## 3 Results and Analysis
### Analysis on the Overall Change Trend of Ecological Quality
The area and proportion of the ecological environmental quality in the study area for the nine periods were statistically analyzed (Table 5, Figure 3). It can be observed that the proportion of good ecological environmental quality was the largest, followed by the moderate category, from 2012 to 2020. The ecological environmental quality structure in 2012-2013 was similar, with the most significant variation occurring in 2014; the proportions of poor, fair, and moderate ecological quality were the highest in the nine-year period, and the MRSEI value was the lowest. Although the MRSEI value in 2015 was the highest in the nine years, there was no significant change in the area of good ecological quality compared to the previous year, and the area of moderate quality was the lowest in the nine years, decreasing by 9.72% compared to 2014. It is worth noting that in 2015, the area of excellent ecological quality reached 21.48%, which was nearly 10% higher than the average proportion of excellent quality in the other seven years, excluding 2014. This can be attributed to the construction period of the highway from 2013 to 2015, with full construction taking place in 2014, stripping the original vegetation and damaging the ecological environment along the route, resulting in a sudden decrease in the MRSEI value in 2014. However, in 2015, thanks to the smooth implementation of the highway ecological environment project, the MRSEI value rapidly increased. After the construction was completed, the construction facilities were removed, construction waste was cleared, depressions were leveled, and the original vegetation and turf were replanted, leading to a  
Figure 2: Spatial distribution of MRSEI values of the Nyingchi–Gongbo’gyamda section of the Lhasa–Nyingchi Motorway from 2012 to 2020.  
significant improvement in the overall ecological environmental quality in a short period of time. Clearly, the ecological environmental quality in the road area and the peripheral areas of the study area has been significantly improved. The proportions of excellent, good, and moderate levels showed a declining trend in 2016 and 2017, indicating a slight rebound in the ecological restoration effect, mainly due to the higher requirements for transplanting vegetation in the high-altitude ecological environment. The ecological environmental quality structure in 2018 was similar to that in 2017, with similar proportions of excellent and good levels, but there was a noticeable increase in the proportion of excellent quality areas in 2018. Looking at Figure 2, it can be further observed that these increased areas were mainly located in the southern part of the eastern segment of the study area. Compared to 2018, the MRSEI value decreased in 2019, and the area of excellent quality decreased by 6.12%. Although there was a slight increase in the proportion of good-quality areas, the combined proportion of excellent and good levels was lower than in 2018. In 2020, the overall ecological quality level was good, with a 1.01% decrease in the area of excellent quality compared to 2012, while the area of good quality increased by 5.46% and had the highest proportion in the nine years. The area of moderate quality decreased by 2.74%, and the combined area of poor and fair levels was the smallest in the nine years. In conclusion, the ecological environmental quality in the study area in 2020 did not show significant changes compared to before the construction of the highway. Figure 3: Changes in the proportion of MRSEI levels in the Nyingchi-Gongbo’gyamda section of the Lhasa–Nyingchi Motorway from 2012 to 2020. \begin{table}
\begin{tabular}{r r r r r r r r r r} \hline \hline  & \multicolumn{2}{c}{**Excellent**} & \multicolumn{2}{c}{**Good**} & \multicolumn{2}{c}{**Moderate**} & \multicolumn{2}{c}{**Fair**} & \multicolumn{2}{c}{**Poor**} \\ \cline{2-10}  & **S/km\  
Furthermore, the sums of the proportions of good and excellent quality (GE%) were separately calculated.