Cooling capacity of urban trees exposed to thermal stress

In arid regions where episodic heatwaves regularly occur, urban heat islands pose a major threat to human and environmental health, requiring city managers to develop strategies to mitigate the effects of excess heat. Urban vegetation is known for mitigating some of these threats by cooling the local surroundings through shade and evapotranspiration. However, choosing the appropriate tree species remains a challenge, as urban planners have little information on species cooling capacities (i.e., shade intensity and area, water use) relative to water resources that are becoming more limited in many urban locations. Here, we quantified the species-specific cooling effect of 14 tree species (native and exotic) commonly found in Phoenix, AZ, USA, one of the worlds warmest metropolitan areas. Specifically, we investigated 1) how plant water use and shade projection differed across species; and 2) how these parameters changed from spring to mid-summer, when temperatures regularly exceed 40 oC. We measured leaf stomatal conductance (gsw), leaf and whole-plant transpiration (Et), coupled with measurements of canopy temperature, NDVI, 3D reconstruction of canopy volume and shade estimation, and microclimate. Measurements were taken from mid-Spring and extended through pre-Monsoon and Monsoon summer (March-July 2021). Contrary to popular belief, exotic tree species did not significantly transpire more than native species on average or across seasons. While gsw declined with vapor pressure deficit and leaf temperature increasing with summer conditions, Et increased substantially for most species. Eleven of the 14 species increased midday Et, contributing to enhanced canopy cooling, while the remaining 3 species suppressed water loss in response to thermal stress (e.g., stomatal closure or thermal damage to photosynthetic tissues). Importantly, species that were highly susceptible to thermal stress had reduced canopy cover that compromised their capability of projecting shade during the warmest and driest period of the year. Ultimately, Pinus eldarica, Cupressus arizonica, Chilopsis linearis, Bauhinia sp., and Fraxinus velutina were proven to be the best tree options for Phoenix landscaping as these species provide intermediate-full shade across seasons, relative to water use under arid conditions.