Temperature Regulation of the Surface Cooling Rate of Urban Trees under Climatic Extremes

The conversion from natural to urban landscapes reshapes the land–atmosphere interactions, biogeochemical cycles, and ecosystems in cities, inducing numerous environmental risks. As an effective mitigation strategy, urban trees have been used extensively for providing multiple ecological services, mainly functioning as a natural cooler to confront the heat stress. Urban trees differ from natural forests in physiology and phenology due to their close interaction with the ambient built environment. Unlike their natural counterparts, the cooling capacity of urban trees in response to thermal extremes remains unclear at macroscale. Here we present the surface cooling rate of urban trees in cold and heat waves for major metropolitan areas in the contiguous United States based on remotely-sensed imagery. The surface cooling rate is expressed as the negative ratio of land surface temperature (LST) changes to fractional tree cover (FTC) changes, i.e. –ΔLST/ΔFTC. Results show that the environmental temperature strongly regulates the cooling effect of urban trees: the cooling rate is up to 1.3 °C/%FTC at extreme high temperatures, but remains marginal during cold spells. The observed cooling rate is dominated by transpiration of urban trees in heat waves, and can be well captured by the relation of liquid water-vapor equilibrium. In addition, large urban forests cool the environment more consistently and stably than isolated trees in built terrains. With predicted increases in probability and intensity of heat waves in future climate, the enhanced cooling capacity of urban trees will make an attractive means of sustainable urban development.