Diurnal stem diameter variations show CAM and C3 photosynthetic modes and CAM-C3 switches in arid South African succulent shrubs

► CAM and C3 modes were determined from foliar organic acid accumulation in six succulent shrubs. ► Stem diameter changes relative to atmospheric vapour pressure deficits were used to distinguish CAM and C3 modes. ► These methods were compared at two arid South African sites in distinguishing CAM and C3 photosynthetic modes. ► Foliar acidification concurred with predicted stem diameter variations for CAM and C3 modes. ► Stem diameter changes revealed CAM-C3 changes that were concealed when measuring CAM activity from total foliar acids.