evaporative sites, were followed in a tropical forest pioneer, Piper aduncum, on two different days in Trinidad during February 1995. ∆ 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 ∆C 18 O 16 O followed evaporative demand. Lower values of ∆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 δH 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.

Concentration and isotopic composition (δC and δO) of ambient CO and water vapour were determined within a Quercus petraea canopy, Northumberland, UK. From continuous measurements made across a 36-h period from three heights within the forest canopy, we generated mixing lines (Keeling plots) for δCO, δ COO and δ HO, to derive the isotopic composition of the signal being released from forest to atmosphere. These were compared directly with measurements of different respective pools within the forest system, i.e. δC of organic matter input for δCO, δO of exchangeable water for δ COO and transpired water vapour for δ HO. [CO] and δCO showed strong coupling, where the released CO was, on average, 4 per mil enriched compared to the organic matter of plant material in the system, suggesting either fractionation of organic material before eventual release as soil-respired CO, or temporal differences in ecosystem discrimination. δ COO was less well coupled to [CO], probably due to the heterogeneity and transient nature of water pools (soil, leaf and moss) within the forest. Similarly, δ HO was less coupled to [HO], again reflecting the transient nature of water transpired to the forest, seen as uncoupling during times of large changes in vapour pressure deficit. The δO of transpired water vapour, inferred from both mixing lines at the canopy scale and direct measurement at the leaf level, approximated that of source water, confirming that an isotopic steady state held for the forest integrated over the daily cycle. This demonstrates that isotopic coupling of CO and water vapour within a forest canopy will depend on absolute differences in the isotopic composition of the respective pools involved in exchange and on the stability of each of these pools with time.

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