Physiological and Morphological Traits and Competence for Carbon Sequestration of Several Green Roof Plants under a Controlled Environmental System

Abstract

The physiological and morphological traits of green roof 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. Moreover, there have been few studies on the relevance of physiological and morphological traits and competence for carbon sequestration of green roof plants. In particular, Sedum L. species are generally regarded as crassulacean acid metabolism (CAM) plants, but several Sedum species are recognized as having an “inducible type of CAM.” These plants 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. 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. The purpose of this study is to compare the physiological and morphological traits of four green roof plants and discuss the influence of these traits on their environmental benefits under a controlled environmental system. In addition, we attempted to clarify the relevance of physiological and morphological traits and competence for carbon sequestration in each plant using growth analysis. We used Sedum mexicanum Briton, Sedum aizoon L., Zoysia matrella (L.) Merr., and Ophiopogon japonicus (Thunb.) Ker Gawl. in this study, and only the two Sedum species were assigned treatments with different frequency of irrigation to investigate physiological and morphological responses and variation in carbon sequestration. The two Sedum species exhibited the C3 photosynthetic pathway in wet and dry treatments, implying the inducible type of CAM. Morphological responses of the two Sedum species were significantly affected by the different frequencies of irrigation; plants responded to increases in below-ground resources (water and nutrients) with increased biomass allocation of leaves and stems. Owing to these physiological and morphological responses in the wet treatment, transpiration rates and competence for carbon sequestration [relative C sequestration rate per whole-plant C content (RGRc)] of the two Sedum species were higher than those of Z. matrella and O. japonicus. This result suggested that the cooling effect and carbon sequestration of Sedum in wet and increased nutrient conditions are equivalent to those observed in other plants. In addition, the net assimilation rate (NARc) and leaf area ratio per whole-plant C content (LARc) were significantly correlated with photosynthetic rates and biomass allocation. Thus, it became clear that growth analysis can reveal the relevance of physiological (NARc) and morphological (LARc) traits and RGRc. Our results will serve as a baseline of the physiological and morphological traits and carbon sequestration of green roof plants and contribute to more suitable design and maintenance of vegetation in green roofs.