Assessing the microclimatic and cool-island benefits of vegetation at a Hong Kong golf course

Abstract

This microclimatic study deciphers the various strengths of air and surface cooling by different vegetation communities to inform incorporation of these natural cooling assets within the compact urban fabric of cities like Hong Kong. This study explored the effect of vegetation type, community structure, seasonal and weather effects on cooling extents compared to worse-case urban scenarios represented by a bare rooftop control plot (Control). Eight treatment plots at a Hong Kong golf course comprised four lawn plots, two tree strips and two woodlands. Data collection spanned 12 months, during which microclimatic parameters (e.g. air temperature, relative humidity, wind, and solar radiation) were recorded around-the-clock. Analyses of overall differences between plots’ microclimatic conditions were conducted. Thermal comfort indexes of universal thermal comfort index (UTCI) and physiological equivalent temperatures (PET) were calculated and analysed for the summer. Air temperature and thermal comfort conditions of a pondside lawn, an open lawn and Control were compared to shed light on the role of blue space in cooling. Correlations of air temperature conditions and parallel with other weather parameters were carried out to analyse key elements for year-round cooling by woodlands. On sunny summer day, woodlands created a maximum cooling index of -4.1 °C compared to Control, followed by moderate cooling by tree strips and then lawn plots. Woodlands’ outstanding cooling ability withstands the effect of season and weather, whereas tree strips’ cooling abilities are largely subject to sun angle. On the hottest days, extreme heat stress levels were found at Control under both UTCI (11.1% incidence) and PET (66.7 % incidence), whereas extreme PET heat stress was also present at lawns and tree strips. Despite cooler air temperature at the pondside lawn, its mean UTCI cooling was weaker by 0.6 °C than the open lawn. Cooling indexes of all plots were suppressed in cloudy, overcast and rainy days and in winter. Slightly warmer conditions were found in woodlands in winter when compared to open lawn due to heat retention biomass structure created by tree trunks and understory vegetation. The tree shade lowers the effect of solar radiation on subcanopy microclimates, which is self-regulated by evapotranspiration of local vegetation, ground cover and community structure. Solutions to creating a thermally amicable urban atmosphere lies in increasing the percentage of urban woodlands, incorporation of large areas of natural surfaces in urban green space and enhancement of the invaluable cooling provision of tree shade throughout the city. Realization of these visions reinforces a city’s resilience against climate change and benefits city dwellers’ quality of life.