Mitigating Urban Heat with Tree Layout in Urban Green Spaces

A recent study found that the shade provided by trees significantly reduces the mean radiant temperature and physiologically equivalent temperature in urban green spaces during summer. While we often associate tree’s reduction of temperature with shade, the major influence of tree layouts on thermal comfort was found to be wind distribution, with low wind regimes created in the downstream area of trees. The study suggests that to improve thermal comfort, trees should be arranged downstream of wind to avoid low winds at the site. This information can be used by landscape architects to design urban green spaces that provide good thermal comfort for visitors during summer by properly arranging the trees in the space.

So what is the key takeaway?

In order to improve summer thermal comfort, it is suggested to arrange trees downstream of the prevailing winds in order to avoid areas of stagnating wind which can negatively impact thermal comfort.

Simulating tree layouts
After collecting data for nearly a month, the study was able to simulate Shanghai’s characteristic southwest winds, temperatures both in the air and at surface, and many other meteorological conditions.The study investigated the effects of different tree layouts on microclimate and outdoor thermal comfort in an "open forest and grassland" type of green space. To avoid the influence of background elements such as buildings, water bodies, and site shapes, generic layouts were used. For the sake of the simulations, the generic layouts were based on a green space at Shanghai Jiao Tong University where 49 Cinnamomum camphora were planted on a 100 m x 100 m land with grass, with a 10 m-wide border around the entire site, making it 120 m x 120 m. The sizes of the trees were uniform, with a height of 7m and a crown diameter of 5m. The study looked at five groups of generic layouts: squared corners, arrowed corners, canopy gaps, determinant plantings, and evenly distributed. The first four groups had four different sublayouts each, while the evenly distributed case had only one sublayout.

Tree arrangements for the simulation tested different cluster densities and locations to determine the influence on heat and wind. 

Trees slow winds

The study found that the wind speed distribution was affected by the different tree layouts, with the most notable change being the creation of a large downstream "cavity" when the trees were located at the upstream area of the site. The trees decelerated the wind not only in the regions under the trees but also behind the trees. The size of the cavity was found to depend on the shape of the tree block, with greater frontal area leading to a larger cavity. The position of the trees did not change the general pattern of wind distribution, but the cavity occurred outside the studied site for some layouts. The "evenly distributed" scenario had a different pattern, with the wind gradually reducing from upstream to downstream. The study also found that the wind was deflected and accelerated when it first encountered the block of trees.

Map of wind speeds. Locating trees to the North and East (downstream from Shanghai’s southwesterly winds) lead to more free-flowing winds. 

This in turn, influences air temperature

The study found that the spatial distribution of air temperature was affected by the different tree layouts, with the lowest air temperature found under the trees, and the highest air temperature found in the downstream area behind the trees. The air temperature distributions were related to the wind speed and radiation. The solar radiation was blocked by the trees which reduced air temperatures under the shade. The air temperature was also reduced due to transpiration. The low wind cavity created behind the trees reduced the convective heat transfer and increased the air temperature. The different tree layouts did not change the general pattern of the air temperature distributions. However, for some layouts, the downstream high air temperature region mainly occurred outside the studied green area. The maximum difference in air temperature within the green area was limited to 1°C, with the highest and lowest air temperatures being 30.9°C and 29.9°C respectively. The spatially averaged air temperatures for different layouts were between 30.4°C and 30.5°C.

Map of air temperatures at 1.4m. Locating trees to the North and East (column 3 and 4) lead to cooler spaces. Trees located to the south or west cause winds to stagnate and retain heat. This is also the case for evenly distributed trees.

Practical implications

The study found that the layout of trees can have an impact on the microclimate in "open forest and grassland" type of green spaces. The layout of the trees changed the air temperature distribution, but the variation of air temperature was limited to 1°C. The distribution of radiation and wind speed, however, were greatly affected. The shading provided by the trees significantly reduced radiation, but due to the high solar altitude in summer Shanghai and the small fraction of trees compared to the entire space, changing the layout of the trees did not greatly increase the shading area, even when the trees were evenly distributed on the site. Therefore, it is not necessary to scatter the trees for better shading as the increase in shaded area by scattering is limited, and clusters of trees can still be used to create an aesthetic appearance. Additionally, the layout of the trees can create a low wind cavity behind the tree clusters, which can negatively affect thermal comfort during the summer. To avoid this, the trees should be placed downstream to avoid a low wind regime.

In summary, this study suggests that when designing "open forest and grassland" types of green spaces, the layout of trees should be taken into consideration to improve microclimate and thermal comfort. The study found that tree shade provided significant reduction in radiation and air temperature, but the layout of trees had limited impact on the shading area. To improve summer thermal comfort, it is suggested to arrange trees downstream of the wind to avoid low wind "cavities" that can negatively impact thermal comfort. Additionally, clusters of trees can still be used to create an aesthetic appearance, and it is not necessary to scatter the trees for better shading.

How can this apply to Sydney? 

Based on the findings of this study, to improve summer thermal comfort in green spaces in Sydney, it could be suggested to arrange trees downstream of the northeasterly winds to avoid low winds at the site. Additionally, it is important to keep in mind that the specific tree species, size, and layout may need to be adjusted to suit the unique microclimatic conditions of Sydney. The study also suggests that it is unnecessary to scatter the trees for better shading, as the increase in shaded area by scattering was limited, and clusters of trees can still be used to create aesthetic appearance.