# Monitoring Forest Diversity under Moso Bamboo Invasion: A Random Forest Approach
## 4 Discussion  
### Importance of Variables  
The selection of appropriate variables for remote sensing is crucial for estimating forest diversity. Variables used as input parameters before modeling can be classified into three categories: spectral indices, vegetation indices, and topographic features. Here, we used RF importance analysis to identify the optimal variables for remote sensing. Four new bands (B5, B6, B7, and B8A) in the red-edge and near-infrared regions provide unprecedented spectral features that are highly sensitive to biophysical and biochemical responses of vegetation, which are essential for mapping vegetation features [50]. However, in this study, the classical and shortwave infrared bands (B2, B3, B11, and B12) also showed a critical role in addition to the B5 band. The superior performance of the B2 and B3 bands was correlated with chlorophyll or leaf nitrogen concentration. A previous study has reported a strong correlation between visible light bands and chlorophyll or leaf nitrogen concentration [51]. In particular, the 440 nm and 573 nm reflectances are most closely related to leaf nitrogen concentration [52]. The near-infrared band of the electromagnetic spectrum is most strongly reflected by vegetation, but it does not reveal any details about the underlying soil, whereas the short-wave infrared bands (B11, B12) can distinguish  
Figure 3: Agreement between predicted and observed values of diversity indices. Abbreviations: R\({}^{2}\) = coefficient of determination, RMSE = root mean squared error, MAE = mean absolute error.  
between vegetation and soil [14]. Theoretically, the reflectance in the short-wave infrared bands (B11, B12) is predominantly affected by leaf water content [15; 29], and around 1450 nm and 1950 nm are most pronounced [53]. According to previous studies, these absorption bands saturate at higher water contents, and the intermediate absorption region around 1650 nm and 2200 nm is sensitive to differences in leaf water content [54; 55]. This coincides with the B11 and B12 bands. Therefore, the observation that B2, B3, B5, B11, and B12 have high importance for biodiversity is plausible.  
CRI1 is an important vegetation index used in this study. It has been found that the reciprocal of spectral reflectance near 510 nm is the most sensitive to carotenoid (Car) content, but chlorophyll (Chl) also affects these spectral bands; therefore, to eliminate the effect of Chl on spectral reflectance near 510 nm, 550 nm was chosen to establish CRI1 for assessing Car content [30]. Previous studies have shown that Car is associated with plant responses to environmental stresses [56].  
In this study, we used slope, slope direction, and elevation terrain feature variables. All three variables showed high importance according to the RF importance analysis. Topography, including slope, aspect, and elevation factors, has a significant effect on species composition in subtropical forests [57]. In addition, the combination of slope and aspect affects plant invasion [58; 59]. In this study, the invasion of Moso bamboo differed significantly at different elevations; Moso bamboo proportion gradually decreased with increasing elevation, which has also been confirmed previously [60]. Thus, topographic factors influence biodiversity in the study area.