162 Armson et al.: Shading Effectiveness of Street Trees in Manchester, UK and hence higher aspect ratios, cast more shade relative to their canopy area because of the inclination of the sun. The shade areas cast by the trees were around 50% greater than canopy area for S. arnoldiana, P. calleryana and Prunus ‘Umineko’, but only around 30% greater for C. laevigata and Malus ‘Ru- dolph’. Therefore the effect of canopy diameter and height cancelled each other out, at least at midday in the summer. It is likely, however, that in the morning and evening, and earlier and later in the year when the sun is at a lower angle, the taller, thinner trees will cast greater shade than the shorter, wider ones. It is clear, therefore, that the area of shade cast by street trees is much larger than their canopy areas, and this will be particu- larly true for tall, thin trees and when the sun is at a lower angle. Air Temperatures Unlike the many other studies reviewed by Bowler et al. (2010), tree shade did not have a significant impact upon the local air temperatures below the tree canopies. To some extent, this may have been due to the thermometers, which have a measurement error of around 0.2°C. However, the effect was clearly far less than the 1°C average effect described by Bowler et al. (2010). This is probably because the tree canopies were so small and the air was not perfectly still. Warm air would have readily been advected into the area of shade beneath the trees’ canopy. Mean Radiant Temperatures Tree shade reduced mean radiant temperatures by around 4°C during both measurement periods, but only in early summer were there any significant differences between the species. It is likely that the difference between the species in early sum- mer is because C. laevigata reached its maximum leaf density earlier than the other species and so reduced solar radiation more in its shade than the other species, which came fully into leaf later. Once all the species were in full leaf during July, the earlier differences between the species were not apparent. The reductions in mean radiant temperature found in this in- vestigation, ranging from 3.8°C to 5°C, were somewhat lower than the maximum reductions of 5°C–7°C researchers previously found for areas in permanent tree shade (Armson et al. 2012). This is probably because of two reasons. First, the air temper- atures in this survey were not as high as those recorded in the earlier study, with mean air temperatures of 22°C rather than 28°C. Second, the large trees and banks of trees in the earlier study probably cast denser shade. Nevertheless, the mean 4°C temperature reduction is large and would be particularly impor- tant to human thermal comfort, as it represents the reduction of perceived heat a person would feel in tree shade. Mean radiant temperatures are a primary determinant of the physiological equivalent temperature (PET) which is used to assess human thermal comfort in a specific area (Matzarakis et al. 1999). De- termining human thermal comfort is a complex and problematic process, however, and there are other indices as well as PET. Calculating PET involves measuring received radiation in three planes, as well as the air temperature, relative humidity, wind speed, and cloud cover. Matzarakis et al. (1999) investigated comfort levels at various PET levels and found that people were most comfortable when PET was between 18°C and 23°C, with slight heat stress beginning above this level and increasing as PET increased further. In this survey, tree shade reduced mean ©2013 International Society of Arboriculture radiant temperatures from around 28°C to values below 24°C. These temperature are almost within the comfort level of Mat- zarakis et al. (1999) and below the 24.5°C discomfort level for air temperature suggested by Wilson et al. (2008). The tree shade would be enough to produce cool “refuge” areas on hot days. Surface Temperatures Tree shade reduced surface temperatures by 12°C, on average, and there were significant differences between the species. The greater cooling provided by C. laevigata and P. calleryana are probably related to the high LAI of these species, which con- sequently would produce denser shade, reducing the incident solar radiation on the surface. This suggestion is backed up by the correlation analysis, which showed that trees with a high- er LAI provide greater surface cooling than trees with a low LAI. Planting trees with a higher LAI should therefore con- tribute more to this aspect of reducing the urban heat island. Once again, the reductions in surface temperature, around 12°C, caused by these small street trees, was smaller than the maximum temperature reductions caused by permanent tree shading of around 20°C (Armson et al. 2012). The differ- ence is partly due to the lighter shade these small trees cast, but another important effect is that the trees only cast shade on a specific surface point for a period of 1–2 hours. Tall, slen- der trees will cast shade for a particularly short time, meaning it was no surprise that temperature reductions were negatively correlated with canopy aspect ratio, at least in early summer. Nevertheless, whatever the canopy form, it is clear that the shade cast by street trees can contribute to significant reduc- tions in surface temperatures, at least locally, and this will have the effect of reducing heat storage in the paved surface. Combined with the evapotranspirational cooling the trees pro- duce, this should help ameliorate the urban heat island effect. It is hoped that results such as these can be incorporated into physical models of the urban environment to help quantify the regional thermal benefits of street trees (Stone and Rog- ers 2001; Stone and Norman 2006; Chen and Wong 2009). CONCLUSION The results of this study have shown that although the shade cast by small street trees is not large enough to cause local reductions in air temperature it can have significant meteoro- logical benefits. First, shade significantly reduces mean radi- ant temperatures and can create “refuge” areas for people on hot days. Second, shade can also reduce surface tempera- tures, which will help reduce storage of heat in hard surfaces and so have a regional effect in reducing the urban heat island effect. The areas of shade that street trees provide are also much greater than their actual canopy area—by 30%–50%, even at midday—so their influence, unlike that of areas of grass, ex- tends outside their canopy. Although the study authors did not find any difference in the area of shade cast between the spe- cies, researchers did find that species with a higher LAI, such as C. laevigata and P. calleryana, do reduce surface tempera- tures more because they cast a denser shade. Trees with a lower aspect ratio also reduce surface temperatures more because they shade a specific point on the ground for longer. This sug- gests that to optimize the benefits of street trees it is best to plant trees that have as broad and dense a canopy as possible.
July 2013
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