Carbon isotope discrimination during branch photosynthesis of fagus sylvatica: Field measurements using laser spectrometry

Abstract

On-line measurements of photosynthetic carbon isotope discrimination ( 13Δ) under field conditions are sparse. Hence, experimental verification of the natural variability of instantaneous 13Δ is scarce, although 13Δ is, explicitly and implicitly, used from leaf to global scales for inferring photosynthetic characteristics. This work presents the first on-line field measurements of 13Δ 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 13Δ was substantial, with mean diurnal amplitudes of ~9‰ and maximum diurnal amplitudes of ~20‰. The highest 13Δ were generally observed during early morning and late afternoon, and the lowest 13Δ during midday. An assessment of propagated standard deviations of 13Δ demonstrated that the observed diurnal variation of 13Δ was not a measurement artefact. Day-to-day variations of 13Δ were summarized with flux-weighted daily means of 13Δ, which ranged from 15‰ to 23‰ in 2009 and from 18‰ to 29‰ in 2010, thus displaying a considerable range of 8-11‰. Generally, 13Δ showed the expected negative relationship with intrinsic water use efficiency. Diurnal and day-to-day variability of 13Δ 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 13Δ, could only be identified under drier conditions in 2009. © The Author 2014.