The influence of the gas exchange and transpiration of C3 and CAM-facultative species on the green roof's cooling performance

Crassulacean acid metabolism (CAM) allows succulent species to survive under drought by reducing the daytime water loss and gas exchange. We compared gas exchange of two CAM-facultative species (Sedum lydium Boiss. and Sedum kamtschaticum Fisch.) and three C3 species of semi-xeric habitats (Lotus corniculatus L., Bromus erectus Huds. and Salvia officinalis L.) during a gradual reduction of substrate moisture content (SMC). The aim was to describe how the C3 and the CAMfacultative species modify their gas exchange rates when the substrate dries out gradually, as well as to identify the critical value of SMC that stops the transpiration. Additionally, we tested the cooling capacity of specie in a week-long glasshouse experiment, in vegetated containers with commercial green roof substrate. The net CO2 assimilation (A) rates of C3 species were significantly higher than those of CAMfacultative species. When SMC fell below 12% v/v, Sedum species showed a rapid decline of A due to the likely switch from C3 to CAM metabolism. However, the high rates of gas exchange of C3 species caused a rapid decline of RWC over the course of the experiment and death of some leaves. The lower substrate temperatures under C3 plants confirmed the positive effect of high gas exchange rates and transpiration on the cooling capacity. We conclude that the use of C3 plants, with sustainable irrigation (to maintain at least, on average, 10% v/v of SMC), has a potential to provide greater cooling to a green roof than the use of Sedum species.