256 Rahman et al.: Effect of Pit Design and Soil Composistion on Performance of Street Trees Arboriculture & Urban Forestry 2013. 39(6): 256–266 Effect of Pit Design and Soil Composition on Performance of Pyrus calleryana Street Trees in the Establishment Period M.A. Rahman, P. Stringer, and A.R. Ennos 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 com- monly 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. Key Words. Evapotranspiration; Manchester; Planting Design; Planting Pit; Pyrus calleryana; Root Aeration; Soil; United Kingdom; Urban Heat Island. The role of urban trees in adapting cities to the urban heat island effect is well understood (Oke 1989; McPherson et al. 1997; Shashua-Bar and Hoffman 2000; Gill et al. 2007; James et al. 2009; Leuzinger et al. 2010; Peters et al. 2010; Armson et al. 2013). However, the appropriate planting design to maxi- mize the shading and cooling benefits of urban street trees and to integrate them into the urban fabric amongst other intensely competitive land uses is still a big challenge. Along with poor quality soil, street trees in urban areas face both above- and be- lowground space competition (Grabosky and Bassuk 1995; Jim 2001). Greater soil volume with better aeration and drainage is very important for better root growth and the uptake of water and nutrients, allowing trees to achieve an optimum size and provide the ecosystem functions and benefits for which they are planted. Poor tree growth incurs a high level of maintenance input and drains resources that could otherwise be devoted to other aspects of urban forestry (Jim 2001). Gilbertson and Bradshaw (1990) reported that around 23% of newly planted trees in Liverpool, England, had died just after three years of planting, mainly due to limited soil volumes and increased soil compaction. In order to overcome this problem, some researchers have suggested mixing of different coarse matrix in the soil used for street tree plant- ing to reduce the soil compaction and increase the load-bearing capacity. For example, Kristoffersen (1998) described the pos- sibility of expanding the rooting zone of street trees by establish- ing a root-friendly, load-bearing growing medium under sealed pavement to carry light traffic. Grabosky and Bassuk (1995; 1996) also suggested rock-based structural soils for improving the rooting condition of urban street trees. Subsequently, sev- eral urban planners have started to incorporate stone or sand in ©2013 International Society of Arboriculture varying proportions with soil, into major landscape improvement projects where the desired outcome is large, fast-growing trees. However, there has been no published study that has inves- tigated the long-term impact of these load-bearing soils or the impact of sealed pavements on tree growth and physiology. Grabosky et al. (2001) showed that street trees grown in struc- tural and non-compacted soils had almost twice the shoot and root extension three years after planting compared to those grown in the standard pavement profiles. Studies have also shown that pavements can have significant impacts on the soil’s physical characteristics, such as soil moisture and aeration (Morgenroth and Buchan 2009). If soil pore volume and soil pore continuity are reduced by compaction or sealing, roots cannot be supplied with oxygen (Herbauts et al. 1996; Horn et al. 2007). There are some studies that showed the effect of compacted soils on the growth and rooting abundance of different tree species (Ran- drup 1996; Smiley et al. 2006; Bartens et al. 2009; Rahman et al. 2011). However, little information is available on how the differ- ent types of paving sealant affect soil gas diffusivity or how urban trees react to soil aeration deficiencies. Weltecke and Gaertig (2012) have shown that relative gas diffusivity at tree planting sites with asphalt, flagstone, or cobblestone sealant was 10 times lower than at those sites without sealing. Moreover, there was no signifi- cant difference between completely sealed (asphalt) surfaces and “water permeable” surfaces with flagstone or cobblestone surfaces with gaps in between. Even if there is sufficient moisture in compact- ed soil underneath the pavement, it might act as a barrier to oxygen diffusion and lead to relatively anaerobic conditions in the deeper soil profile (Morgenroth and Buchan 2009). This condition may also affect the soil fauna and microbial activity, leading to nutrient defi-
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