Carbon isotope discrimination during branch photosynthesis of Fagus sylvatica: field measurements using laser spectrometry

Photosynthetic carbon isotope discrimination of Fagus sylvatica was measured online and under field conditions using branch bags and laser spectrometers. A substantial variability was observed. Its potential drivers were investigated.On-line measurements of photosynthetic carbon isotope discrimination ((13)) under field conditions are sparse. Hence, experimental verification of the natural variability of instantaneous (13) is scarce, although (13) 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) 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) was substantial, with mean diurnal amplitudes of similar to 9 and maximum diurnal amplitudes of similar to 20. The highest (13) were generally observed during early morning and late afternoon, and the lowest (13) during midday. An assessment of propagated standard deviations of (13) demonstrated that the observed diurnal variation of (13) was not a measurement artefact. Day-to-day variations of (13) were summarized with flux-weighted daily means of (13), which ranged from 15 to 23 in 2009 and from 18 to 29 in 2010, thus displaying a considerable range of 811. Generally, (13) showed the expected negative relationship with intrinsic water use efficiency. Diurnal and day-to-day variability of (13) 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), could only be identified under drier conditions in 2009.