Diurnal variation of Δ13CO2, Δ18O16O and evaporative site enrichment of δH218O in Piper aduncum under field conditions in Trinidad

Abstract

Concurrent measurements of gas exchange, instantaneous isotope discrimination (Δ) against 13CO2 and C18O16O, and extent of 18O enrichment in H2O at the evaporative sites, were followed in a tropical forest pioneer, Piper aduncum, on two different days in Trinidad during February 1995. Δ13CO2 differed from that predicted from measurements of internal:external CO2 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 CO2 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 ΔC18O16O followed evaporative demand. Lower values of ΔC18O16O 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 δH218O 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.