Exploring the paradox of densification and greening in China’ s old cities

Abstract

Urban densification is widely regarded as a key strategy for curbing urban sprawl and optimizing resource efficiency. However, it still threatens urban greenspaces by altering the original land use. The imperative to preserve and restore urban greenspaces has gained increased attention, aligning with the global spotlight on urban resilience. This prompts the crucial question: How will urban densification impact urban greening? Focusing on the old city areas of 159 cities with over one million population in China, we depicted the dynamics of urban greenness using remote sensing data and explored greening trends under urban densification on both horizontal and vertical scales by generalized additive model (GAM). The findings are as follows: (1) As urban horizontal density increased from 68.4 % to 93.1 %, a significant U-shaped trend in urban greenness emerged within the old city areas over the past three decades. Notably, 80 % of these cities experienced turning points before 2010. (2) Almost half of the cities have recovered the urban greenness to 30 years ago, with a growth of 6.58 % on average. While urban densification led to a loss of greenness (-0.0197±0.0182), urban greenspaces countered this with a gain of 0.0049±0.0075, and the original built-up area contributed a gain of 0.0027±0.0284. (3) We decomposed urban densification into horizontal and vertical growth. The horizontal growth coincides with the greenness loss initially, but the rebound occurs when the horizontal density approaches 83 %. Vertical growth, on the other hand, is negatively correlated with urban greenness. Our study depicts the turning point of urban greening in old city areas unveiling the ecological effect of urban densification, which provides a reference for the sustainable development of future urbanization.