Oct. 20, 2021
Oct. 7, 2021
With global warming and its effects becoming increasingly apparent, all possible measures must be explored to reduce its impact on human life. A growing number of communities find themselves dealing with increased temperatures not taken into account in earlier city planning. Urban cooling measures can make life easier for these residents and also mitigate climate change. Hence, the findings of an Australian study that tested tree canopy and infrastructure surface features are important for city planners and authorities moving forward.
It is hotter in cities than in surrounding rural areas and natural ecosystems. People living in cities, or those traveling from adjacent areas into cities, will have noticed the temperature difference, called the “Urban Heat Island Effect” (UHIE).
This effect is increasing due to many factors. In addition to climate change, exacerbating factors include paved roadways and areas, man-made structures that absorb and reflect sunlight, and an increase in air-conditioning and artificial lightning, adding to “waste heat” in urban areas. Most of the heat is, however, retained by the buildings and paved areas, as they have a low albedo (part of the light reflected by a surface or body) and high heat retention, leading to a higher night temperature.
Trees are known to mitigate UHIE by changing the microclimate. It is cooler under or near trees due to shade and evapotranspiration. Tree shade prevents sunlight from reaching buildings and infrastructure, therefore preventing a subsequent rise in temperature. Urban trees reduce residential energy consumption for cooling by 7.2% annually.
An earlier study had shown that shade is more important than evapotranspiration in improving microclimate and causing urban cooling. Many factors can affect the quality and quantity of shade a tree provides.
To better understand these factors, three scientists from the departments of environment and social science at Western Sydney University, Australia, conducted the following experiment.
They reasoned that shade qulaity is mainly influenced by the density of the canopy, which can be measured by Leaf Area Index (LAI). Canopies with low LAI are open and provide less shade, while high LAI indicates dense canopy, which provides more shade. Besides density, the size of a tree canopy can also be important to improve the microclimate. The Australian scientists also decided to examine how surfaces made of different materials reacted to shading.
Figure 1. “The picture shows examples of common tree species and surface types tested in the experiment. (A) Crepe myrtle (Lagerstroemia L.) with brick pavers and grass in sunlight and shade. (B) Jacaranda (Jacaranda mimosifolia D.Don) with asphalt and grass in sunlight and shade. (C) Paperbark (Melaleuca quinquenervia (Cav.) S.T.Blake) with bark mulch in sunlight and shade. (D) Queensland box (Lophostemon confertus (R.Br.) Peter G.Wilson & J.T.Waterh.) with grass, bark mulch, and asphalt in sunlight and shade,” Kaluarachichi, et al., (2020). (Image credits: https://doi.org/10.3390/f11111141)
The experiment was conducted in Parramatta, a city in Greater Sydney, which has recorded the highest UHIE in the state of New South Wales. The city has a population of 2.2 million, and growing urbanization has decreased tree cover by 0.83% between 2009 to 2016.
The scientists sampled 471 trees belonging to thirteen species, which were native, exotic, evergreen, and deciduous. These trees are usually planted in parks and streets. The surfaces tested were grass, asphalt, bark, and mulch in sunlight and shade. Figure 1 shows the most common species and surfaces tested in the experiment.
Tree data collected were stem diameter at breast height (DBH), total height, and crown size [measured by vertical crown projected area (Ac)]. Leaf area index was also measured, using CI-110 Plant Canopy Imager, a plant canopy analyzer. Leaf Area Index is the one-sided leaf area per meter square of ground area.
The scientists recorded the Leaf Area Index from the ground in two random spots under each tree. The plant canopy analyzer manufactured by CID Bio Science Inc. has a long arm studded with 24 PAR sensors and has a fisheye camera that takes 150o. A big advantage of the plant canopy analyzer was that it required no above-canopy measurements. The images are divided into zenith and azimuthal divisions, and the scientists could choose which divisions they wanted to investigate.
The instrument has software that uses the Gap Fraction Method to make non-destructive measurements of the Leaf Area Index. This indicates how much sky a person can see on the ground through the tree canopy. A value of “0” means no sky is visible, and a value of “1” means only the sky is visible and there is no canopy. Fractions indicate partial canopy cover.
