Photosynthetic and Transpiration Rates of Three Sedum Species Used for Green Roofs

Sedum species are the most commonly used vegetative plants in extensive green roofs worldwide. Green roof technology provides several environmental benefits, for example, cooling and insulation of buildings (Wong et al., 2003; Sailor, 2008), mitigation of the urban heat-island effect (Susca et al., 2011), stormwater management (Villarreal and Bengtsson, 2005; Getter et al., 2007; Rowe, 2011), carbon sequestration and reduction of air pollution (Yang et al., 2008; Getter et al., 2009), and habitat provision for other organisms (Kadas, 2006). These benefits result from the presence of living plants and growth medium on green roofs. Studying the physiological traits of green roof plants is the key to understanding the environmental benefits of green roofs. Sedum species use the crassulacean acid metabolism (CAM) photosynthetic pathway, which plays a crucial role in their growth under drought conditions (Yamori et al., 2014; Way and Yamori, 2014). In addition, several Sedum species have been described as “inducible” CAM plants (Lee and Griffiths, 1987; Gravatt and Martin, 1992). They are actually C3 plants with an ability to switch their carbon metabolism to the CAM pathway in response to drought stress (Sayed, 2001). This suggests that the physiological responses of Sedum species to soil water regimes severely affect the carbon sequestration and the cooling effect of a Sedum green roof. However, the investigation of Sedum species in green roofs has tended to focus primarily on their drought tolerance (Monterusso et al., 2005; VanWoert et al., 2005; Nagase and Dunnett, 2010; Thuring et al., 2010), and less attention has been paid to their physiological traits in wet conditions. The aim of the present study was to investigate the physiological responses of three Sedum species to different soil water regimes, and compare their photosynthetic and transpiration rates with those of two other commonly used green roof plants. In addition, we discuss the influence of soil water regimes on the environmental benefits of the Sedum green roofs.