Role of plant cover in the performances of Extensive Green Roofs in sub mediterranean environment

Green roofs provide many benefits to the urban environment, but an adequate plant cover is crucial in order to achieve optimal performances from these infrastructures. Plant cover improves the roof insulation in winter and reduces summer temperatures by shading and transpiration. Succulent species, such as Sedum species, are widely used on green roofs, thanks to their drought resistance due to CAM metabolism, as well as to the reduced maintenance they need. CAM metabolism, however, without diurnal transpiration, is responsible for the poor cooling performances in summer. In addition, many succulent species have slow colonization rates and/or are subject to freezing injury during the winter. A reduced plant cover has direct negative consequences both on the cooling by shading and on the winter insulation. We investigated how to create green roofs with a good plant cover, optimal summer and winter thermal performances and reduced management needs. Primarily, we compared, thought indoor experiments, the water losses, the net CO2 assimilation rates and the cooling performances of two CAM-facultative, three C3 and one C4 species, under well-watered and drought conditions. Our results confirmed that C3 and C4 tested species continue to transpire also when Sedum species adopt the CAM metabolism, suppressing the diurnal transpiration. C4 metabolism allows interesting transpiration performances, even under very droughty conditions. This has a positive effect on the substrate temperatures, but, at the same time, a good canopy cover is necessary. It is fundamental for shadow effect in summer, but also to increase the minimal temperatures of substrate in winter. We also monitored the way of growth of six native perennial species, cultivated on an experimental green roof under a low-input management. The six species, thanks to their different growth behaviors, could provide the good coverage necessary to guarantee the main green roof benefits.