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

摘要

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