158 Armson et al.: Shading Effectiveness of Street Trees in Manchester, UK Although street trees should be particularly effective at providing cooling benefits, the effectiveness of different species is likely to differ. In general, more drought-tolerant and slow-growing trees have less dense canopies, and so reduce radiation less than faster-growing species (Heisler 1986a; Heisler 1986b; Shashua-Bar and Hoffman 2000), as well as providing less evapotranspirational cooling. How- ever, surprisingly little work has been done to compare the amount and depth of shade cast by individual street tree species and their likely effects on the cooling of people and the surfaces beneath them. Perhaps this is because it is harder to measure light levels meaningfully beneath isolated trees than beneath a continuous forest canopy. Of course, light levels beneath street trees also depend on how they have been managed and whether they have been pruned. A wide range of species are grown in cities across central and northwestern Europe, but the European tree survey has shown that three to five genera account for 50% to 70% of all street trees planted (Pauleit 2003). In England, UK, a sur- vey of urban street trees showed that six species account for 37% of all trees and shrubs planted within cities (Britt and Johnston 2008). The main six species were Leyland cypress (× Cupressocyparis Leylandii), hawthorn (Crataegus spp.), sycamore (Acer pseudoplatanus), silver birch (Betula pendu- la), common ash (Fraxinus excelsior), and privet (Ligustrum spp.). Increasingly, smaller tree species are being planted as street trees because they fit better into narrow pavements and are easier to manage (Britt and Johnston 2008). Particularly common genera are members of the family Rosaceae, namely Pyrus, Prunus, Crateagus, Malus, and Sorbus. Therefore, understanding the shading effects of these small tree species is critical in maximizing the climatic benefits of street trees. This study was designed to compare the shading benefits of five commonly planted street trees in Manchester, UK, and to investigate how differences in their canopy charac- teristics might affect their performance. To do this, the size and canopy shape of trees of each species were measured, along with their leaf area index (LAI), and the air, mean radi- ant, and surface temperatures were measured both in the sun and in the shade beneath them. The aim was that this study would give a broad comparison of the climatic benefits of the five species, and indicate what factors make for a good street tree. This study was conducted in Manchester because it is a good example of a temperate city with a maritime climate and a pronounced heat island effect of 3°C–5°C (Smith et al. 2011) and it allowed us to compare the effects of short-term shade provided by street trees with those of permanent deep tree shade (Armson et al. 2012) measured in the same city. MATERIALS AND METHODS Location and Selection of Sample Trees Investigations into the benefits of urban street trees were con- ducted on five tree species in six residential locations within southern Manchester, UK. These trees were sampled twice dur- ing the summer of 2011, between 11:30 am and 4:30 pm, when temperatures were near their maximum, on clear warm days. Because of the large sample size and the poor summer weather conditions, sampling could not be carried out over a single day; days often clouded over after a sunny morning before all the trees could be monitored. Therefore the trees were measured over two days in early summer, on May 25 and June 3 (referred to as early summer) and over three days during July 14, 15, and 25 (referred to as mid-summer). During these days, mean maximum temperatures were around 22°C, similar to the “average” days for summers 2009 and 2010 (Armson et al. 2012), but this was largely because there was no heat wave during the poor summer; temperatures were low during the preceding days. Nevertheless, the periods of measurement were sunny, with peak short wave radiation of around 800 W m-2 , meaning the cooling effects of the tree shade should have been close to the maximum possible. In total, 51 trees were examined; of which twelve were Crataegus laevigata, ten Sorbus arnoldiana, ten Prunus ‘Umineko’, ten Pyrus calleryana and nine Malus ‘Ru- dolph’. The trees were distributed over six locations: twenty three at Manley Road (53°26’49”N, 2°15’39”W), eleven at Cringle Road (53°26’10”N, 2°10’53”W), eight at Vic- toria Road (53°27’05”N, 2°15’33”W), three at Granville Avenue (53°26’50”N, 2°15’55”W), five at Thorncliffe Grove (53°26’36”N, 2°10’51”W), and one at Beech Range (53°26’37”N, 2°11’49”W) (see Table 1 for species distribu- tion). The sites were all of similar morphology, being com- posed of rows of terraced houses, with a road width of 4.5 m to 7 m, and paved with dark asphalt. The selected trees had been planted in 2005–2006 at an age of 4–5 years, and were planted in the same rooting conditions, each tree being in a 1.5 m2 open tree pit located along the curb edge of the pavement. All trees were planted a minimum of two meters away from any buildings, were seven to ten meters apart from each other, were free of damage, dieback, and visible disease and were in areas that were free from shadow from dawn till 4:30 pm. Morphological Measurements At each of the sample trees, a standard set of morphologi- cal measurements were taken. Tree canopy spread and bole height were measured using a standard tape measure. East and Table 1. The distribution of C. laevigata (n = 12), S. arnoldiana (n = 10), Prunus ‘Umineko’ (n = 10), P. calleryana (n = 10), and Malus ‘Rudolph’ (n = 9) over the six sample locations in Manchester, UK. Street Manley Road Cringle Road Victoria Road Granville Avenue Thorncliffe Grove Beech Range Distribution of tree species over the sample area C. laevigata 1 4 4 3 0 0 S. arnoldiana 6 4 0 0 0 0 0 0 0 0 0 Prunus ‘Umineko’ P. calleryana 10 6 0 4 0 0 0 Malus ‘Rudolph’ Total number of trees 0 3 0 0 5 1 23 11 8 3 5 1 ©2013 International Society of Arboriculture
July 2013
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