Microclimatic resilience of subtropical woodlands and urban-forest benefits

Abstract

With compact development mode and subtropical climate, urban Hong Kong suffers from the urban heat island exacerbated by climate change. In recent years, record-high summer air temperatures (Ta) and heat stress were recorded, which discourage outdoor exercise and nature-based enjoyment. The thermal overload could be mitigated by urban woodlands that bring effective cooling through evapotranspiration and shading by diverse species composition, growth forms and biomass structure. This study measured microclimatic parameters over 12 months at four experimental plots in a peri-urban golf course, including simple woodland with limited species and single-layer structure, complex woodland with diverse species and multiple-tier structure, unshaded lawn, and bare-concrete serving as control. Woodland cooling was most prominent on summer and sunny days, and weaker in winter and cloudy conditions. Daytime Ta in woodlands was cooler, whereas nighttime warming was observed mainly at the simple woodland. Despite seasonal variations of cooling strengths, Ta was consistently suppressed on cloudy day vis-a-vis sunny day. On summer sunny day, concrete and lawn plots registered scorching air and surface temperatures (Ts), whereas woodland plots demonstrated summer cooling amplification with maximum −4.1 °C Ta depression versus concrete and −3.9 °C versus lawn. Comprehensive evaluations and comparisons were made between the magnitude and variations of the woodland cooling effect in different seasons and weather conditions. The benefits and feasibility of creating urban woodland patches in Hong Kong to improve urban climate and thermal comfort were discussed.