Arboriculture & Urban Forestry 46(3): May 2020 2100 (Denia 2015). Such increases in ambient tem- peratures will increase the frequency of urban heat waves that will potentially induce health problems coupled with heat-related illnesses and deaths. Such effects were already seen in the 2018 Northeast Asian heat wave, during which 65 deaths were recorded in a single week. In addition, there were more than 70,000 hospitalisations, all of which were concentrated in urban areas induced by the extreme heat (NHK World-News Japan 2018). While the effects of UHI had received much atten- tion, there has also been some early as well as recent work conducted in the tropics to understand the ben- efits of vegetation (e.g., the cooling effect of parks) in mitigating the effects of UHI (Jauregui 1990, 1991; Chow et al. 2016; Elmes et al. 2017; Hwang et al. 2017). These studies were generally focused on examining if the incorporation of vegetation into urban areas would potentially mitigate the effects of UHI and to quantify the cooling effects of vegetation within the urban environment. It is noteworthy, how- ever, that there are problems with such measurements as no two cities, or sites within cities, will be identi- cal. The one exception, however, will be the amount of green spaces. With these inconsistencies, it rules out the possibility of using conventional experimen- tal approaches. Therefore, the preferred method will be to compare air temperatures across various parks (Potchter et al. 2006; Yu and Hien 2006; Chang et al. 2007; Jansson and Gustafsson 2007; Bowler et al. 2010) and small green spaces (Heidt and Neef 2008; Shashua-Bar et al. 2009; Hamada and Ohta 2010; Oliveira et al. 2011) surrounded by streets or roadways. The limitation with such studies is that the sites vary in location, so the results are expected to exhibit differences according to the site conditions. For example, a meta-analysis carried out by Bowler et al. (2010) found that parks exhibited an average daytime temperature of 0.9 °C cooler than the surrounding urban temperature. This relatively small effect is probably because warm air is absorbed into the park from the surrounding streets and roadways (Hamada and Ohta 2010). A more effective method of determining the cool- ing effect of vegetation is to observe surface tempera- tures. Also noteworthy is that diurnal surface temperatures of vegetation and paved areas have rarely been monitored over a long period. Except for a study in Basel, Switzerland, which showed that 229 during summer at midday, the hard surface tempera- ture was 12 °C warmer than air. By contrast, the can- opy of trees was found to be 4 °C warmer than air (Leuzinger et al. 2010). Other research has employed the use of the energy-balance modelling approach (Tso et al. 1990, 1991). For instance, Gill et al. (2007) modelled the maximum summer surface temperature of areas of different surface types in Manchester, United Kingdom, and predicted maximum tempera- tures of 43 °C for concrete, compared to a maximum temperature of 18 °C for woodland and grass (Gill 2006). These results are comparable to results for tree canopies conducted by Leuzinger et al. (2010). Another area of importance associated with the effects of UHI which has received some attention is that of shade from trees (Kotzen 2003; Shahidan et al. 2010; Abreu-Harbich et al. 2015). This is important for two reasons. Firstly, the surface temperature of the shaded area is expected to be reduced as a result of reduced heat storage and convection; secondly, shad- ing positively impacts on human comfort, and this is achieved through an altered perception of tempera- ture (Matzarakis et al. 2007). A shaded person should feel cooler than one exposed to the sun, and a person standing on a hotter surface should feel warmer than one standing on a cooler surface (Monteith and Unsworth 1990). A measure of perceived heat is therefore obtained, not from an air thermometer but from a globe thermometer (an instrument reading both convection and radiation)(Thorsson et al. 2007). The element of shade is critical, as Rosenzweig et al. (2009) had shown in their work with urban forests and green roofs that when the effect of shading was eliminated in the climate model, reductions in surface air temperatures were underestimated. Importantly, there is evidence to suggest that the urban forest can be managed to impact positively on the UHI, but an evidence base will need to be developed to elucidate the ecosystem service benefits urban trees can pro- vide (Feyisa et al. 2014; Livesley et al. 2016) to miti- gate the UHI. Therefore, this present study had two aims: firstly, to measure the effect of turf cover and tree shade on surface temperatures; and secondly, to measure the effect of turf cover and tree shade on globe tempera- tures. Globe temperature was measured to provide insights into human comfort as it changes alongside variation in surface temperatures. Hence, although these variables are discussed in this study as separate ©2020 International Society of Arboriculture
May 2020
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