• For accurate interpretation of oxygen isotopes in tree rings (δ¹⁸O), it is necessary to disentangle the mechanisms underlying the variations in the tree's internal water cycle and to understand the transfer of source versus leaf water δ¹⁸O to phloem sugars and stem wood.
• We studied the seasonal transfer of oxygen isotopes from precipitation and soil water through the xylem, needles and phloem to the tree rings of Larix decidua at two alpine sites in the Lötschental (Switzerland). Weekly resolved δ¹⁸O records of precipitation, soil water, xylem and needle water, phloem organic matter and tree rings were developed.
•  Week-to-week variations in needle-water ¹⁸O enrichment were strongly controlled by weather conditions during the growing season. These short-term variations were, however, not significantly fingerprinted in tree-ring δ¹⁸O. Instead, seasonal trends in tree-ring δ¹⁸O predominantly mirrored trends in the source water, including recent precipitation and soil water pools. Modelling results support these findings: seasonal tree-ring δ¹⁸O variations are captured best when the week-to-week variations of the leaf water signal are suppressed.
•  Our results suggest that climate signals in tree-ring δ¹⁸O variations should be strongest at temperate sites with humid conditions and precipitation maxima during the growing season.