Background/Question/Methods Urban greening is one of the most advocated strategies to reduce the adverse effects of urban heat islands during the hottest periods of the year. Vegetation and especially trees are expected to cool the local environment through evapotranspiration and shade provision. Literature shows a varying air temperature cooling potential due to urban trees in different climates and times of the day. These differences have been difficult to interpret so far, as separating the effects of tree shade, evapotranspiration, and tree-wind interactions is challenging and impossible to do with observations alone. Here, we conduct numerical experiments including and excluding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model for four cities characterized by distinct climates (Phoenix, Singapore, Melbourne, Zurich). We analyze the seasonal and diurnal cycles of air and surface temperatures. Results/Conclusions The combination of all tree effects results in a distinctive diurnal pattern of urban air temperature modification, namely higher air temperature decrease during either morning or evening hours, or both, compared to midday in warm climates. In other words, trees provide air temperature cooling during the majority of the time but cooling can be limited or even reversed at times of extreme heat. Such an outcome is mediated by the physiological properties of vegetation. Specifically, transpirative cooling is limited when radiative warming is largest as leaves are hydraulically stressed by high VPD values. This effect could be significant to the point of cancelling the temperature reduction expected by planting urban trees. The occurrence of soil water stress in combination with high VPD values can further amplify this phenomenon. However, state-of-the-art stomatal responses to VPD included in ecohydrological models have uncertainty, as they cannot represent the entire range of observed plant responses. For instance, the adaptability of urban vegetation to cope with high VPD when well irrigated in the urban environment remains an open question. Regardless of this uncertainty, we suggest that when microclimate regulation is sought during heat waves, species selection is important and using plants with an anisohydric behaviour might be much more beneficial and provide a larger cooling potential than isohydric plants that tightly regulate leaf water potential with increasing VPD.
