## 3 Results and Analysis
### Spectral Characteristics of Quercus aquifolioides with Different Altitude Gradient
The red edge slope is the maximum reflectance in the red band, which can better reflect the chlorophyll content of plants and is often closely related to the photosynthetic rate of plants [42]. Figure 4: Spectral reflectance ratio of _Quercus aquifolioides_ leaves at different altitudes. From Figures 5 and 6, it can be seen that the general trend of the first derivative spectra of _Quercus aquifolioides_ leaves, at different altitudes, was basically the same, but there were some differences in values. As shown in Figure 6a, the red edge position and the red valley position of the leaf surface spectral curve move toward the short wave direction as the altitude increases. This showed that, with the increase in altitude, the effect on the position of red edge and red valley on leaf surface became more severe. However, the yellow edge position and the green peak position move to the long wave direction first and then to the short wave direction with the elevation. As shown in Figure 6b, with the elevation increasing, the water stress band reflectance and green peak reflectance first increased and then decreased, while the red valley reflectance increased. The slope of red edge, yellow edge and blue edge all decreased with the elevation. As can be seen from Figure 6c, with the elevation increasing, the red edge area first decreases and then increases, the yellow edge area decreases and the blue edge area increases. Previous studies generally believed that chlorophyll is one of the important parameters in determining the characteristics of plant spectral reflectance curves. When the vegetation is in a healthy growth state and has a high chlorophyll content, the red edge position generally moves towards the long wave direction. However, when vegetation is subjected to external environmental stress, such as drought stress, high temperature stress or pest damage, the red edge position tends to move towards short wave direction [41; 42]. In this study, the red edge of _Quercus aquifolioides_ showed a decreasing trend with the elevation. This just proves the indication effect of red edge position on chlorophyll content. \begin{table}
\begin{tabular}{c c} \hline \hline
**Spectral Parameters** & **Definition** \\ \hline Red edge position, REP & The wavelength position corresponding to RES \\ Red edge slope, RES & The largest first-order derivative value in the red edge (680–750 nm) \\ Blue edge position, BEP & The wavelength position corresponding to BES \\ Blue edge slope, BES & The largest first-order derivative value in the blue edge (490–530 nm) \\ Yellow edge position, YEP & The wavelength position corresponding to YES \\ Yellow edge slope, YES & The largest first-order derivative value in the yellow edge (560–640 nm) \\ Red valley position, RVP & The wavelength position corresponding to RRV \\ Reflectance of red valley, RRV & Minimum reflectance in the wavelength range of 640–700 nm \\ Green peak position, GPP & The wavelength position corresponding to RGP \\ Reflectance of green peak, RGP & Maximum reflectance in the wavelength range of 510–580 nm \\ Reflectance of water stress band, RWSB & Maximum reflectivity in the wavelength range of 1550–1750 nm \\ Red edge area, REA & Sum of first derivative in red edge (680–750 nm) \\ Yellow edge area, YEA & Sum of first derivative in yellow edge (490–530 nm) \\ Blue edge area, BEA & Sum of first derivative in blue edge (560–640 nm) \\ Leaf chlorophyll index, LCI & (R\({}_{850}\)–R\({}_{710}\))/(R\({}_{850}\)–R\({}_{850}\)) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of spectral parameters. Figure 5: The first derivative spectral curves of the _Quercus aquifolioides_ leaves at different altitudes. Figure 6: Spectral parameters of _Quercus aquifoloides_ leaves at different altitudes. (**a**) spectral position, (**b**) spectral reflectance, (**c**) spectral area parameter.