Arboriculture & Urban Forestry 39(4): July 2013 Arboriculture & Urban Forestry 2013. 39(4): 157–164 157 A Comparison of the Shading Effectiveness of Five Different Street Tree Species in Manchester, UK David Armson, Mohammad Asrafur Rahman, and Anthony Roland Ennos Abstract. One major benefit of urban trees is the shade they provide on sunny days; this reduces the heat stored in engineered surfaces and lowers the heat load on people, increasing their comfort. This study compared the shading effectiveness of five small street tree species within the urban landscape of Manchester, UK. The area of shade produced by each tree during early and midsummer 2012 was calculated from morphological measurements, such as canopy height, width, and aspect ratio. The effect of tree shade on air, mean radiant and surface temperatures was also compared and related to the leaf area index (LAI) of the canopy. It was found that tree shade reduced mean radiant temperatures by an average of 4°C, though neither tree species nor LAI had a significant effect. Tree shade reduced surface temperatures by an average of 12°C, and the tree species and LAI both had sig- nificant effects. Tree species with higher LAI, Crataegus laevigata and Pyrus calleryana, provided significantly more cooling than the other species, and surface temperature reduction was positively correlated with LAI. This study has shown that trees are useful in improving both human thermal comfort and reducing surface temperatures in urban areas, and that selection of tree species with high LAI can maximize the benefits they provide. Key Words. England; Human Thermal Comfort; Manchester; Mean Radiant Temperature; Surface Temperature; Tree Shade; United Kingdom; Urban Heat. Trees provide two major benefits to the urban climate. First, like other forms of vegetation, the leaves intercept sunlight and much of its energy; the heat is used for transpiration. As a conse- quence, the leaves and the surrounding air are cooled compared to areas covered with built surfaces (Pauleit and Duhme 2000; Leuzinger et al. 2010). Leaf temperatures have been measured as being 11°C–30°C cooler (Pauleit and Duhme 2000; Leuz- inger et al. 2010) than surrounding built surfaces, the effect be- ing greatest at midday during hot sunny weather, and greater in trees than green roofs or grass. However, the effects on air temperature tend to be small; for example, parks are only 1°C cooler on average (Bowler et al. 2010) than the surrounding ar- eas, partly because of the poor coupling between surfaces and air, and because warm air is readily advected into them (Oke 1978; Bowler et al. 2010). A better way of quantifying the cool- ing benefits of vegetation is to measure the rate of evapotrans- piration, which in trees can be performed by measuring weight loss (Miller 1980; Kjelgren and Montague 1998) of containerized trees, using porometry (Rahman et al. 2011), or in recent years by using sap flow gauges (Pataki et al. 2011; Peters et al. 2011). The second benefit of trees is that they also provide shade because of their elevated and extensive canopies. This reduces the receipt of solar radiation beneath the tree; the effect is maxi- mized in the summer when deciduous trees are in leaf and solar radiation can be reduced by up to 90% in tree shade (Heisler 1986a). This is important for two reasons: The first reason is that tree shade helps people to cool down because it is the radiation exchange between a person and the surrounding local environ- ment, not convective transfer, that most affects a person’s ther- mal comfort, as quantified by measures such as their perceived or physiologically equivalent temperature (PET) (Matzarakis et al. 2007). As a result, a person in the shade feels cooler than a person in the sun (Monteith and Unsworth 1990). Measuring PET accurately is a complex procedure as it is affected by sev- eral factors, including the surface temperature of surrounding areas, air temperature, and wind speed. However, it has been found that globe thermometers provide a good indication of the mean radiant temperature around a person (Thorsson et al. 2007), so using globe thermometers in and out of tree shade can offer a good indication of how it affects human thermal comfort. In a previous study, the authors have shown that con- stant tree shade can reduce mean radiant temperatures by 5°C– 7°C in the summer in Manchester, UK (Armson et al. 2012). The second major benefit of tree shade is that it can reduce the amount of heat gained by the surface in shade, furthering the cooling effect that trees provide. Many studies have shown that tree-shaded areas of built surface can be much cooler than those in the sun (Akbari et al. 1997; Scott et al. 1999; Akbari et al. 2001; Streiling and Matzarakis 2003; Wong et al. 2003), though the ef- fect is somewhat smaller than the completely dense shade cast by buildings. In Manchester, UK, it was found that concrete surfaces shaded permanently by a bank of trees can be cooled by up to 20°C in the summer (Armson et al. 2012). Shaded built surfaces can be almost as cool as surfaces covered by evapotranspiring grass (Armson et al. 2012). Trees have the added advantage over grass swards, moreover, that they provide surface cooling while retaining most of the resilient built surface; the planting pit is gen- erally much smaller than their canopy, and the shade the canopy provides is cast over a greater surface area than the canopy area, because the zenith angle of the sun is generally below 90 degrees. ©2013 International Society of Arboriculture
July 2013
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
