Diurnal variation of <i>Δ</i>¹³CO₂, <i>Δ</i>C¹⁸O¹⁶O and evaporative site enrichment of <i>δ</i>H₂¹⁸O in <i>Piper aduncum</i> under field conditions in Trinidad

Abstract

Concurrent measurements of gas exchange, instantaneous isotope discrimination ( Δ ) against 13 CO 2 and C 18 O 16 O, and extent of 18 O enrichment in H 2 O at the evaporative sites, were followed in a tropical forest pioneer, Piper aduncum, on two different days in Trinidad during February 1995. Δ 13 CO 2 differed from that predicted from measurements of internal:external CO 2 concentration ( C i / C a ) and showed a wide range of values which decreased throughout the course of the day. Derivation of C c (the CO 2 concentration at the carboxylation site) was not possible using carbon isotope discrimination under field conditions in situ and was derived assuming a constant value of internal transfer conductance ( g w ). Under low rates of assimilation the derived C c / C a , like C i / C a , remained relatively stable over the course of both days and Δ C 18 O 16 O followed evaporative demand. Lower values of Δ C 18 O 16 O on day 2 occurred in response to the indirect effect of increased leaf‐to‐air vapour pressure deficits (VPD) and reduced stomatal conductance. For the first time, direct determination of the δ H 2 18 O of transpired water vapour ( δ t ) allowed derivation of evaporative site enrichment without the prerequisite of isotopic steady state (ISS) defined in the Craig and Gordon model. Generally, δ t was less enriched than the source water ( δ s ) in the morning and more enriched in the afternoon, which would be predicted from an increase and decrease in ambient VPD, respectively. On both days, leaves of P. aduncum approached ISS (indicated where δ t ≈ δ s ) between 1300 and 1500 h. Evaporative site enrichment was maintained into the late afternoon, despite a decrease in ambient VPD. The data presented provide a greater insight into the natural variation in isotopic discrimination under field conditions, which may help to refine models of terrestrial biome discrimination.