240 al. 2011). By contrast, in smaller plots, enhanced cooling was possible due to the buildup of a microcli- mate, which was cooler than the surrounding air. This is usually achieved through the flow of warm air from the surrounding areas into the plot by means of advec- tion. This, in turn, increases evapotranspiration and cools the plots (oasis effect)(Georgescu et al. 2011). These results have, therefore, confirmed that increas- ing the presence of small green spaces in urban areas can mitigate the UHI effects. In fact, the data is sug- gestive that many small patches of turf will be more effective than one large area of vegetation (e.g., park). Still, more intense and concurrent observations of surface temperatures across small and larger turf plots would be necessary to validate this finding. Shading through tree canopies is another approach to cool surface temperatures. Although the impact of shading on surface temperatures was 50% less as compared to the presence of vegetation, the combina- tion of shade and vegetation within small plots was found to be most effective. This finding was consis- tent with that of Bowler et al. (2010). It was observed that shade in a park was able to reduce surface tem- peratures across all surface types by 1.1 to 1.8 °C (Figure 6b), and the reduction was most pronounced over asphalt. This outcome supports the planting of trees coupled with careful selection of canopies that can maximise the potential for shade as a practical solution to mitigate the UHI in cities. Some recent research has, in fact, confirmed that urban trees are more useful than turf in UHI mitigation because trees have higher evapotranspiration rates than an equiva- lent area of turf (Onishi et al. 2010; Ng et al. 2012). However, other research had reported otherwise (Peters et al. 2011). Nevertheless, because the cooling effect of trees comes primarily from transpiration, a reliable estimate of the cooling effect will be to mea- sure leaf temperatures across the entire canopy or, better still, obtain transpiration rates, which we will consider in future work. In a recent review by Nuruz- zaman (2015), it was concluded that across green roofs, shade trees, green vegetation, and water bod- ies, the most effective and least costly strategy to mit- igate UHI was that of green vegetation. Therefore, it is apparent that the implementation of trees and turf would be beneficial to mitigate the UHI over having just one or the other. Ow et al: Benefits of Tree Shade and Turf in an Urban Environment Globe Temperatures It was important to study globe temperatures because of the high temperature and humidity in the tropics, which will impact on outdoor thermal comfort. Gen- erally, the outcomes were similar to surface tempera- tures. Globe temperatures rose from dawn, peaked at noon, and gradually declined. The difference between globe and surface temperatures was that surface tem- peratures continued to increase until 14:00 hrs while globe temperatures started to decline at 12:00 hrs (Figure 4). Interestingly, a consistent 6 °C increase in globe temperatures was found across all surface types as well as sun and shade conditions when compared against surface temperatures (Figure 4). Additionally, the data for globe temperatures indicated that the effect of shade and turf was less pronounced as the tempera- ture readings were consistently higher than the pre- vailing air temperatures (Figure 4). This was different from surface temperatures where the presence of shade and turf resulted in temperature readings that were fairly similar to prevailing air temperatures. The effect of shade in mitigating the effects of UHI should not be neglected, where the temperature reduction was between 4.7 and 5.6 °C when shade was available (Figure 4). A temperature reduction of between 8 and 10 °C was observed for turf, indicating that vegetation had a key role in heat mitigation strat- egies (Figure 4). The temperature differential between globe and surrounding urban air temperature for a park indi- cated that shade was able to reduce temperatures within the park by 0.5 to 1.8 °C across all surface types (Figure 6c). These reductions were provided by the canopy cover of trees in the park, which would have facilitated the reduction in short wave penetra- tion (Rizwan et al. 2008). This was most pronounced with asphalt and concrete. Apart from the beneficial effects of shade, the data also ascertained the benefits of turf because asphalt and concrete were found to be on average 70% warmer than turf (Figure 6c). Lastly, we are not aware of much research, especially within a tropical urban context, that has studied the effects of shade and vegetation on temperature. We are not sug- gesting that the results can be applied across all urban sites given the variable conditions such as urban typologies and the wind environment, but the results provide valuable insights into the diurnal temperature environment for plots of smaller and larger extents of greenery. Direct and relative measures of various tree ©2020 International Society of Arboriculture
May 2020
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