Quantifying environmental trade-offs of urban form-function compactness

Abstract Compact urban form is increasingly embedded in global sustainability agendas, yet its environmental trade-offs are still poorly understood. Previous research has mainly emphasized physical form and single outcomes, neglecting functional dimensions and potential ecological trade-offs due to the lack of fine-grained data. Here, we introduce a multi-source data fusion-based urban functional compactness (UFC) indicator and integrate it with urban physical compactness (UPC) into a form-function coupling framework. Using structural equation modeling across 176 Chinese cities, we reveal dual impacts: stronger UFC enhances land use efficiency and public transport performance but is associated with reduced green space provision and weaker waste treatment. A 1% increase in UFC raises infrastructure utilization efficiency by about 2% while also indirectly elevating environmental pollution by 0.74%. In contrast, a 1% increase in UPC directly enhances water conservation by 3% and reduces energy consumption by 6% but indirectly increases pollution impacts by 2.59%. These findings challenge the assumption that compactness is inherently green, demonstrating instead that form–function coupling produces divergent pathways: efficiency gains coexisting with amplified ecological risks.