Temperature Reduction in Urban Surface Materials through Tree Shading Depends on Surface Type Not Tree Species

Abstract

Trees play a vital role in urban cooling. The present study tested if key canopy characteristics related to tree shade could be used to predict the cooling potential across a range of urban surface materials. During the austral summer of 2018–2019, tree and canopy characteristics of 471 free-standing trees from 13 species were recorded across Greater Sydney, Australia. Stem girth and tree height, as well as leaf area index and ground-projected crown area was measured for every tree. Surface temperatures were recorded between noon (daylight saving time) and 3:00 p.m. under the canopy of each tree in the shade and in full sun to calculate the temperature differential between adjacent sunlit and shaded surfaces (∆Ts). The limited control over environmental parameters was addressed by using a large number of randomly selected trees and measurement points of surface temperatures. Analyses revealed that no systematic relationship existed among canopy characteristics and ∆Ts for any surface material. However, highly significant differences (p < 0.001) in ∆Ts existed among surface materials. The largest cooling potential of tree shade was found by shading bark mulch (∆Ts = −24.8 °C ± 7.1), followed by bare soil (∆Ts = −22.1 °C ± 5.5), bitumen (∆Ts = −20.9 °C ± 5.8), grass (∆Ts = −18.5 °C ± 4.8) and concrete pavers (∆Ts = −17.5 °C ± 6.0). The results indicate that surface material, but not the tree species, matters for shade cooling of common urban surfaces. Shading bark mulch, bare soil or bitumen will provide the largest reductions in surface temperature, which in turn results in effective mitigation of radiant heat. This refined understanding of the capacity of trees to reduce thermal loads in urban space can increase the effectiveness of urban cooling strategies.

Cooling Effect of Trees with Different Attributes and Layouts on the Surface Heat Island of Urban Street Canyons in Summer

“…Tun et al discover that shading reduces surface temperatures by an average of 20 °C, with the maximum temperature decreasing by 40 °C. Their study site had higher temperatures than ours, with the highest temperature in New South Wales, Australia, reaching 48.9 °C in January 2020 [50]. Therefore, the cooling effect of trees is largely influenced by urban heatwaves.…”

Comparison of Various Tree Features for Blocking Solar Radiation in the Arid Urban Environment

“…The influence on SR control, such as thermal and UVR, changes according to the canopy dimensions of height, diameter, and trunk height on the one hand and the leaves' sizes, shapes, and surface structures on the other hand (Grant et al, 2003). These effects have been studied in many types of research (Shahidan et al, 2010;Sanusi et al, 2017;Armson et al, 2013;Grant and Heisler, 2006;Zheng et al, 2018;Kong et al, 2017;TUN and MG 2020;Deng et al, 2020).…”

“…The Leaf Area Index (LAI) is a well-established measurement parameter in forestry, agriculture, and ecology, LAI is defined as the total projected area of leaves of a single tree or group of trees over a unit of land (TUN and MG, 2020). It is an essential aspect of tree canopies affecting SR penetration and microclimate below canopies (Sanusi et al, 2017;Fahmy et al, 2010).…”

Thermal Comfort Assessment in Urban Green Spaces: Contribution of Thermography to the Study of Thermal Variation between Tree Canopies and Air Temperature

“…On the other hand, assessing temperatures at heights above the canopies allows for an understanding of heat dissipation and air circulation in the permit area. This is particularly important for assessing the influence of UTC on urban ventilation and the dissipation of accumulated heat [44][45][46][47]. The presence of trees helps to reduce daytime temperatures by absorbing solar radiation, providing shade, and increasing evapotranspiration through the cooling effect of urban trees [48][49][50].…”