Spatiotemporal dynamics and coupling coordination between urban expansion and carbon storage in the chengdu-chongqing urban agglomeration

Urban expansion, a prominent feature of global land-use change, profoundly influences terrestrial carbon dynamics and regional ecological security. Under the framework of green and low-carbon development, understanding the multiscale spatiotemporal coupling between urban expansion and carbon storage is vital for informing sustainable urbanization strategies. Taking the Chengdu-Chongqing urban agglomeration in western China as a representative case of rapid urbanization, this study integrates multitemporal remote-sensing data from 2000 to 2020 to investigate the dynamic patterns and interactions between urban growth and carbon storage. Carbon storage was quantified by integrating the InVEST model with the biomass expansion factor (BEF) method, which replaces the uniform class-mean carbon densities used in conventional InVEST applications with stock-volume-derived, province-specific forest biomass densities. The results indicate that: (1) built-up area increased from 3,140.3 km² in 2000 to 15,550.5 km² in 2020. After a slight slowdown during 2005–2010, absolute expansion accelerated markedly and reached its highest rate during 2015–2020, while the spatial pattern remained dominated by Chengdu and Chongqing; (2) regional carbon storage showed an overall increasing trend with significant spatial heterogeneity, with high-value zones primarily located in the northeastern and southwestern parts of the study area; (3) the coupling coordination degree (CCD) between urban expansion and carbon storage exhibited scale-dependent trajectories: the city-scale mean increased slightly from 0.368 in 2000 to 0.394 in 2020, whereas the county-scale mean decreased continuously from 0.557 to 0.490, with high-CCD counties concentrated in the forest-dominated periphery and low-CCD counties in the urbanized cores. These results provide an empirical basis for balancing urban development and ecological conservation in rapidly urbanizing regions.