Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait

Abstract. Rapid increases in heat exposure in urban areas, fueled by both climate change and urban heat islands (UHI), are manifesting as a pressing concern. Planting and conserving urban trees is one of the pivotal strategies in mitigating outdoor heat and optimizing thermal comfort. We present an integrated review and meta-analysis of 131 studies conducted in the past 13 years, investigating the cooling effects of trees across 15 climate types in 85 global cities or regions. The cooling efficacy of trees is mainly determined by interconnecting urban morphology, tree traits, and, critically, the prevailing background climates. Our meta-analysis reveals that the cooling effects of urban trees observed in hot climates are significant due to low latitudes, along with their substantial solar radiation blockage and pronounced transpirational cooling. Moreover, an optimal level of transpirational cooling can be achieved at relatively lower humidity levels. However, in tropical and arid climates, extreme conditions involving high temperatures and vapor pressure deficits may trigger stomata closure in leaves, thereby impeding transpirational cooling. Our review further underscores the guiding principles of optimizing urban morphology by arranging buildings and trees, as well as selecting suitable tree species according to their traits to enhance the cooling effects of trees in different climates. The cooling effects of trees demonstrate a nonlinear increase in correlation with higher leaf area index (LAI), leaf area density (LAD), tree canopy coverage, and, inversely, a lower sky view factor (SVF). This systematic review and meta-analysis serve as a critical resource for researchers, urban planners, and policymakers striving to mitigate urban heat by strategically using urban trees.