Non-steady state effects in diurnal 18O discrimination by Picea sitchensis branches in the field

We report diurnal variations in O-18 discrimination ((18)Delta) during photosynthesis ((18)Delta(A)) and respiration ((18)Delta(R)) of Picea sitchensis branches measured in branch chambers in the field. These observations were compared with predicted (18)A ((18)Delta(pred)) based on concurrent measurements of branch gas exchange to evaluate steady state and non-steady state (NSS) models of foliage water O-18 enrichment for predicting the impact of this ecosystem on the PO of atmospheric CO2. The non-steady state approach substantially improved the agreement between (18)Delta(Pred) and observed (18)Delta ((18)Delta(obs)) compared with the assumption of isotopic steady state (ISS) for the delta O-18 signature of foliage water. In addition, we found direct observational evidence for NSS effects: extremely high apparent (18)Delta values at dusk, dawn and during nocturnal respiration. Our experiments also show the importance of bidirectional foliage gas exchange at night (isotopic equilibration in addition to the net flux). Taken together, neglecting these effects leads to an underestimation of daily net canopy isofluxes from this forest by up to 30%. We expect NSS effects to be most pronounced in species with high specific leaf water content such as conifers and when stomata are open at night or when there is high relative humidity, and we suggest modifications to ecosystem and global models of delta O-18 Of CO2.