## 4 Discussion  
### Applying Vegetation Indices to Extract Suaeda salsa from UAV Imagery  
To assess the spectral separability between _Suaeda salsa_ and background vegetation dominated by _Phragmites australis_ (reeds), NDVI and RVI values were calculated from red and near-infrared bands acquired by a UAV-mounted multispectral sensor, while the SVI (_Suaeda salsa_ Vegetation Index, defined as SVI = green \(-\) (blue + red)/2) was derived from RGB imagery captured by a visible-light camera. Approximately 400,000 pixels of _Suaeda salsa_ and 600,000 pixels of reeds were randomly sampled from their respective distribution areas for spectral analysis.  
As shown in Figure 10a, the NDVI values of both _Suaeda salsa_ and reeds exhibited a clear normal distribution. However, the value ranges largely overlapped, making it difficult to reliably differentiate the two species using the NDVI alone. Specifically, NDVI values for _Suaeda salsa_ ranged from 0.00002 to 0.44736 (mean = 0.22369), while those for reeds ranged from 0.00006 to 0.48732 (mean = 0.24369). A similar pattern was observed in the RVI results (Figure 10b), with _Suaeda salsa_ values ranging from 1.00005 to 2.61901 (mean = 1.80953) and those for reeds ranging from 1.00013 to 2.90110 (mean = 1.95061). These results indicate that the NDVI and RVI alone are insufficient for reliably distinguishing _Suaeda salsa_ from reeds.  
The SVI, as a vegetation index specifically designed for identifying _Suaeda salsa_, is typically used after the NDVI has been applied to extract all vegetation pixels and serves to further separate _Suaeda salsa_ from other vegetation such as reeds [8]. In this study, SVI values for _Suaeda salsa_ ranged from \(-\)0.07115 to 0.00404 (mean = \(-\)0.03356), whereas those for reeds ranged from \(-\)0.03273 to 0.03903 (mean = 0.00315), clearly higher than those of _Suaeda salsa_ (Figure 10c). While the SVI offers some ability to distinguish between the two vegetation types, the presence of overlapping regions and the proximity of peak values reduce classification accuracy.  
In contrast, hue angle values exhibited a more distinct separation between _Suaeda salsa_ and reeds (Figure 10d). The peaks were well separated, with minimal overlap, suggesting  
Figure 10: Pixel counts for _Suaada salsa_ and reeds using (**a**) NDVI, (**b**) RVI, (**c**) SVI, and (**d**) hue angle.  
superior classification performance. The hue angle values for _Suueda salsa_ ranged from 217.4529 to 278.181 (mean = 247.817), while those for reeds ranged from 206.2238 to 254.7813 (mean = 230.5025). Further analysis suggests that although the selected regions had high vegetation coverage, small gaps of exposed soil were inevitable. The minor overlap observed in Figure 10d is likely due to soil pixels within both sampling regions, which can be effectively excluded during hue angle-based classification through thresholding. In contrast, the overlap observed in Figure 10c may be partially due to soil pixels but primarily results from the spectral similarity between _Suueda salsa_ and reeds.  
In summary, the NDVI and RVI fail to provide effective differentiation between _Suueda salsa_ and reeds. Although the SVI demonstrates improved performance, it still suffers from overlapping distributions and close peak values. Hue angle offers the most distinct separation, with the widest gap between distribution peaks and the smallest overlapping region, making it the most effective indicator for distinguishing _Suueda salsa_ from reeds in this study.