Green roof plant physiological water demand for transpiration under extreme heat

Green roofs are a sustainable strategy for improving the eco-environment in urban areas. However, plants on green roofs are increasingly threatened by extreme heat and drought due to climate change. There is a lack of comprehensive understanding of the physiological water demand of green roof vegetation under extreme heat. Therefore, this exploratory study investigated two commonly used Sedum species for urban green roofs (Sedum spectabile Boreau and Sedum alfredii Hance) under extreme heat, considering two treatments: irrigation and nonirrigation. The results indicated that under extreme heat, if without irrigation, the two Sedum species' mean daily water consumption by transpiration was approximately 4.68 g per Sedum plant. However, under irrigation, the diurnal variation of stomatal conductance of the two Sedum species allowed the stomata to remain open throughout the day, resulting in an over-twentyfold increase in mean daily water consumption through transpiration (120.34 g per Sedum plant) and twofold increase in leaf area index. A random forest model showed that multi-environmental factors explained 67.53 % of the variability in stomatal behavior. Photosynthetically active radiation and soil moisture are the primary environmental factors that directly affect stomatal conductance. The irrigation induces stomatal opening throughout the diurnal cycle, ensuring the continuation of vegetation growth and the maintenance of physiological functions under extreme heat, which then contributes to keeping its ecoenvironmental functions and providing sustainable services, such as cooling and carbon sequestration. The findings of this study can guide planning and managing green roofs in urban areas that face extreme heat events.