Effect of rooting conditions on the growth and cooling ability of Pyrus calleryana

Urban forests appear to be an excellent way of mitigating the urban heat island and adapting cities to climate change, as trees provide cooling by evapotranspiration. However, the effects of urban growing conditions on tree growth and cooling performance have not been widely investigated. The current study addresses this shortcoming by studying the growth and leaf physiology of the commonly planted urban tree Pyrus calleryana ‘Chanticleer’. The study was carried out between February and November, 2010 on streets in Manchester, UK, where P. calleryana trees had been growing for five to six years under three contrasting growth conditions: in pavement; in grass verges; and in Amsterdam soil. Trees in Amsterdam soil had grown almost twice as fast as those in pavements, the difference being related to their lower degree of soil compaction, and hence lower shear strength. Trees grown in Amsterdam soil also had better performance in leaf physiological parameters such as stomatal conductance, leaf water potential, and foliar nutrient status. Phenological observations were also consistent with the observed differences in growth. The lower soil moisture content at 20 cm depth in Amsterdam soil also suggested there was a higher infiltration rate and more moisture available to plant roots. The enhanced growth and physiological performance of trees grown in Amsterdam soil meant they provided peak evapotranspirational cooling of up to 7 kW, 5 times higher than those grown in pavements.