Arboriculture & Urban Forestry 46(3): May 2020 In this study, two experiments were carried out involving variable vegetation intensities with the aim to understand the UHI effect in the city-state. The first experiment was in an open field environment with the objective to examine the effect of small patches of vegetation on globe and surface measurements. These plots were comprised either of turf or concrete, and measurements were made in full sun or in full shade provided by the tree canopies. Diurnal measurements from 2015 to 2017 were carried out. The second experiment was to examine the effects of a larger extent of vegetation, such as an urban park, on globe and surface temperatures. Diurnal and midday mea- surements were made above green and hard surfaces exposed to the sun and also in the shade. This data was later compared against the air temperature in the external park environment. Green areas and parks nestled within high-density housing areas were selected randomly but still close enough to possess consistent climatic conditions. Effect of Small Patches of Vegetation on the Urban Heat Island Effect The area used to study the effect of a small patch of vegetation on UHI was comprised of a mixture of turf and hard surfaces measuring approximately 30 × 25 m, which had a row of Samanea saman trees spaced some 3 m apart with umbrella-shaped canopies that overlapped each other and provided the site with shade throughout the course of the day until 15:00 hrs, when the position of the sun moved and light from the sun was able to heat up the area. The area was not in direct contact with buildings, roads (e.g., traffic), or any form of infrastructure that might have an influ- ence on the data collected. The trees were medium- sized trees measuring 10 to 12 m in height with a crown volume of between 250 and 300 m3 . The shade provided by these trees was significant and remained consistent across the experimental period. This spe- cies was selected for evaluation as it is commonly planted in many tropical urban cities. It is widely sought after for the shade it provides as a result of its umbrella-shaped crown. In addition, it is fast grow- ing, has a high rate of transplant success, and adapts effectively to poor-quality urban soils. Two plots exposed to shade by these trees comprised of turf and concrete were set aside for shade observations. Approximately 10 m adjacent to the shaded plots, a further two plots in full sun with no shade throughout 231 the day were set up. The turf and concrete plots in the sun and shade were spaced approximately 5 m apart. The plots with turf were made up of Stenotaphrum secundatum. Mowing was generally avoided as it would alter the density and impact on the data. How- ever, some pruning (minimal) of the turf was under- taken every once in a while to ensure that the plot was looking tidy. The plots with Stenotaphrum secunda- tum were generally healthy despite not receiving any irrigation. The concrete plots were a footpath used by pedestrians and were comprised of concrete blocks measuring 2 × 2 m (Figure 1). Diurnal air temperature, wind speed, and rainfall were recorded. The meteorological data was recorded with a weather hawk 916 wireless weather station powered by a solar panel (Scientific Sales, Inc. NJ, USA). Readings were made at 5-min intervals. Air temperature, wind speed, and rainfall data were important for comparative analysis against the ambi- ent meteorological conditions experienced each day. The surface temperatures in each plot were recorded at 5-min intervals with the 41382LF2 temperature/ humidity sensor coupled with a data logger (Scien- tific Sales, Inc. NJ, USA). Each sensor was set in the middle of every test plot. For the turf plots, the sen- sors were set into a shallow 8 to 10 mm crevice made to ensure that the sensor was in contact with the soil surface. The sensors for the concrete plots were set into similar 8 to 10 mm crevices (Figure 1). The key objective of setting the sensors within the crevices was to avoid direct solar radiation, which may alter the temperatures recorded by the sensors. Similarly, globe temperatures in each plot were recorded at 5-min intervals. This monitoring was aimed at estimating the effect of temperature, humid- ity, wind speed, and visible as well as infrared radia- tion on thermal comfort. The globe thermometers (Testo, SMI Sdn. Bhd. Malaysia) were placed at a height of 1.2 m in the centre of each plot (Figure 1). Care was taken to ensure that the area beneath the thermometer was covered by grass with a subtended 2 π solid angle. It is noteworthy that for the concrete test plots, because of the limited 2 × 2 m concrete slabs, a subtended solid angle of 75% beneath the thermometer was comprised of concrete while the rest was made up of turf. This was suggestive of some underestimation in the globe temperatures over concrete, though the effect is expected to be negligible. The globe thermometers used had a hollow 150-mm-diameter ©2020 International Society of Arboriculture
May 2020
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