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

Variations of oxygen isotopes δ¹⁸O in tree rings provide critical insights into past climate and tree physiological processes, yet the mechanisms shaping the intra-annual δ¹⁸O signals remain incompletely understood. To address this gap, we investigated how seasonal changes in source water, leaf water, and sugars influence δ¹⁸O recorded along the tree rings of Pinus sylvestris in Finland. We conducted a seasonal analysis measuring δ¹⁸O from needle water, source water, and phloem sugars and investigated the fraction of oxygen isotope exchange during wood formation. We found that seasonal δ¹⁸O amplitudes are significantly reduced from leaf water to tree rings, driven by opposing seasonal patterns in increasing source water δ¹⁸O and decreasing evaporative enrichment as relative humidity increases. Additionally, the isotope exchange between source water and phloem sugars further dampens seasonal δ¹⁸O signals in the rings. Our findings show that oxygen isotope exchange is critical in shaping δ¹⁸O signals, influencing the role of source water and relative humidity recorded on intra-annual resolution. This refined understanding helps interpret tree physiological responses under changing conditions and improves climate reconstructions based on tree rings using intra-annual resolution.