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Rapid urbanization and climate change require stronger microclimatic regulation by urban trees, and maximizing their cooling, humidifying, and shading functions requires an exact understanding of the underlying mechanisms affected by climatic conditions and the forest characteristics. By measuring different aspects of microclimate regulation by urban trees in 165 plots in Harbin city and measuring climatic conditions, tree size, and compositional differences, we define changes in patterns along various urban-rural gradients (ring-road development and urban history) and for different forest types and decoupling the complex associations among them. We found that the horizontal cooling (1.7 degrees C to 4.0 degrees C) was larger than the vertical cooling (-1.71 degrees C to 0.33 degrees C) and soil cooling (0.28 degrees C to 2.17 degrees C); The humidifying effect (RH) ranged from -0.34% to 7.30%, and total radiation intercepted (Delta E) ranged from 11.07 kLux to 45.95 kLux. We also found higher under-branch height, larger canopy, and higher percentage of Ulmaceae, but lower percentage of Salicaceae in more urbanized regions. The relative importance of tree compositions and size on microclimatic regulation was shown using redundancy analysis (RDA), and RDA variation partitioning showed that tree sizes explained 24.7% of the variations in the microclimate regulations, and tree composition and their interactions with climatic conditions explained 9.5% and 25.4% of the variations, respectively. Our findings reveal that maximizing microclimatic regulation by urban forests in northeastern China could possibly be achieved through specific-function-oriented afforestation and an increase conservation of large existing trees, and the data in this paper could favor policy decision of urban forest manager and local administration of urban green infrastructure.