A preliminary exploration of the cooling effect of tree shade in urban landscapes

Qiuyan Yu,Wenjie Ji,Ruiliang Pu,Shawn Landry,Michael Acheampong,Jarlath O’ Neil-Dunne,Zhibin Ren,Shakhawat Hosen Tanim

doi:10.1016/j.jag.2020.102161

Abstract

Mitigating urban heat island (UHI) effects, especially under climate change, is necessary for the promotion of urban sustainability. Shade is one of the most important functions provided by urban trees for mitigating UHI. However, the cooling effect of tree shade has not been adequately investigated. In this study, we used a simple and straightforward method to quantify the spatial and temporal variation of tree shade and examined its effect on land surface temperature (LST). We used the hillshade function in a geographic information system to quantify the spatiotemporal patterns of tree shade by integrating sun location and tree height. Relationships between shade and LST were then compared in two cities, Tampa, Florida and New York City (NYC), New York. We found that: (1) Hillshade function combining the sun location and tree height can accurately capture the spatial and temporal variation of tree shade; (2) Tree shade, particularly at 07:30, has significant cooling effect on LST in Tampa and NYC; and (3) Shade has a stronger cooling effect in Tampa than in NYC, which is most likely due to the differences in the ratio of tree canopy to impervious surface cover, the spatial arrangements of trees and buildings, and their relative heights. Comparing the cooling effects of tree shade in two cities, this study provides important insights for urban planners for UHI mitigation in different cities.

Highlights

Urban heat island (UHI) effect as a result of urbanization, has become a major problem for the sustainable development of modern societies (Alavipanah et al, 2015; Giorgio et al, 2017; Murgante et al, 2011; Kim, 1992)
Temporal variability of tree shade was captured by the hillshade function as the cover and direction of shade all changed from sunrise to sunset
Examining the cooling effect of tree shade at a city scale provides both an avenue to improve the understanding of the ecosystem services of urban forest and to aid the development of efficient UHI mitigation strategies

Summary

Introduction

Urban heat island (UHI) effect as a result of urbanization, has become a major problem for the sustainable development of modern societies (Alavipanah et al, 2015; Giorgio et al, 2017; Murgante et al, 2011; Kim, 1992). About 27% of cities and 65% of urban population is currently experiencing warmer temperature (0.6 °C) than the world average (Estrada et al, 2017), as the impacts of climate change in cities can be exacerbated by UHI effects (Campbell, 1996; Huang et al, 2015; Tam et al, 2015). These phenomena threaten the sustainability of future urban development to support growing populations. In this regard, mitigating UHI effects is necessary for the promotion of urban sustainability, especially under climate change (Akbari et al, 2001)

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Discussion

Conclusion