A study of the cooling effect of urban trees: Influencing factors, assessment methods, planning strategies, and impacts

Global climate change and urbanization are causing temperatures to rise in cities, and the resulting heat island effect is becoming increasingly challenging. Urban trees play an essential part in a city’s ecosystems by offering a cooling impact through shading and transpiration; this results in reduced urban temperatures, increased thermal comfort for walkers, and an improved urban microclimate. This study integrates the cooling mechanisms, assessment methods, and socio-environmental effects of urban trees, proposing a comprehensive framework encompassing biophysical mechanisms, assessment models, and planning strategies to bridge the gap between research and practice. This review follows the PRISMA systematic review framework and synthesizes 128 eligible studies on the cooling effects of urban trees. Results show that trees reduce air temperature by approximately 0.5–5.8 °C and surface temperature by 2–12 °C, depending on canopy density, climate type, and measurement method. Morphological traits such as leaf area index (LAI > 3) and canopy density were the strongest predictors of cooling in humid and temperate climates, while physiological processes including transpiration enhanced cooling effectiveness under arid conditions. These findings provide quantitative thresholds and a conceptual framework that may support climate-adaptive urban planning and design.