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Zheng, S.; He, C.; Guldmann, J.; Xu, H.; Liu, X. Heat Mitigation Benefits of Urban Trees. Encyclopedia. Available online: https://encyclopedia.pub/entry/53117 (accessed on 02 July 2024).
Zheng S, He C, Guldmann J, Xu H, Liu X. Heat Mitigation Benefits of Urban Trees. Encyclopedia. Available at: https://encyclopedia.pub/entry/53117. Accessed July 02, 2024.
Zheng, Senlin, Caiwei He, Jean-Michel Guldmann, Haodong Xu, Xiao Liu. "Heat Mitigation Benefits of Urban Trees" Encyclopedia, https://encyclopedia.pub/entry/53117 (accessed July 02, 2024).
Zheng, S., He, C., Guldmann, J., Xu, H., & Liu, X. (2023, December 25). Heat Mitigation Benefits of Urban Trees. In Encyclopedia. https://encyclopedia.pub/entry/53117
Zheng, Senlin, et al. "Heat Mitigation Benefits of Urban Trees." Encyclopedia. Web. 25 December, 2023.
Heat Mitigation Benefits of Urban Trees
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This entry is adapted from the peer-reviewed paper 10.3390/f14122280

Modeling, validating, and simulating are three essential parts in investigating the heat mitigation benefits of urban trees (BUT). Therefore, 81 relevant studies from the last ten years are reviewed, analyzed. Three main ways for urban trees to adjust the environment are summarized, including shade creation and radiation modification, cooling effects of transpiration, and airflow blocking and modification effects. 

