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

The distributions of CAM and C₃ epiphytic bromeliads across an altitudinal gradient in western Panama were identified from carbon isotope (δ¹³C) signals, and epiphyte water balance was investigated via oxygen isotopes (δ¹⁸O) across wet and dry seasons. There were significant seasonal differences in leaf water (δ¹⁸O_lw), precipitation, stored 'tank' water and water vapour. Values of δ¹⁸O_lw were evaporatively enriched at low altitude in the dry season for the C₃ epiphytes, associated with low relative humidity (RH) during the day. Crassulacean acid metabolism (CAM) δ¹⁸O_lw values were relatively depleted, consistent with water vapour uptake during gas exchange under high RH at night. At high altitude, cloudforest locations, C₃ δ¹⁸O_lw also reflected water vapour uptake by day. A mesocosm experiment with Tillandsia fasciculata (CAM) and Werauhia sanguinolenta (C₃) was combined with simulations using a non-steady-state oxygen isotope leaf water model. For both C₃ and CAM bromeliads, δ¹⁸O_lw became progressively depleted under saturating water vapour by day and night, although evaporative enrichment was restored in the C₃ W. sanguinolenta under low humidity by day. Source water in the overlapping leaf base 'tank' was also modified by evaporative δ¹⁸O exchanges. The results demonstrate how stable isotopes in leaf water provide insights for atmospheric water vapour exchanges for both C₃ and CAM systems. The paper defines the niche segregation of C₃ and CAM photosynthetic pathways for epiphytic bromeliads along an altitudinal gradient in Panama. Measurement of the leaf water oxygen (¹⁸O) stable isotope composition supports transpiration by day or night, since under high humidity water vapour influx resets the leaf water ¹⁸O signal. [ABSTRACT FROM AUTHOR]