The use of stable isotope proxies in combination with tree-ring parameters has become a well-established tool to unravel plants’ responses to a changing environment. However, while there have been many studies on intra-annual wood formation processes, the specific details of the fractionation of stable isotopes in high-resolution time scales -knowing the exact date of fractionation- remain unknown. Such a time scale mismatch, provides obstacles to investigate the timing, sensitivity and interactions among important ecophysiological processes (e.g., photosynthesis, stomatal conductance) that drive responses over shorter time scales. Consequently, process-based modeling remains poorly constrained, casting important uncertainties on the prediction of forest responses to meteorological variability. Here, we circumvent this difficulty by analysing weekly wood anatomical features jointly with carbon stable isotope fractionation (δ13C), measured on black spruce (Picea mariana) trees, in eastern Canada, during two consecutive growing seasons (2020 and 2021). The results illustrate a positive correlation between weekly fractionation of δ13C and cell production (number of cells per week), showing a rising trend from the beginning to the end of the growing season. There is also a positive correlation between cell wall thickness and fractionation of δ13C during the growing season. Both of these correlations indicate that fractionation of δ13C may be a good proxy for intra-annual, photosynthesis-driven variations in tree productivity during the growing season. These results may also help to improve modelling of the ecophysiological response of black spruce forests in the context of climate variability.