The green roof is known to be a beneficial technology in urban environments. Some of the benefits include cooling and insulation of buildings (Sailor, 2008; Wong et al., 2003), mitigation of the urban heat-island effect (Susca et al., 2011), stormwater management (Getter et al., 2007; Villarreal and Bengtsson, 2005), carbon sequestration and air pollution reduction (Getter et al., 2009; Yang et al., 2008), and habitat provision for other organisms (Kadas, 2006). Owing to these environmental benefits, the green roof has been adopted in many countries. These benefits depend on the presence of living plants and the growing medium. For example, the boundary layer of air over the roof is cooled by evapotranspiration, shading from plants, and the trapping of cool air by the vegetation layer (Dimoudi and Nikolopoulou, 2003; Solecki et al., 2006). In addition, air pollution is reduced through plant photosynthesis and trapping of the gases by vegetation and soil layers (Garland, 1977). Consequently, the physiological and morphological traits of the plants are key to understanding the environmental benefits of green roofs. However, the comparative investigation of physiological and morphological traits of green roof plants is limited (Lundholm et al., 2015). Moreover, there have been few studies examining the relevance of physiological and morphological traits and competence for carbon sequestration in green roof plants.
Warm-season turfgrasses and Sedum species are the most common vegetation in Japan. Green roofs composed of warm-season turfgrasses or other perennials are generally installed with irrigation systems to prevent drought stress. In contrast, Sedum green roofs are not always installed with irrigation because these plants can adapt to periodic drought. They are regarded as CAM plants whose stomata remain closed during the day, with gas exchange occurring at night (VanWoert et al., 2005). This physiological pathway plays a crucial role in the plant’s response to drought stress; other CAM plants have also been investigated for utilization in green roofs (Lin and Lin, 2011). In addition, several Sedum species are recognized as having an “inducible type of CAM” (Gravatt and Martin, 1992; Lee and Griffiths, 1987). They are C3 and C4 plants with the ability to switch their carbon metabolism to the CAM pathway. In the case of Sedum, the inducer of CAM is drought stress (Sayed, 2001). This observation suggests that differences in water regimes result in physiological and morphological changes that may have a considerable effect on the environmental benefits of Sedum green roofs (e.g., carbon sequestration, air pollution reduction, and cooling effects). Further, although a Sedum green roof is likely to be exposed to drought conditions, the substrate may be wet during the rainy season in southeast Asian countries (Chen, 2013). Van Mechelen et al. (2015) also suggested that irrigation is necessary on green roofs in (semi)-arid climates for the survival and success of extensive green roof plantings. Therefore, it is necessary to investigate the physiological and morphological responses of Sedum to various water conditions resulting from different maintenance practices and climates.
The purpose of this study is to compare the physiological and morphological traits of several green roof plants and discuss the influence of these traits on their environmental benefits. In addition, we attempted to clarify the relevance of physiological and morphological traits and competence for carbon sequestration in each plant using growth analysis. In particular, Sedum species were assigned different water regimes to investigate the physiological and morphological responses and variation in carbon sequestration.
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