## 3 Results
### Impact of Continuous Urbanization Growth on Regional Environment in Beijing-Tianjin-Hebei  
Figure 9 reveals the dynamic coupling relationship between urbanization processes and ecological vulnerability. The long-term time series ISC indicators not only effectively  
Figure 8: Spatiotemporal variations of EVI in four ISC density regions from 2005 to 2020. (**a**,**b**) Low EVI area. (**c**,**d**) Medium EVI area. (**e**,**f**) High EVI area. depict the spatial expansion pathways of urbanization, but also provide a basis for understanding its impact on ecosystem stability and resilience. Figure 9a shows the changing trends of ecological vulnerability in different ISC density areas (high density and low density) from 2005 to 2020. The results indicate that the EVI mean values in high ISC density areas increased significantly (R\({}^{2}\) = 0.7047), growing from approximately 0.39 in 2005 to 0.44 in 2015, followed by a slight decline. This trend may reflect that urban center areas, during rapid urbanization processes, have alleviated ecological vulnerability to some extent through artificial greening and ecological engineering measures. The EVI fluctuations in low ISC density areas were smaller, showing an overall slight decline (R\({}^{2}\) = 0.1077), indicating that as urban peripheries expand outward, the original ecosystems in these areas are gradually being eroded, with ecological vulnerability somewhat intensified. Statistical hypothesis testing revealed significant differences in EVI values between high and low ISC density areas across all study years (\(p\) < 0.001, indicated in Figure 9b), confirming the validity of our density-based classification approach and the distinct ecological vulnerability patterns in different urbanization zones. Figure 9c further reveals the quantitative relationship between impervious surface expansion and ecological vulnerability changes. The figure divides the ISC increment from 2005 to 2020 into several intervals and statistics the corresponding EVI differences within each interval. The results show that as the magnitude of ISC growth increases, EVI change values tend toward the negative direction, with mean values continuously declining. Significant differences were observed between different ISC increment groups (\(p\) < 0.001, Figure 9c), indicating that impervious surface expansion driven by urbanization significantly intensifies ecosystem vulnerability, meaning that the more intense urban development is, the greater the degree of regional ecosystem degradation. Figure 9: Temporal coupling relationship between urbanization process and ecological vulnerability. (**a**) Trends of EVI changes in different density zones. (**b**) Statistical comparison of EVI between high- and low-density ISC areas. (**c**) Boxplot of EVI changes under ISC variation zones. (\(p\) < 0.001, indicated by ***). We further studied landscape pattern changes under different ISC density regions. Forests and grasslands are affected differently by urbanization, and the changes in forests and grasslands in regions with different degrees of urbanization also vary. Two indicators reflect the landscape pattern changes in different density regions during China's urbanization process, showing the changing trends in the quantity and area of grassland and forest patches, particularly the significant increase in average forest area in high-density regions, reflecting the strengthening of urban greening processes. Although the grassland patch density in low-density areas has declined somewhat, it remains the highest, indicating that the natural landscapes in these areas maintain a relatively dispersed pattern. Figure 10 uses two landscape indices, Mean Patch Area (AREA_MN) and Patch Density (PD), to reflect the vegetation patch area and patch fragmentation degree, respectively. Combined with the above-divided ISC density levels from T_1 to T_4, the changes in landscape quality of different urbanization level regions are calculated in the Beijing-Tianjin-Hebei area.