Temporal variations in microclimate cooling induced by urban trees in Mainz, Germany

•Spatial temperature variations are compared with transpiration in a mid-sized city.•Urban tree transpiration significantly enhances microclimate cooling.•Nocturnal transpiration induced strong cooling due to limited air mixing at night.•Transpiration cooling effect strongest in an enclosed courtyard environment.

Global warming is likely to increase the frequency and magnitude of heat waves. As the urban geometry and material amplifies warming, city dwellers will face an intensification of heat-induced health problems and mortality. Although increased vegetation cover is frequently used in urban planning to mitigate excessive heat, temporal variations, as well as the influence of synoptic weather conditions and surrounding urban geometry on the vegetation cooling effect, are still unclear. In this study, we monitored the transpiration-induced cooling from trees over two summers in five urban settings characterized by varying levels of greenness and urban geometry in the city of Mainz (Germany). Differences in air temperature and humidity patterns were compared with estimates of tree transpiration derived from high-resolution stem size and sap flow measurements. Results from the five urban sites indicate significant cooling due to transpiration, but with large variability depending on time of day and weather conditions. The cooling effect is strongest during periods of high transpiration demand, and in the stable nocturnal boundary layer when air mixing is limited. The strongest transpiration cooling was found in an enclosed courtyard structure. These findings reveal that a few trees can substantially mitigate urban excess heat, but that the urban geometry, time of the day, and prevailing weather conditions considerably modulate this effect.