Abstract
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.
Highlights
Cities are the most important living space of humans, at least as measured by the proportion of the global population living in them
This growing urbanization causes huge changes in the urban environment [1]. One result of this transformation is the intensification of the Urban Heat Island (UHI) effect
The UHI effect is a systemic characteristic of urban climate and shows in higher air and surface temperatures compared to Forests 2020, 11, 1064; doi:10.3390/f11101064
Summary
Cities are the most important living space of humans, at least as measured by the proportion of the global population living in them. This growing urbanization causes huge changes in the urban environment [1] One result of this transformation is the intensification of the Urban Heat Island (UHI) effect. Mean values of the annual number of summer days (≥25 ◦ C) are expected to increase by 20–50 days in the decades ahead compared to the period 1971–2000, depending on the climate scenario (RCP4.5 vs 8.5) [4]. The severity of this change is shown by a health risk assessment study in the UK which found a 2.1% increase in mortality for every 1 ◦ C rise in temperature [5]
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