The scientists also measured surface and black globe temperature in the afternoon, at the warmest times of the day (1200 to 1500 hours) under tree shade and adjacent open areas. The black globe temperature indirectly measures human thermal comfort. It is a composite metric of air temperature, sunlight, radiant heat, relative humidity, and wind speed measured by a tripod-mounted weather station. The scientists used the weather station to take measurements for 15 minutes, both under the tree shade and outside the shade.
Figure 2: “Distribution of the surface temperature differential (ΔTS; panel (A)) and Leaf Area Index (LAI; panel (B)) in each tree species. Distribution of LAI in each species is arranged from the highest mean LAI to the lowest mean LAI,” Kaluarachichi, et al., (2020). (Image credits: https://doi.org/10.3390/f11111141)
The difference in surface temperature, or surface temperature differential (ΔTS), was used to judge how effective shade was in cooling surfaces.
Trees reduced the surface temperature by 20oC on average. Trees such as Lophostemon confertus, Pyrus calleryana, and Liquidambar styraciflua reduced the temperatures by a whooping 40oC.
Contrary to expectations, there was no statistically significant effect of LAI and crown size (Ac) on the surface temperature differential. For example, Waterhouse floribunda had the second-highest Leaf Area Index but the least effect on ΔTS; see Figure 2. Even a globe temperature reduction of 13oC was not correlated to Leaf Area Index and crown size.
Figure 3: “Box whisker plot illustrates the distribution of surface temperature differential (ΔTS) in surface types: bare soil, bark mulch, bitumen, grass, and pavers. Levels not connected by the same letter are significantly different,” Kaluarachichi, et al., (2020). (Image credits: https://doi.org/10.3390/f11111141)
How effective shade is in urban cooling is determined by the surface material of the infrastructure. There was a significant difference in the surface temperatures under the sun, shade, and between the surfaces tested.
Surface temperatures under the shade ranged from 20.4oC to 54.7oC; bitumen had the highest temperatures, followed by bark mulch, concrete pavers, bare soil, and grass. In the sun, the surface temperature ranged from 30.1oC to 76.9oC, where bark mulch had the highest temperature followed by bare soil, bitumen, grass, and concrete pavers.
Bark mulch (24.8 ± 7.1 oC) saw the greatest reduction in surface temperatures due to shading. Bare soil recorded the second-highest surface temperature differential (22.1 ± 5.5 oC), followed by bitumen ( 20.9 ± 5.8 oC), grass (18.5 ± 4.8 oC), and concrete pavers (17.5 ± 6.0 oC). The scientists explain this in the context of previous studies that have shown that bark mulch has a low albedo of 0.05, compared to bitumen (0.2–0.05), concrete pavement (0.13–0.1), and bare soil (0.26–0.16). However, they suggest further experiments to find all the factors that are behind the differences seen on the surface.
Globe temperature under shade and the difference with sunny areas are also not connected to species or tree canopy characteristics. Shaded globe temperature ranges from 26.3 to 44.5 oC, with the bark mulch showing the highest values followed by bitumen, pavers, bare soil, and grass. The same trend was seen for globe temperature under the sun, where the range was 28.4 to 54.1 oC. Consequently, bark mulch showed the greatest difference in temperature due to shading and grass showed the least difference.
Grass was had the lowest surface and globe temperatures both under the sun and shade. Grass had the highest albedo of the surfaces tested in the experiment and does not store heat. Moreover, the scientists think that evapotranspiration and the use of light for photosynthesis by grass could have also played a role.
One of the limitations that the scientists noted was that the city had mostly young mature trees. There were only a few older trees with wide canopies. Tree canopy size increases with maturity, and overlapping canopies can have an increased cooling effect, which could not be measured in this study.
There is an advantage if urban cooling depends on the surface material and not the tree species and canopy features, such as size and density of foliage. The choice of species that can be used to cool urban spaces increases, and trees that are fast-growing species can be chosen. This can help individuals and municipal authorities get shade faster to improve living quality and fight climate change. Moreover, the planners and municipal authorities should consider their choice of material for constructing infrastructure to keep the city cooler.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Britannica, T. Editors of Encyclopaedia (2020, July 9). Albedo. Encyclopedia Britannica. https://www.britannica.com/science/albedo
Kaluarachichi, T.U.N., Tjoelker, M.G., & Pfautsch, S. (2020). Temperature Reduction in Urban Surface Materials through Tree Shading Depends on Surface Type Not Tree Species. Forests, 11(11), 1141; https://doi.org/10.3390/f11111141
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