human thermal comfort microclimate tree radiation modification urban heat island

1. Microclimate Benefit Performance Evaluation through Measurement

The experimental research on trees in the field of urban microclimate mainly concerns microclimatic data (air temperature, humidity, solar radiation, wind direction, and wind speed, etc.) at measurement points and compares them with a tree’s physiological parameters (three-dimensional green quantity, leaf area index, canopy cover, canopy closure, plant coverage, average leaf inclination, etc.), as presented in Figure 1 and Table 1 [1][2]. Some scholars have also combined thermal environment simulation software, such as Envi-met 4.2 and Airpak 3.0, to analyze the impact of landscape design methods on the microclimate [3][4][5].
Forests 14 02280 g006
Figure 1. Layout of measuring points and instruments to obtain trees’ cooling effects. (a) is a schematic diagram; (b) is field measurement chart. A: weather station to obtain meteorological parameters in open areas; B and C: sensors to obtain meteorological parameters in shaded areas [6].
Table 1. Commonly used microclimatic measuring parameters and instruments [7].
Test Parameter Test Equipment Factory Owners Accurate Test Range
Air temperature
Relative humidity		 HOBO pro v2 data logger
(U23-001)		 Onset Computer Corporation, Bourne, MA, USA ±0.2 °C
(0~50 h)		 −40~70 °C
Wind Speed, Wind Direction
Black sphere temperature		 Ultrasonic anemometer
sensor (Model 81000)		 M. Young Company,
Traverse, MI, USA		 ±1% ± 0.05 m/s 0~40 m/s
Meteorological parameters Davis Vantage Pro2 Davis Company,
Boston, MA, USA		 ±0.5 °C (Ta)
±5% (v)		 −40~65 °C (Ta)
0–1800 W/m2 (S)
Transpiration rate
Leaf surface temperature		 Photosynthesis apparatus
Li-6400		 Decagon Company,
Pullman, WA, USA		 ±0.007 mmol/mol 0~75 mol
Solar radiation
Long-wave radiation		 4-component net radiation sensor NR01 Hukseflux Company,
Delft, The Netherlands		 7–25 μV/W/m2 0~2000 W/m2
Soil temperature T type thermocouple Sensors Company,
Wuxi, China		 ±0.05 °C −200~260 °C
Thermal imaging Thermal infrared imager Kaise Company,
Ueda, Japan		 ±2 °C −40~500 °C
Leaf reflectance Spectrophotometer (U-4100) Hitachi Company
Tokyo, Japan		 / 175~2600 nm
Root depth, root width and root density Tree Radar (TRU-100) Tree Radar Company,
Silver Spring, MD, USA		 1 cm /
There are three main ways in which trees adjust the environmental microclimate: solar radiation modification, transpiration, and blocking effect on airflow.
The solar radiation attenuation by the canopy is mainly affected by physical factors, such as branches and leaves, which differ somewhat across tree species. Therefore, the solar radiation attenuation performance of different tree species varies greatly, especially in different climate regions. Kotzen et al. [8] tested this attenuation effect for street trees common in tropical regions, and they analyzed the effects of solar radiation intensity, incident angle characteristics, and canopy leaf area density on solar radiation attenuation. Based on measured data, Akbari [9] discussed radiation occlusion and transmission mechanisms, indicating that planting design needs to take into account tree canopy density, tree height, canopy transmittance in different seasons, and canopy structure levels.
The transpiration of trees is an important influencing factor on the surface energy balance and cooling effects [10][11]. In order to quantify the cooling and humidification caused by trees, researchers rneed to accurately obtain a tree’s transpiration rate. At present, there are two main methods for measuring tree transpiration rates [9][12]: measuring the convective mass transfer coefficient (a) and air humidity on the blade surface. However, it is difficult to obtain the convective mass transfer coefficient in heterogeneous urban environments. Another is the trunk runoff method, which can use the trunk runoff meter to measure the liquid flow for a long time, but the instrument is usually very expensive and causes significant damage to the tree. Due to the limitations of the above methods, there are few experimental studies on tree transpiration rates. Akbari [9] found that trees can evaporate about 100 gallons of water per day in dry and hot climates. If evapotranspiration is combined with proper layout and shade, the temperature drop caused by nearby trees can reach 9 °C. Chen et al. [13] established a regression model for calculating the biomass of garden plants by measuring the daily transpiration rates of common trees in Beijing. Han [14] tested the transpiration rate, ecological effect, and utilization of light energy of common tree species in severely cold areas. By simplifying the calculation of the transpiration heat transfer, the cooling effect of different tree species in different months was obtained.
A tree’s blocking effects on airflow are not only related to its location and surrounding environment but also to the tree’s characteristics, such as size, orientation, porosity, and canopy density. Many researchers at home and abroad have conducted studies on the effects of trees on the near-surface wind environment. Shahidan et al. [15] found that, in a typical urban area, the physical parameters (leaf area index, crown width, and branches, etc.) of different trees yield large differences in impacts on airflow, and they also have a great impact on the wind environment, especially the leaf area index (LAI) and crown width. Heisler [16] found that the canopy blocking effect on wind speed in residential areas depends on the density of the canopy. Increasing the density by 10% can reduce wind speed by 10% to 20%, and increasing it by 30% can reduce wind speed by 15% to 35%.
In addition, the control of wind pressure and direction by trees will further affect the urban microclimate. Zheng [17] found that, if a site is located in the downwind direction of the plant coverage area, trees can play a role in reducing wind speed and wind pressure. Planting a dense row of trees can concentrate and strengthen the airflow under the canopy and improve ventilation conditions at the ground level under the trees. Dimoudi and Nikolopoulou [18] found that in the mainstream wind direction, the influencing distance of a tree on temperature is about five-times its height, and a rational tree layout can effectively improve the thermal comfort of pedestrians around a building. However, when the tree is in the non-mainstream wind direction, the impact is not obvious. Therefore, many studies have suggested that pedestrian comfort should be improved by combining urban greening with the main ventilation channels of urban areas.

