## 3 Results and Analysis
### Analysis on the Overall Change Trend of Ecological Quality  
As shown in Table 4, the MRSEI values for the Lhasa-Nyingchi Motorway segment from Nyingch to Gongbo'gyamda in the years 2012-2020 are as follows: 0.5885, 0.5951, 0.5296, 0.6202, 0.59, 0.5777, 0.5898, 0.5703, and 0.5987. Overall, compared to before the construction of the highway in 2012, the MRSEI value in 2020 has slightly increased, but there is significant fluctuation in the MRSEI values each year. The MRSEI values showed a noticeable decrease from 2013 to 2014, which is closely related to highway construction. The MRSEI values then increased significantly from 2014 to 2015, mainly due to the implementation of ecological restoration projects. From 2015 to 2017, there was a slow decline in the MRSEI values, but from 2017 to 2020, the MRSEI values showed a stair-step growth trend, indicating that the ecological restoration work was challenging but that the ecological environmental quality was improving overall. Figure 2 shows the spatiotemporal changes in the ecological environmental quality of the study area. The MRSEI values were divided into five categories: poor, fair, moderate, \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **2012** & **2013** & **2014** & **2015** & **2016** & **2017** & **2018** & **2019** & **2019** \\ \hline MRSEI & 0.5885 & 0.5951 & 0.5296 & 0.6202 & 0.59 & 0.5777 & 0.5898 & 0.5703 & 0.5987 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: MRSEI from 2012 to 2020. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Index**} & \multicolumn{4}{c}{**2012**} & \multicolumn{4}{c}{**2020**} \\ \cline{2-7}  & **PC1** & **PC2** & **PC3** & **PC1** & **PC2** & **PC3** \\ \hline FVC & 0.847 & \(-\)0.201 & \(-\)0.066 & 0.858 & \(-\)0.241 & 0.056 \\ LAI & 0.909 & \(-\)0.005 & \(-\)0.002 & 0.894 & \(-\)0.134 & \(-\)0.128 \\ GPP & 0.86 & \(-\)0.095 & 0.25 & 0.882 & \(-\)0.103 & \(-\)0.24 \\ Wet & 0.081 & 0.806 & 0.572 & 0.226 & 0.88 & \(-\)0.404 \\ LST & 0.293 & 0.65 & \(-\)0.693 & 0.496 & 0.439 & 0.745 \\ Eigenvalues & 2.375 & 1.123 & 0.874 & 2.61 & 1.053 & 0.794 \\ Variance Contribution & 47.5 & 22.454 & 17.479 & 52.201 & 21.059 & 15.884 \\ Rate (\%) & 47.5 & 69.954 & 87.433 & 52.201 & 73.26 & 89.144 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of PCA in 2012 and 2020. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**2012**} & \multicolumn{3}{c}{**2020**} \\ \cline{2-7}
**Index** & **OW** & **SW** & **CW** & **OW** & **SW** & **CW** \\ \hline FVC & 0.1750 & 0.3178 & 0.2412 & 0.1884 & 0.3178 & 0.2575 \\ LAI & 0.2367 & 0.1761 & 0.2086 & 0.1890 & 0.1761 & 0.1821 \\ GPP & 0.2477 & 0.1761 & 0.2145 & 0.1752 & 0.1761 & 0.1757 \\ Wet & 0.2570 & 0.165 & 0.2143 & 0.1439 & 0.165 & 0.1552 \\ LST & 0.0836 & 0.165 & 0.1214 & 0.3036 & 0.165 & 0.2295 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Index weight in 2012 and 2020.  
good, and excellent, with intervals of 0.2. It can be observed that in 2015, the ecological environmental quality was rated as good, while the rest of the years were rated as moderate. In terms of the spatial distribution of the MRSEI values in the nine periods, the ecological environmental quality gradually decreased from the eastern segment to the western segment. The areas with good and excellent ecological quality were mainly located in the southeastern part of the study area, where the humidity and temperature conditions were favorable and the vegetation coverage was relatively high. The areas with poor, fair, and moderate ecological quality were primarily located in the road area and the peripheral areas of the central and western segments. This is mainly due to the higher altitude and colder, drier climate in these areas, resulting in relatively lower vegetation coverage. The relatively poor ecological environmental quality in the western segment may also be related to the denser distribution of local residential areas.