Carbon isotope discrimination during branch photosynthesis of F agus sylvatica : a B ayesian modelling approach

Field measurements of photosynthetic carbon isotope discrimination ( 13 Δ) of F agus sylvatica , conducted with branch bags and laser spectrometry, revealed a high variability of 13 Δ, both on diurnal and day‐to‐day timescales. We tested the prediction capability of three versions of a commonly used model for 13 Δ [called here comprehensive ( 13 Δ comp ), simplified ( 13 Δ simple ) and revised ( 13 Δ revised ) versions]. A B ayesian approach was used to calibrate major model parameters. Constrained estimates were found for the fractionation during CO 2 fixation in 13 Δ comp , but not in 13 Δ simple , and partially for the mesophyll conductance for CO 2 ( g i ). No constrained estimates were found for fractionations during mitochondrial and photorespiration, and for a diurnally variable apparent fractionation between current assimilates and mitochondrial respiration, specific to 13 Δ revised . A quantification of parameter estimation uncertainties and interdependencies further helped explore model structure and behaviour. We found that 13 Δ comp usually outperformed 13 Δ simple because of the explicit consideration of g i and the photorespiratory fractionation in 13 Δ comp that enabled a better description of the large observed diurnal variation (≈9‰) of 13 Δ. Flux‐weighted daily means of 13 Δ were also better predicted with 13 Δ comp than with 13 Δ simple .