The importance of street trees for human thermal comfort in a warming climate

Abstract

© 2015 Dr. Ruzana Adibah Mohd SanusiTree microclimate and human thermal comfort are important for urban residents in summer as trees can improve street microclimate for pedestrians through shade and transpiration. It is important to know how trees provide microclimate and human thermal benefits and in what circumstances they provide the most benefit to the urban environment. This thesis provides greater understanding as to the influence of extreme heat weather events on street microclimate and human thermal comfort. The assessment of street microclimate modification under the trees was done in all studies through measurements of air temperature, relative humidity, solar radiation, Mean Radiant Temperature (Tmrt) and wind speed. The human-biometeorological index of Physiological Equivalent Temperature (PET) was used for the assessment of human thermal comfort. Cooling benefits from trees depend upon the percentage of canopy cover, street orientation, the side of the street being assessed and time of the day. The air temperature in treed streets could be 2.1°C cooler, and the human thermal comfort perceived by pedestrians could be 4.6°C cooler in high-canopy cover streets than in low-canopy cover streets. These microclimate benefits were greatly influenced by the street orientation due to the influence of sun’s zenith. In addition, the differences in cooling benefit amongst three different tree species were primarily determined by the difference in the cumulative shade provided by leaves, branches and twigs are important in determining the microclimate and human thermal comfort benefits from trees. These differences were captured and quantified using digital full frame photography from which Plant Canopy Area Index (PCAI) was estimated underneath individual tree canopies. Platanus x acerifolia trees were the most prone to leaf loss due to a heat wave event compared to Ulmus procera and Eucalyptus scoparia with 44% of the canopy lost. This canopy loss of P. acerifolia due to the heat wave event also changed the cooling benefits of the P. acerifolia trees and lead to a PET benefit reduction from 6.6°C to 1.6°C after the heat wave. This thesis has been successful in identifying and quantifying some of the key factors that influence street microclimate and human thermal comfort benefits provided by trees. The research outcomes from this thesis contribute to the knowledge and future direction of urban forest planning and management in particular for the provision of ecosystem services provided by street trees in the form of microclimate and human thermal comfort benefits