accessibility principles, a total of 452 plots were designed; The Global Positioning System was used to record each plot of position. In this study, based on the Chinese Forest Biodiversity Monitoring Network (CForBio) [26], all trees with a diameter at breast height (DBH) greater than 10 cm were identified, while trees with DBH less than 10 cm, shrubs and grasslands were not investigated considering the effect of dense canopy. In addition, the spatial distribution of different forest types was also obtained through the 9th National Forest Inventory of China (2018).

#### *2.3. Data Source and Processing*
