Tree cooling effects and human thermal comfort under contrasting species and sites

Research Highlights: This paper presents a cross-city empirical study on micro-climatic thermal benefits of urban trees, using machine-learning analysis to identify the importance of several in situ measured tree physiognomy traits for cooling. Background and Objectives: Green infrastructure and trees in particular play a key role in mitigating the urban heat island (UHI) effect. A more detailed understanding of the cooling potential of urban trees and specific tree traits is necessary to support tree management decisions for cooling our progressively hot cities. The goal of this study was to identify the influence and importance of various tree traits and site conditions. Materials and Methods: Surface temperature, air temperature at 1.1 m and at tree crown height, as well as wet bulb globe-temperature of shaded and fully sun-exposed reference areas, were used to study the cooling effect of seven different urban tree species. For all 100 individuals, tree height, crown base, trunk circumference, crown volume, crown area, leaf area index (LAI) and leaf area density (LAD) were measured. Measurements were conducted in the cities of Dresden, Salzburg, Szeged, and Vienna as representatives for middle European cities in different climate zones. Results: Beside site conditions, tree species, height, height of crown base, as well as trunk circumference, have a great influence on the cooling effect for city dwellers. The trunk circumference is a very valuable indicator for estimating climate regulating ecosystem services and therefore a highly robust estimator for policy makers and tree management practitioners when planning and managing urban green areas for improving the availability and provision of ecosystem services.

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“…al. [34] described average temperature differences of only 0.4-0.8 °C or 1.0-1.3 °C. However, Golden et.…”

Section: Cooling Of Air Temperature At 11 M Heightmentioning

confidence: 98%

Identifying Tree Traits for Cooling Urban Heat Islands—A Cross-City Empirical Analysis

Helletsgruber

Gillner

Gulyás

et al. 2020

Forests

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“…Evidence for the positive contribution trees make to society is extensive. Thermal comfort of urban areas can be improved as a result of the increased shelter, shading, and transpirational cooling provided by trees (Deak Sjöman et al., 2016; Moss et al., 2019; Rahman et al., 2020). The interception and storage of rainfall associated with trees alleviates flood events in areas with extensive canopy cover (Berland et al., 2017).…”

Section: Introductionmentioning

confidence: 99%

Using botanic gardens and arboreta to help identify urban trees for the future

Hirons

Watkins

Baxter

et al. 2020

Plants People Planet

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Diversification of urban forests is essential to enhance their resilience to future biotic threats as well as those posed by a changing climate. Arboreta and botanic gardens host a wide range of plant material that can be evaluated to inform tree selection policy. This study demonstrates that plant functional traits, such as the water potential at leaf turgor loss, can be highly instructive when developing evidence‐based recommendations for urban environments. However, if botanic collections are to fulfill a critical role in understanding plant response to environment, they should not be managed solely as visitor attractions but must have scientific objectives at the forefront of management policy.

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“…This effect might significantly affect the immediate environment. For example, the results of experimental studies showed that evaporative cooling alone contributes to an air temperature reduction up to 3° within the canopies (Rahman et al 2017b , 2020b ) and the canopy-to-air temperature difference depends on meteorological conditions, tree species and, urban site-specific characteristics (Meier and Scherer 2012 ). The cooling effect by transpiration depends to a large extent on stomatal conductance (Tan et al 2018 ) and our results are in agreement with measured values at midday on Tilia europaea in Sweden (0.1–0.2 mol m −2 s −1 ) (Konarska et al 2016 ).…”

Section: Discussionmentioning

confidence: 99%

A single tree model to consistently simulate cooling, shading, and pollution uptake of urban trees

et al. 2020

Self Cite

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Extremely high temperatures, which negatively affect the human health and plant performances, are becoming more frequent in cities. Urban green infrastructure, particularly trees, can mitigate this issue through cooling due to transpiration, and shading. Temperature regulation by trees depends on feedbacks among the climate, water supply, and plant physiology. However, in contrast to forest or general ecosystem models, most current urban tree models still lack basic processes, such as the consideration of soil water limitation, or have not been evaluated sufficiently. In this study, we present a new model that couples the soil water balance with energy calculations to assess the physiological responses and microclimate effects of a common urban street-tree species (Tilia cordata Mill.) on temperature regulation. We contrast two urban sites in Munich, Germany, with different degree of surface sealing at which microclimate and transpiration had been measured. Simulations indicate that differences in wind speed and soil water supply can be made responsible for the differences in transpiration. Nevertheless, the calculation of the overall energy balance showed that the shading effect, which depends on the leaf area index and canopy cover, contributes the most to the temperature reduction at midday. Finally, we demonstrate that the consideration of soil water availability for stomatal conductance has realistic impacts on the calculation of gaseous pollutant uptake (e.g., ozone). In conclusion, the presented model has demonstrated its ability to quantify two major ecosystem services (temperature mitigation and air pollution removal) consistently in dependence on meteorological and site conditions.

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Tree cooling effects and human thermal comfort under contrasting species and sites

Cited by 87 publications

References 48 publications

Identifying Tree Traits for Cooling Urban Heat Islands—A Cross-City Empirical Analysis

Identifying Tree Traits for Cooling Urban Heat Islands—A Cross-City Empirical Analysis

Using botanic gardens and arboreta to help identify urban trees for the future

A single tree model to consistently simulate cooling, shading, and pollution uptake of urban trees

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