Opposing seasonal trends in source water and sugar dampen intra-annual variability in tree rings oxygen isotopes

Intra-annual variations of oxygen isotope composition (δ¹⁸O) in tree rings offer insights into tree ecophysiology and how trees respond to climate. In this study, we focused on the interplay between the δ¹⁸O from source-water, leaf-water and photosynthates to understand how seasonal trends are integrated in tree rings. We conducted a seasonal analysis of Pinus sylvestris in Finland. Our findings reveal a significant reduction in the seasonal variability and trends of δ¹⁸O from needle-water to tree rings. This dampening effect on the seasonal trends is due to the opposing seasonal patterns: source-water δ¹⁸O increases from early spring to late summer, while the evaporative enrichment of ¹⁸O recorded in the δ¹⁸O of photosynthates at the leaf level decreases over the same period. Additionally, oxygen isotope exchange between source water and phloem sugars during wood formation  contributes to the dampening of the evaporative δ¹⁸O signals in tree rings. These findings enhance our understanding of how δ¹⁸O integrates into tree rings, particularly the seasonal signals preserved along the tree growing season. This study offers new perspectives on how intra-annual δ¹⁸O proxies capture seasonal environmental variations over time, which is crucial for refining climate reconstructions and improving our knowledge of tree physiological responses to climate.