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Non‐steady state effects in diurnal 18 O discrimination by Picea sitchensis branches in the field
Author(s) -
SEIBT U.,
WINGATE L.,
BERRY J. A.,
LLOYD J.
Publication year - 2006
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1111/j.1365-3040.2005.01474.x
Subject(s) - steady state (chemistry) , field (mathematics) , chemistry , state (computer science) , analytical chemistry (journal) , physics , mathematics , chromatography , algorithm , pure mathematics
We report diurnal variations in 18 O discrimination ( 18 Δ ) during photosynthesis ( 18 Δ A ) and respiration ( 18 Δ R ) of Picea sitchensis branches measured in branch chambers in the field. These observations were compared with predicted 18 Δ ( 18 Δ pred ) based on concurrent measurements of branch gas exchange to evaluate steady state and non‐steady state (NSS) models of foliage water 18 O enrichment for predicting the impact of this ecosystem on the δ 18 O of atmospheric CO 2 . The non‐steady state approach substantially improved the agreement between 18 Δ pred and observed 18 Δ ( 18 Δ obs ) compared with the assumption of isotopic steady state (ISS) for the δ 18 O signature of foliage water. In addition, we found direct observational evidence for NSS effects: extremely high apparent 18 Δ 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 δ 18 O of CO 2 .