Water vapour isotopic exchange by epiphytic bromeliads in tropical dry forests reflects niche differentiation and climatic signals

The O-18 signals in leaf water (delta O-18(lw)) and organic material were dominated by atmospheric water vapour O-18 signals (delta O-18(vap)) in tank and atmospheric life forms of epiphytic bromeliads with crassulacean acid metabolism (CAM), from a seasonally dry forest in Mexico. Under field conditions, the mean delta O-18(lw) for all species was constant during the course of the day and systematically increased from wet to dry seasons (from 0 to +6 parts per thousand), when relative water content (RWC) diminished from 70 to 30%. In the greenhouse, progressive enrichment from base to leaf tip was observed at low night-time humidity; under high humidity, the leaf tip equilibrated faster with delta O-18(vap) than the other leaf sections. Laboratory manipulations using an isotopically depleted water source showed that delta O-18(vap) was more rapidly incorporated than liquid water. Our data were consistent with a Craig-Gordon (C-G) model as modified by Helliker and Griffiths predicting that the influx and exchange of delta O-18(vap) control delta O-18(lw) in certain epiphytic life forms, despite progressive tissue water loss. We use delta O-18(lw) signals to define water-use strategies for the coexisting species which are consistent with habitat preference under natural conditions and life form. Bulk organic matter (delta O-18(org)) is used to predict the delta O-18(vap) signal at the time of leaf expansion.