Distinct dominant factors of spatially heterogenous urban tree cooling identified in cities across China

• High-resolution data and nonlinear fitting reveal spatial heterogeneity of cooling. • Quantified marginal cooling effect of urban trees under varying tree cover conditions. • 75 % of cities show stronger tree CE in both low and high tree cover areas. • Natural factors outweigh human influence on tree CE at 70 % tree cover percentile. • Building height and air temperature impact CE differently across tree cover levels. Trees are recognized as a pivotal nature-based solution for mitigating urban thermal stress. Urban tree cooling efficiency (CE), defined as the temperature reduction per 1 % increase in tree coverage percentage (TCP), serves as an effective metric to optimize the cooling benefits of trees given limited urban spaces. Despite vast literature on CE of urban trees, however, the spatial heterogeneity of CE within and across cities remains poorly understood. To address this gap, this study proposed a unified framework to quantify CE as a function of TCP. We used land cover dataset with high resolution and Landsat land surface temperature (LST), coupled with a nonlinear modeling approach to assess CE across 302 Chinese cities. The results reveal a U-shaped pattern of CE as TCP increases within a city: areas with low and high tree coverage exhibit notable CE, while CE diminishes in areas with moderate tree cover. This U-shaped pattern is observed in over 75 % of the studied cities, with over 80 % in Eastern and Southwest China, while a lower percentage of 40% is shown in Northern China. CE variations across cities are driven by different factors as TCP changes. In areas with less than 70 % tree cover, human-induced factors of the cities such as socioeconomic development and urban morphology dominate. Whereas, in areas with higher tree cover, natural factors including air temperature, precipitation and urban greenness play a more significant role. Among these factors, the impacts of building height and air temperature on CE differ significantly between high-TCP areas and medium-to low-TCP areas. Our findings provide novel insights into the marginal cooling effect of increasing urban trees at varying tree cover levels and the dynamic interplay between anthropogenic environment and local climate. These insights provide valuable guidance for urban planning and management strategies aimed at promoting thermally sustainable cities.