Effect of Pit Design and Soil Composition on Performance of Pyrus calleryana Street Trees in the Establishment Period

Abstract

Evapotranspirational cooling from urban trees is an effective way of reducing the urban heat island. However, the appropriate planting design to maximize the cooling benefit of street trees has not been widely examined. The current study investigated the growth and physiology of a commonly planted urban tree, Pyrus calleryana, in Manchester, UK. Trees were planted in April 2010 using three standard planting techniques: in a small open pit, and in small or large closed pits with non-compacted load-bearing soils and sealed with permeable paving slabs. The growth rate, leaf area index, and stomatal conductance were monitored over the next three growing seasons, together with chlorophyll analysis and fluorescence and leaf water potential, allowing researchers to determine tree health, water status, and evapotranspirational cooling. Trees in the open pits grew twice as fast as those in small covered pits and 1.5 times as fast as trees in large covered pits. Having significantly higher canopy density, canopy spread, and stomatal conductivity, the trees in the open pits provided up to 1 kW of cooling, compared to around 350 and 650 W by the small and large covered pits, respectively. Phenological observations, chlorophyll fluorescence, total chlorophyll, and foliar nutrient content confirmed that the trees in open pits were healthier. However, the leaf water potential of trees in the covered pits was less negative, showing that they were not suffering from water stress. Instead, limited aeration probably affected their root respiration and nutrient uptake, impairing their growth and physiological performance.