Leaf water δ18O reflects water vapour exchange and uptake by C3 and CAM epiphytic bromeliads in Panama

The distributions of CAM and C-3 epiphytic bromeliads across an altitudinal gradient in western Panama were identified from carbon isotope (delta C-13) signals, and epiphyte water balance was investigated via oxygen isotopes (delta O-18) across wet and dry seasons. There were significant seasonal differences in leaf water (delta O-18(lw)), precipitation, stored 'tank' water and water vapour. Values of delta O-18(lw) were evaporatively enriched at low altitude in the dry season for the C-3 epiphytes, associated with low relative humidity (RH) during the day. Crassulacean acid metabolism (CAM) delta O-18(lw) values were relatively depleted, consistent with water vapour uptake during gas exchange under high RH at night. At high altitude, cloudforest locations, C-3 delta O-18(lw) also reflected water vapour uptake by day. A mesocosm experiment with Tillandsia fasciculata (CAM) and Werauhia sanguinolenta (C-3) was combined with simulations using a non-steady-state oxygen isotope leaf water model. For both C-3 and CAM bromeliads, delta O-18(lw) became progressively depleted under saturating water vapour by day and night, although evaporative enrichment was restored in the C-3 W. sanguinolenta under low humidity by day. Source water in the overlapping leaf base 'tank' was also modified by evaporative delta O-18 exchanges. The results demonstrate how stable isotopes in leaf water provide insights for atmospheric water vapour exchanges for both C-3 and CAM systems.