title: Carbon isotope discrimination during branch photosynthesis of Fagus sylvatica: field measurements using laser spectrometry.
abstract: On-line measurements of photosynthetic carbon isotope discrimination ((13)Δ) under field conditions are sparse. Hence, experimental verification of the natural variability of instantaneous (13)Δ is scarce, although (13)Δ is, explicitly and implicitly, used from leaf to global scales for inferring photosynthetic characteristics. This work presents the first on-line field measurements of (13)Δ of Fagus sylvatica branches, at hourly resolution, using three open branch bags and a laser spectrometer for CO₂ isotopologue measurements (QCLAS-ISO). Data from two August/September field campaigns, in 2009 and 2010, in a temperate forest in Switzerland are shown. Diurnal variability of (13)Δ was substantial, with mean diurnal amplitudes of ~9‰ and maximum diurnal amplitudes of ~20‰. The highest (13)Δ were generally observed during early morning and late afternoon, and the lowest (13)Δ during midday. An assessment of propagated standard deviations of (13)Δ demonstrated that the observed diurnal variation of (13)Δ was not a measurement artefact. Day-to-day variations of (13)Δ were summarized with flux-weighted daily means of (13)Δ, which ranged from 15‰ to 23‰ in 2009 and from 18‰ to 29‰ in 2010, thus displaying a considerable range of 8-11‰. Generally, (13)Δ showed the expected negative relationship with intrinsic water use efficiency. Diurnal and day-to-day variability of (13)Δ was, however, always better predicted by that of net CO₂ assimilation, especially in 2010 when soil moisture was high and vapour pressure deficit was low. Stomatal control of leaf gas exchange, and consequently (13)Δ, could only be identified under drier conditions in 2009.
authors.affiliation: Institute of Agricultural Sciences, ETH Zurich, Universitätsstrasse 2, 8092 Zurich, Switzerland.
authors.forename: Lydia
authors.initials: L
authors.lastname: Gentsch
authors.forename: Patrick
authors.initials: P
authors.lastname: Sturm
authors.forename: Albin
authors.initials: A
authors.lastname: Hammerle
authors.forename: Rolf
authors.initials: R
authors.lastname: Siegwolf
authors.forename: Lisa
authors.initials: L
authors.lastname: Wingate
authors.forename: Jérôme
authors.initials: J
authors.lastname: Ogée
authors.forename: Thomas
authors.initials: T
authors.lastname: Baur
authors.forename: Peter
authors.initials: P
authors.lastname: Plüss
authors.forename: Matti
authors.initials: M
authors.lastname: Barthel
authors.forename: Nina
authors.initials: N
authors.lastname: Buchmann
authors.forename: Alexander
authors.initials: A
authors.lastname: Knohl
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes / Q000032:analysis; D029964:Fagus / Q000502:physiology*; D007834:Lasers; D010788:Photosynthesis; D018515:Plant Leaves / Q000502:physiology; D018547:Plant Stems / Q000502:physiology; D018526:Plant Transpiration / Q000502:physiology*; D013057:Spectrum Analysis
chemical_list: D002247:Carbon Isotopes; D002245:Carbon Dioxide
keywords: Branch bags; CO2; Fagus sylvatica; laser spectrometers; leaf gas exchange; open chambers; photosynthetic carbon isotope discrimination; stem respiration.