2. Simulation and Prediction of Thermal Performance

In recent years, with the continuous improvements in computer performance, numerical simulation has become the main research method for the quantitative prediction and evaluation of urban thermal environments. It is necessary to include urban trees in numerical simulations to accurately calculate the urban surface energy balance and achieve accurate simulation of the UHI [19][20]. Because of the complexity of trees’ heat and moisture transfer processes and the diversity of their geometric shapes and spatial locations, it is very difficult to create 3D tree models in a given urban street environment, accounting for their spatial locations and sizes [20]. In order to meet this challenge, the commonly used approach is to use existing simulation software.
ENVI-met and ANSYS Fluent are CFD models that are widely used in microclimate simulation and outdoor thermal comfort studies. ENVI-met is a three-dimensional urban microclimate simulation software developed by Bruse and Fleer in 1998. It is based on heat transfer and computational fluid dynamics, and it is mainly used to simulate at the urban block scale, across ground, buildings, vegetation, and the atmosphere [21][22]. ANSYS Fluent, a general-purpose CFD platform based on the Finite Volume, provides comprehensive modelling of fluid flows under steady or transient conditions [23]. Since ANSYS Fluent requires the user to formulate a specific problem via user-defined functions, it requires a high level of physics expertise [24]. Until now, most numerical simulations of the impact of trees on the outdoor thermal environment have been carried out using ENVI-met. Zhang [11] used ENVI-met to simulate the arrangement of eight tree species in residential areas in summer and winter and found that the tree spacing ratio is essential to improve the outdoor thermal environment. Duarte [10] used ENVI-met to explore the influence of trees on air temperature and found that densely planted street trees are cooler than central and pocket parks. Chen [25] studied the impact of common tree species in humid and hot areas by coupling the energy consumption simulation software EQUEST 2.0 with ENVI-met 4.2.
However, because of the complexity of trees’ impact on the outdoor environment, ENVI-met simplifies the tree model as follows [7][19][26]: (1) in terms of solar radiation, ENVI-met only considers the attenuation of direct solar radiation by the tree canopy and does not consider the influence of trees on long-wave radiation and heat transfer between trees and the surrounding environment; (2) ENVI-met adopts an empirical resistance coefficient (0.2), which cannot be modified according to the actual species. These simplifications may cause ENVI-met to inaccurately simulate heat and mass exchanges between a tree and its surrounding environment.

3. Verification of Modeling Prediction Based on Measurement

Many researchers have evaluated the ENVI-met tree model in their own climate zones [21][22][27]. Zheng et al. [19][26] verified ENVI-met accuracy in hot and humid areas, showing that ENVI-met greatly simplifies the calculation processes of radiation, convection, and transpiration between trees and the environment, with large deviations in the simulation of radiation attenuation, wind speed, and transpiration rate. The root mean square error between the simulated and measured values of solar radiation under a tree reaches 256 W/m2.
With the development of computational fluid dynamics (CFD) technology, some studies have represented the effect of trees on airflow and heat and moisture transfer by adding source terms to the Navier–Stokes (N-S) equation [28]. Upreti [29] studied trees’ radiation attenuation, canopy flow, and heat and mass transfer, using the Monte Carlo method to calculate solar radiation and long-wave radiation attenuation by the tree canopy with a structured grid, simplifying the calculation of canopy radiation transmission by the method of spherical crown envelope surface. Gao and Long [30] coupled the CFD simulation of airflow with outdoor radiation calculation but did not calculate long- and short-wave radiation, nor did they involve the coupling of the tree canopy energy equation with the convection, heat transfer, and radiation equations of the surrounding environment. Argiro [18] simplified the microclimate model of trees by using fixed solar transmittance and transpiration rates. Using this model, they analyzed the microclimatic benefits of trees in the urban environment and conducted a parameter sensitivity analysis of the simplified tree model, which assumes that the sunlight transmittance of a tree canopy is constant and does not consider long-wave radiation.

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Subjects: Green & Sustainable Science & Technology
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Senlin Zheng
,
Caiwei He
,
Jean-Michel Guldmann
,
Haodong Xu
,
Xiao Liu
View Times: 114
Revisions: 2 times (View History)
Update Date: 26 Dec 2023
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