Intra-annual variability of wood formation and δ13C in tree-rings at Hyytiälä, Finland

• A newly developed approach explore the tree ring δ13C – wood growth relationships.
• The timing of cambial activity was related to thresholds of PAR and temperature.
• The Gompertz-function approach can reproduce the distribution of cambial activity.
• Inclusion of cambial cell lifetime improves intra-annual tree-ring δ13C modelling.
• A shift of importance of the environmental controls occurs during the growing season.

Investigation of the relationship between tree-ring stable carbon isotope composition (δ13C) and environmental variables at the intra-seasonal scale can inform on the understanding of the environmental forcing affecting trees during the active period of radial growth. Recent progress in the measurement techniques for assessing the δ13C signature of tree rings at high spatial resolution provides an opportunity to derive tree physiological information at fine temporal scale, within a given year. Three δ13C time series of resin-extracted wholewood from tree-rings of Scots pine trees (Pinus sylvestris L.) sampled at Hyytiälä (Finland) have been produced by using laser-ablation stable isotope mass spectrometry. These intra-seasonal stable isotope series exhibit a strong common signal (EPS = 0.96) demonstrating the capacity of trees within a stand to preserve a common intra-seasonal response to external controls in a similar manner as found with annual measurements. To estimate when wood cells are active and responsive to environmental information, a Gompertz approach, assessed against microcore data, was adopted to model the timing of wood formation. The addition of a cell lifetime function into environment-growth models may evaluate more completely, the environmental effect on intra-annual tree-ring δ13C values and that during the growing season. Statistical analysis of the resulting tree-ring δ13C intra-annual signal implies a shift in importance from the influence of the environmental variables through out the growing season.