Transpirational cooling and physiological responses of trees to heat

Abstract

Heatwaves cause severe adverse effects on urban human life. Green spaces that cool the urban environment through shading and transpiration can improve the urban thermal environment. However, there is currently a lack of quantitative evaluation of the transpirational cooling effect and physiological response of trees to heat. Therefore, this exploratory study investigated two pairs of trees (Cinnamomum camphora and Cedrus deodara), which are commonly used for urban greening in Nanjing, China. Transpirational cooling was estimated using the heat diffusion method for xylem sap flow in four trees. The morphological characteristics and environmental factors of each forest land were measured, and the transpirational cooling of the four trees was analyzed statistically. The results indicated that the canopy transpriation of C. deodara samples decreased substantially during extreme heat days. At the unit leaf area scale, the transpirational cooling effect of C. camphora samples was more pronounced than that of C. deodara samples during extreme heat days. Water vapor pressure and soil heat flux at 20 cm underground were the main drivers affecting the canopy transpiration of C. camphora samples, whereas air temperature and soil temperature at 5 cm underground were the main drivers affecting the canopy transpiration of C. deodara samples. To analyze the physiological cooling effects of trees, a new index (Transpirational Cooling Efficiency, TCE) is proposed to quantitatively evaluate the relationship between tree transpirational cooling and sap flow. The findings of this study may enrich the response patterns of trees to transpiration under extreme heat and provide guidance for the planning and designing of green spaces to improve thermal environments.