Pine and larch tracheids capture seasonal variations of climatic signal at moisture-limited sites

Seasonal dynamics of the timing and rate in cell production and differentiation imprint climate signals into intra-ring variations of anatomical wood structure (e.g. intra-annual density fluctuations). Despite recent methodological advances in quantitative wood anatomy, our understanding of xylem response to climate at the finest scale of intra-ring resolution is incomplete. The goal of this study is to investigate intra-ring changes of tracheid dimensions (cell radial diameter and wall thickness) controlled by moisture stress. Anatomical wood parameters of Pinus sylvestris and Larix sibirica from two drought-susceptible locations in Khakassia, South Siberia, were analysed. We found that inter-annual variation of tracheid parameters regularly exceeds the variation between radial tracheid files. This suggests that the climatic signal is recorded throughout the entire ring. However, each cell parameter has a specific zone in the ring where its climatic response reaches the maximum. The climatic response of the radial cell diameter has a temporal shift across the ring, which is particularly apparent in pine rings. The climatic response of cell wall thickness at the intra-ring scale has a more complex pattern. Our results facilitate investigation of the climate impact on tree rings at the finest intra-ring scale by quantifying the timing of climatic impact on ring structure and identifying specifically when climate impacts the formation of a particular cell