The Best Urban Trees for Daytime Cooling Leave Nights Slightly Warmer

Agnieszka Wujeska-Klause,Sebastian Pfautsch

doi:10.3390/f11090945

Abstract

Summer air temperatures will continue to rise in metropolitan regions due to climate change and urbanization, intensifying daytime and nighttime air temperatures and result in greater thermal discomfort for city dwellers. Urban heat may be reduced by trees which provide shade, decreasing air and surface temperatures underneath their canopies. We asked whether tree height and canopy density can help to identify species that provide greater microclimate benefits during day and night. We also asked if increased canopy cover of street trees provides similar microclimate benefits. We used continuous measurements of near-surface air temperatures under 36 park trees and from two urban streets to assess these questions. In the park, trees were grouped according to their height (<10 m, 10–20 m, >20 m) and canopy density (low, high), while the effect of canopy cover was tested using streets with high (31%) and low (11%) cover. Daytime near-surface air temperature declined with increasing height and canopy density providing significant cooling benefits. However, this trend was reversed at night when tall trees with dense canopies restricted longwave radiative cooling and trapped warm air beneath their crowns. High canopy cover of street trees reduced daytime air temperatures more, resulting in a lower number of days with hot (>35 °C) and extreme (>40 °C) air temperatures compared to the street that had low canopy cover. These findings suggest that tree species and streetscapes with dense canopy cover improve local thermal conditions during the day but do not seem ideal to allow for nighttime cooling, creating potential discomfort for residents during hot summer nights. Our results indicate that classifying trees using a simple metric can assist in selecting tree species that can alleviate the local negative effect of urban heat during the day, but at the same time, their effect in preventing optimal longwave radiative cooling during the night must be factored into planting strategies.

Highlights

Given that most of the world population resides in cities, increasing urban air temperatures as a result of human activity is a concern for public health and wellbeing
These findings suggest that tree species and streetscapes with dense canopy cover improve local thermal conditions during the day but do not seem ideal to allow for nighttime cooling, creating potential discomfort for residents during hot summer nights
Our results indicate that classifying trees using a simple metric can assist in selecting tree species that can alleviate the local negative effect of urban heat during the day, but at the same time, their effect in preventing optimal longwave radiative cooling during the night must be factored into planting strategies

Summary

Introduction

Given that most of the world population resides in cities, increasing urban air temperatures as a result of human activity is a concern for public health and wellbeing. Since urban space is already warmer than non-urban areas [2,3], further increases in air temperatures have adverse effects on the urban environment and its residents as signified by the positive relationship between heat and human mortality rates [4]. As population growth and urban expansion continue, cost-effective strategies that help to limit the negative effects of rising air temperatures in our cities are needed. Replacement of natural vegetation by buildings and other low albedo surfaces in metropolitan areas affects the surface energy exchange at daytime and nighttime [5,6,7,8].

Methods

Results

Discussion

Conclusion