A single-substrate model to interpret intra-annual stable isotope signals in tree-ring cellulose

The carbon and oxygen stable isotope composition of wood cellulose ( δ ¹³C_cellulose and δ ¹⁸O_cellulose, respectively) reveal well-defined seasonal variations that contain valuable records of past climate, leaf gas exchange and carbon allocation dynamics within the trees. Here, we present a single-substrate model for wood growth to interpret seasonal isotopic signals collected in an even-aged maritime pine plantation growing in South-west France, where climate, soil and flux variables were also monitored. Observed seasonal patterns in δ ¹³C_cellulose and δ ¹⁸O_cellulose were different between years and individuals, and mostly captured by the model, suggesting that the single-substrate hypothesis is a good approximation for tree ring studies on Pinus pinaster , at least for the environmental conditions covered by this study. A sensitivity analysis revealed that the model was mostly affected by five isotopic discrimination factors and two leaf gas-exchange parameters. Modelled early wood signals were also very sensitive to the date when cell wall thickening begins ( t _wt). Our model could therefore be used to reconstruct t _wt time series and improve our understanding of how climate influences this key parameter of xylogenesis.