Spatial-temporal pattern in the cooling effect of a large urban forest and the factors driving it

Abstract

This study evaluated the cooling effect of the 30.07 km2 Purple Mountain Forest (PMF) in Nanjing City, China. Data from fixed and mobile measurements of air temperature inside and outside of the PMF over 37 days were used to conduct a holistic analysis of spatial/temporal patterns in the cooling effect of forest and the factors driving this variation. The forest exhibited a maximum cooling effect of 8.4 °C throughout the study period. The average daytime cooling intensity was 4.1 °C in the central undeveloped area, decreased to 1.8 °C at the highly developed tourism zone, and further to 1.3 °C at the boundary zone and 0.3 °C in the outside built-up area. Cool air could extend 267 and 883 m into nearby built-up areas along two roads radiating from the forest boundary. The cooling intensity could increase by 1 °C for every 950 m reduction in distance to the central area, every 95 m increase in altitude, or for every 0.8 increase in leaf area index. Cooling was more pronounced in the nighttime than in the daytime and on sunny days than on cloudy and rainy days. Solar radiation and relative humidity were key factors affecting the cooling intensity. Rainfall could considerably increase the cooling intensity of subsequent sunny days at the tourism and boundary zones of the forest. The results of this research enhance our understanding of the cooling effect of large urban forests. Our results also have implications for urban planning and forest management in the context of global warming.