The urban heat island mitigation potential of vegetation depends on local surface type and shade

Abstract

Urban areas face multiple environmental challenges that interact with climate change, including the urban heat island (UHI) effect. Plants can be a nature-based solution for UHI-effect mitigation, alongside various artificial materials, but their performance is only commonly compared to concrete or asphalt. Given shade's ubiquity in the urban environment, it also has a strong potential to interact with and change urban spaces' thermal performances but is rarely included in plant UHI effect-mitigation analyses. We completed an experiment to record the UHI effect-mitigation potential of 10 plant species (turf grasses, shrubs and climber/creeper plants) and eight materials (including three with high albedo) with and without shade on a rooftop in tropical Singapore across multiple sunny days in May and June 2018. We explained the infrared thermography-recorded surface temperatures of plants and materials with a surface type–time interaction in a linear regression model. Our model predicted that, without shade, except for concrete coated with white paint or infrared-reflecting white paint, plants' exterior surfaces were cooler than all artificial materials by at least ∼11 °C at 13:30 (the peak surface temperature of artificial materials). Unshaded plant species had comparable surface temperatures to shaded artificial materials and were marginally cooler than unshaded high-albedo materials. Shading caused no significant reduction of temperatures for nine of the 10 plant species studied. Our results suggest that high-albedo materials and plants can mitigate the UHI effect, either on the ground or as sources of shade to offset heat gain, and that planting up shaded surfaces would generate marginal additional cooling. Our findings should be integrated into broader trade-off analyses on the economic and health value of replacing unshaded artificial materials with plants, as UHI-effect mitigation is one of many ecosystem services provided by plants.