A comparison of the growth and cooling effectiveness of five commonly planted urban tree species

It is often claimed that evapotranspiration from urban trees can mitigate the urban heat island and adapt our cities to climate change; however, the relative effectiveness of different tree species has rarely been investigated. The current study addressed this shortcoming by comparing the growth and physiology of five commonly planted tree species: Sorbus arnoldiana, Crataegus laevigata, Malus ‘Rudolph’, Pyrus calleryana and Prunus ‘Umineko’. The study was conducted between March and November, 2011 in eight different streets of Manchester, UK where trees had been growing for 6 years in the same growing conditions. The study showed that evapotranspirational cooling is closely related to the growth and stress tolerance of tree species. Of the species tested, Prunus ‘Umineko’ and P. calleryana were the fastest growing and Malus ‘Rudolph’ was the slowest growing. In general faster growing species showed higher leaf area index (LAI) and higher stomatal conductivity and so provided more cooling. However, Prunus ‘Umineko’ had surprisingly low cooling and showed signs of drought stress. P. calleryana showed up to 100 % higher stomatal conductance than the other tree species. Combining the higher LAI and wider canopy, P. calleryana and C. laevigata provided cooling up to 2.2 kW tree⁻¹, 3 to 4 times of cooling to that of Prunus ‘Umineko’ and S. arnoldiana and showed no signs of drought stress. Malus ‘Rudolph’ showed stress tolerance but provided low cooling. Prunus ‘Umineko’ and S. arnoldiana with their thin and sparse canopy provided low cooling and showed susceptibility to urban stress.