Critical thresholds in urbanization-vegetation dynamics: Multiscale remote sensing evidence from Chinese cities

Urban vegetation greenness underpins essential ecosystem services, yet its spatial patterns are being profoundly reshaped by rapid urbanization. However, the nonlinear interactions and multiscale threshold effects between urban expansion and vegetation dynamics remain underexplored. Here, we integrated multi-source remote sensing data (MODIS NDVI and Landsat impervious surfaces) across 370 Chinese cities from 2001 to 2022 to quantify human-induced vegetation trends (Sen_HNDVI) and their responses to urbanization intensity (impervious surface proportion, β). Results reveal a significant national greening trend (+0.009 per decade, p < 0.05), with pronounced regional disparities: widespread improvement in northwestern regions but degradation in eastern coastal zones. Along the rural-urban gradient, vegetation trends shifted from positive in rural areas to negative in suburbs, followed by partial recovery in urban cores, highlighting strong spatial heterogeneity. Our analysis reveals a predominant "V-shaped" relationship, with a national-scale recovery threshold occurring at an urbanization intensity of β ≈ 0.88. This threshold is scale-dependent, appearing at lower intensity levels in higher-tier cities, suggesting earlier ecological recovery potential in more developed urban areas. Furthermore, we identified four distinct urbanization-vegetation interaction patterns: persistent degradation, gradual recovery, stabilization, and significant improvement. These findings uncover critical thresholds in urban ecosystems and highlight the inherently nonlinear, scale-dependent nature of urbanization-vegetation dynamics, providing a robust scientific basis for integrating ecological thresholds into urban planning and sustainable development strategies.