title: Determinants of isotopic coupling of CO2and water vapour within a Quercus petraea forest canopy.
abstract: Concentration and isotopic composition (δ13C and δ18O) of ambient CO2and 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 δa13CO2, δaC18O16O and δaH218O, 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. δ13C of organic matter input for δa13CO2, δ18O of exchangeable water for δaC18O16O and transpired water vapour for δaH218O. [CO2] and δa13CO2showed strong coupling, where the released CO2was, 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 CO2, or temporal differences in ecosystem discrimination. δaC18O16O was less well coupled to [CO2], probably due to the heterogeneity and transient nature of water pools (soil, leaf and moss) within the forest. Similarly, δaH218O was less coupled to [H2O], again reflecting the transient nature of water transpired to the forest, seen as uncoupling during times of large changes in vapour pressure deficit. The δ18O 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 CO2and 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.
authors.affiliation: Department of Agricultural and Environmental Science, Ridley Building, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne, NE1 7RU, UK e-mail: k.g.harwood@newcastle.ac.uk Tel.: +44-191-2225493, Fax: +44-191-2225229, , , , , , GB.
authors.forename: K G
authors.initials: KG
authors.lastname: Harwood
authors.affiliation: Department of Agricultural and Environmental Science, Ridley Building, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne, NE1 7RU, UK e-mail: k.g.harwood@newcastle.ac.uk Tel.: +44-191-2225493, Fax: +44-191-2225229, , , , , , GB.
authors.forename: J S
authors.initials: JS
authors.lastname: Gillon
authors.affiliation: Department of Agricultural and Environmental Science, Ridley Building, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne, NE1 7RU, UK e-mail: k.g.harwood@newcastle.ac.uk Tel.: +44-191-2225493, Fax: +44-191-2225229, , , , , , GB.
authors.forename: A
authors.initials: A
authors.lastname: Roberts
authors.affiliation: Department of Agricultural and Environmental Science, Ridley Building, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne, NE1 7RU, UK e-mail: k.g.harwood@newcastle.ac.uk Tel.: +44-191-2225493, Fax: +44-191-2225229, , , , , , GB.
authors.forename: H
authors.initials: H
authors.lastname: Griffiths
keywords: CO2; Carbon; Forest canopies; Key words Stable isotopes; Water vapour