Effects of Water Availability and Temperature on CAM Expression and Water Use Efficiency by Sedum album and Sedum kamtschaticum

Though varying degrees of CAM have been documented for Sedum species, less is known about the implications of this variation for total water and carbon budgets.  This study tracks the carbon gain and water use for the more extensively studied Sedum album and for Sedum kamtschaticum, for which CAM has been sparsely documented.  Whole plant gas exchange chambers were used to document the diurnal flux of CO₂ and evapotranspiration from four replicates of each species.  Plants were watered at the beginning of the experiment and again after 21 days.  Whole plant gas exchange and evapotranspiration was continuously measured within eight acrylic chambers, placed in one of two Conviron (Winnipeg, Canada) growth chambers, using a datalogger (CR10T; Campbell Scientific, Logan, UT).   Air flow through each gas exchange chamber (≈17 mmol·s⁻¹) was measured with mass flow meters (HFM200; Teledyne Hasting Inst., Hampton, VA) and the difference in CO₂ concentration between the air entering and exiting the gas exchange chambers was measured with an infrared gas analyzer in differential mode (LI-6262; LI-COR, Lincoln, NE).  Most likely as a result of differential Crassuleacean Acid metabolism (CAM), S. album fixed C at 2.55 (± 0.30) mmol  and S kamtschaticum fixed C at 4.68 (± 0.74) mmol.  During this same period, S. kamtschaticum used 68.87 (± 8.15) grams of water in contrast to 45.82 (±4.15) grams of water used by S. album.   Malic acid analysis indicated that S. album was in CAM throughout the course of the study, but S. kamtschticum only began CAM idling weakly around day 9.  These findings suggest that S. album was the more drought tolerant species under the environmental conditions of this experiment.  For geographic areas with extended dry periods, S. album may be the preferred species compared to S. kamtschaticum.  In cooler, wetter climates, this species may not transpire water fast enough to contribute substantially to green roof stormwater efficiency.  Nevertheless, having a species that can tolerate periods of drought stress is important for overall system function, especially when ongoing green roof management or renovation costs are considered.