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.

Bias in estimated online leaf carbon isotope discrimination due to woody tissues

“…Furthermore, it assumes (3) that the effect of decreasing transpiration on R wt,int due to defoliation leads to an underestimation of R wt , and therefore F wt and its isotope-related effect on 13 C l , because both stem photosynthesis [26] as well as CO 2 efflux rates [27] are positively related to sap flow (but see [11] for confounding effects). However, despite this potential underestimation, our modelled R wt rates for foliated beech of the first experimental phase were in the range of CO 2 efflux rates previously reported for (foliated) beech [3,8], demonstrating the validity of using the model also for foliated trees during the light.…”

“…These δ 13 C values were more negative than reported in other studies for beech ( − 22 ‰ to − 26 ‰; [28,29]), most probably due to the low light conditions in the climate chambers (maximum values around 600 µmol photons m −2 s −1 ), which increase photosynthetic carbon isotope discrimination and thus lead to more negative δ 13 [3,15]. More important than the absolute δ 13 C values of woody tissue net CO 2 efflux is the associated isotope discrimination 13 C R wt , as any F wt only affects (and potentially biases) leaf 13 C apparent , when it is associated with a noticeable 13 C fractionation.…”

“…Carbon fluxes and associated carbon isotope discrimination of trees are often measured using branch or shoot chambers under steady-state conditions, integrating signals from leaves, and woody tissues [1][2][3][4][5]. Generally, it is assumed that the measured ratio of 13 CO 2 / 12 CO 2 is solely influenced by leaf processes.…”

“…Therefore, it must be assumed that the CO 2 efflux of woody tissues and its isotopic composition affect leaf 13 C apparent , but it remains ambiguous to what extent. Although there is increasing awareness about the effect of woody tissue respiration on leaf 13 C apparent [3,4], the effect of F wt on leaf 13 C apparent has not yet been quantified, despite its relevance for reliable estimates of leaf discrimination. Therefore, this study aims to quantify the carbon isotope-related effect of woody tissue CO 2 fluxes on leaf 13 C apparent estimated from shoot chamber measurements.…”

Strong Coupling of Shoot Assimilation and Soil Respiration during Drought and Recovery Periods in Beech As Indicated by Natural Abundance δ13C Measurements

“…These measurable changes in the carbon isotope composition of soil and ecosystem respired CO 2 were detected using discrete instead of continuous isotope measurements. However, first in situ real-time measurements revealed that such conclusions are not that straightforward as Δ 13 C displays diurnals changes (Gentsch et al, 2014) which are not always reflected in the carbon isotope signature of SR, especially under wet conditions (Wingate et al, 2010). …”