Photosynthesis and water use by two Sedum species in green roof substrate

• Sedum album and Sedum kamtschaticum stands were observed during a gradual drydown.
• Gas exchange and water use efficiency were monitored continuously for three weeks.
• Substrate volumetric water content (VWC) and conductance were also documented.
• Sedum kamtschaticum had greater water use efficiency at VWC above 0.08 m3 m−3.
• Physiological trade-offs associated with CAM cycling were demonstrated.

Different succulent plants have been shown to exhibit varying degrees of crassulacean acid metabolism (CAM). This variation has not been consistently shown to relate to water use efficiency (WUE) by these species. This raises fundamental questions about the ecophysiological implications of switching between C3 photosynthesis and CAM metabolism. More information is also needed for Sedum species that utilize CAM to varying degrees on green roofs so that plant water use can be included in water balance equations that predict green roof performance. We monitored carbon dioxide exchange and water use by two common green roof species, Sedum album and Sedum kamtschaticum, for three weeks without water under strictly controlled environmental conditions in order to (1) compare daily carbon gain, rates of evapotranspiration (ET), and WUE for these two species and (2) assess the effect of CAM cycling on WUE with increasing drought for entire plant stands. Under similar environmental conditions, S. album and S. kamtschaticum canopies exhibited CAM metabolism to different extents with different ecophysiological consequences. S. kamtschaticum exhibited C3 photosynthesis for the first 14 days without water and outperformed S. album in terms of carbon gain and WUE above a volumetric water content (VWC) of 0.08 m3 m−3. S. album was the more drought tolerant species under the conditions of this experiment, gaining at least as much carbon and using water more efficiently at VWC below 0.08 m3 m−3. By switching from C3 to CAM at lower VWC, S. kamtschaticum had an overall greater WUE even though this species used as much if not more water overall compared to S. album. These data further support suggestions that a major adaptive function of CAM is to prolong plant-available water while maintaining a consistent, albeit low level of carbon gain. These findings also demonstrate why Sedum species should be treated independently of each other when considered in green roof designs.