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

tGlobal warming is likely to increase the frequency and magnitude of heat waves. As the urban geometryand material amplifies warming, city dwellers will face an intensification of heat-induced health prob-lems and mortality. Although increased vegetation cover is frequently used in urban planning to mitigateexcessive heat, temporal variations, as well as the influence of synoptic weather conditions and surround-ing urban geometry on the vegetation cooling effect, are still unclear. In this study, we monitored thetranspiration-induced cooling from trees over two summers in five urban settings characterized by vary-ing levels of greenness and urban geometry in the city of Mainz (Germany).

Differences in air temperatureand humidity patterns were compared with estimates of tree transpiration derived from high-resolutionstem size and sap flow measurements. Results from the five urban sites indicate significant cooling dueto transpiration, but with large variability depending on time of day and weather conditions.

The coolingeffect is strongest during periods of high transpiration demand, and in the stable nocturnal boundarylayer when air mixing is limited. The strongest transpiration cooling was found in an enclosed courtyardstructure. These findings reveal that a few trees can substantially mitigate urban excess heat, but that theurban geometry, time of the day, and prevailing weather conditions considerably modulate this effect.