Study on the Trade-Off and Synergy Between Urban Polycentric Structure and Ecological Environment Quality in the Yangtze River Delta Region, China

The dynamic interplay between urban polycentric structure and habitat quality profoundly shapes the pathways and outcomes of urban sustainable development. Based on the nighttime light index and economic aggregate data of 289 county-level units in the Yangtze River Delta region (2008–2023), this study identifies a polycentric urban structure and measures the polycentricity index of 41 prefecture-level cities. An ecological environment quality evaluation index system is constructed to analyze their influencing factors, followed by an exploration of the trade-off and synergy relationship between the two systems. The main findings are: (1) Both the nighttime light-based urban structure index and economic aggregate index exhibited a circularly decreasing pattern centered on Shanghai. (2) The “nighttime light–economic aggregate” polycentricity index showed a significant spatial “point–core” distribution, with gradually expanding outward diffusion over time. (3) The ecological environment quality achieved significant overall improvement, with better conditions in the southeast than the northwest. (4) Pearson correlation analysis confirms a significant positive correlation between a polycentric urban structure and ecological environment quality; the relationship is categorized into four types: strong trade-off, weak trade-off, weak synergy, and strong synergy. This study provides solid theoretical support and scientific decision-making guidance for urban future development planning and ecological protection practices.