## 3 Results and Analysis
### Spectral Characteristics of Quercus aquifolioides with Different Altitude Gradient  
As shown in Figure 3, the trend changes of leaf spectral reflectance curves of _Quercus aquifolioides_ were generally consistent under different altitude gradients. In the visible light band range (350-700 nm), there are obvious differences in spectral reflectance. In the range of 350-700 nm, the reflectivity of each band of 3500 m is greater than 3300 m and the reflectivity of 2905 m. As shown in Figure 4, their maximum reflectance ratios were 1.39, and 1.26, respectively. In the range of 350-500 nm, the reflectance of 3300 m was greater than that of 2905 m, and the maximum ratio of reflectance was 0.95. In the range of 500-700 nm, the reflectivity of 2905 m was greater than that of 3300 m, and the reflectivity ratio was 1.14 at the maximum. In the near infrared band (750-1100 nm), the reflectivity of 2905 m was higher than that of 3500 m and 3300 m, and their maximum reflectivity ratios were 1.56, and 1.75, respectively. The closer the reflectance ratio was to 1, the smaller the difference between them was. In the range of 350-1800 nm, there were 4 main reflection peaks and 5 main absorption valleys in the leaf surface spectral reflection curve of _Quercus aquifolioides_, and their positions were basically the same. The reflection peaks were located at 550 nm, 1096 nm, 1273 nm, and 1656 nm, respectively, and the absorption valleys were located at 350-500 nm, 600-750 nm, 950-1050 nm, 1200-1250 nm, and 1400-1500 nm, respectively. As shown in Figures 3 and 4, the spectral reflectance of _Quercus aquifolioides_ leaves at different altitudes in the visible light to near infrared band (350-1800 nm) was the most distinguishable in the range of 750-1400 nm, indicating that this band was the sensitive range of spectral response of plants to different altitudes. In addition, we can see that the slope of the spectral reflectance curve of _Quercus aquifolioides_ leaves has a sharp increase trend in the range of 700-780 nm. Studies have shown that the phenomenon of plants increasing suddenly in this wave band belongs to the typical \"red edge effect\" characteristic of plants [16; 33; 34]. Figure 3: Reflectance spectral characteristics of _Quercus aquifolioides_ leaves at different altitudes. Dynamic Changes of Spectral Characteristic Parameters of Quercus aquifolioides at Different Altitudes  
Research shows that the original spectrum can eliminate the influence of background noise on the spectrum, and reduce the scattering and absorption of light by atmosphere in the process of spectrum acquisition, after first-order differential processing [35; 36; 37]. Representative hyperspectral characteristic parameters selected in this study were shown in Table 1[38]. The \"trilateration\" parameter is an important in the first derivative spectrum, and is one of the hot spots and focuses in hyperspectral research in recent years. Among them, the red edge is the interval in which the reflection spectrum of plants suddenly changes obviously in the red light band (680-750 nm), which is the most significant mark in the spectrum curve, and is widely used as the characterization parameters of plant growth, leaf water content, nutritional status, chlorophyll content and other indicators [39]. Red edge includes two important concepts: the position of red edge and the other is the slope of red edge. The red edge position is the wavelength position corresponding to the maximum reflectance in the red wavelength band. It represents the degree of stress of plants to the external environment well [39; 40; 41]. For example, under the conditions of environmental pollution, disease threat, drought stress and so on, its position tends to shift to a certain extent. Generally speaking, there are two main directions for the shift of the red edge position, one is to shift towards the long wave direction, while the other is to shift towards the short wave direction.