Carbon isotope discrimination during branch photosynthesis of Fagus sylvatica: a Bayesian modelling approach

Field measurements of photosynthetic carbon isotope discrimination ((13)Delta) of Fagus sylvatica, conducted with branch bags and laser spectrometry, revealed a high variability of (13)Delta, both on diurnal and day-to-day timescales. We tested the prediction capability of three versions of a commonly used model for (13)Delta [called here comprehensive ((13)Delta(comp)), simplified ((13)Delta(simple)) and revised ((13)Delta(revised)) versions]. A Bayesian approach was used to calibrate major model parameters. Constrained estimates were found for the fractionation during CO2 fixation in (13)Delta(comp), but not in (13)Delta(simple), and partially for the mesophyll conductance for CO2 (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)Delta(revised). A quantification of parameter estimation uncertainties and interdependencies further helped explore model structure and behaviour. We found that (13)Delta(comp) usually outperformed (13)Delta(simple) because of the explicit consideration of g(i) and the photorespiratory fractionation in (13)Delta(comp) that enabled a better description of the large observed diurnal variation (approximate to 9 parts per thousand) of (13)Delta. Flux-weighted daily means of (13)Delta were also better predicted with (13)Delta(comp) than with (13)Delta(simple).