*4.3. Spatial Variability of the Predicted Diversity Indices*

Based on the four regression models, we plotted the spatial variation of diversity indices and predicted variables within forests. The spatial distribution of the three diversity indices for the RF result is displayed in Figure 4, while the other results are displayed in Appendix A, Figure A3. Visually, the predicted maps show strong spatial agreements between the *H* and *J* indices, which are negatively related to the λ index in most parts of a forest. The *H* and λ indices account for species richness (i.e., number of different species) and abundance (i.e., number of individual trees per species), while *J* index accounts for species evenness (i.e., the numerical dominance of a few abundant tree species). Generally speaking, forest diversity was higher in the north than in the south, especially in the northeast. It is worth noting that the forest diversity of the sparse woods in the southwest area is significantly lower than that of other regions.

**Figure 4.** Predicted maps and pixel statistics of forest diversity by the three indices (**a**) λ index, (**b**) *H* index, (**c**) *J* index using the RF model.

There are notable differences in tree species diversity according to the various forest types obtained by the 9th National Forest Inventory of China (2018). The diversity of the secondary forest regions (the right part of Figure 5a) could be easily distinguished from the natural forests based on predicted variables (Figure 5b–d). However, the performance varies amongst the three indices. Compared with other regions, the diversity of areas along rivers and roads did not significantly differ, but the *J* index along rivers expressed relatively low values (Figure 5f–h,j–l). Although the best prediction results were obtained by testing four regression models, we found that a single indicator does not adequately characterize diversity. For example, on the right side of the road in Figure 5f–h, there are significant differences in the three diversity indices, which forced us to obtain a more comprehensive assessment of diversity.

**Figure 5.** Zoom-in examples of true color Sentinel-2 images (RGB = bands 4, 3, 2) and forest diversity predictions under different forest environments. Sentinel-2 image (**a**) contains two forest types, secondary forest (right) and natural forest (lift); Images (**e**,**i**) show natural forests traversed by rivers and roads. The (**b**–**d**), (**f**–**h**), and (**j**–**l**) indicate three diversity index results corresponding to images (**a**,**e**,**i**).
