AbstractWe developed novel approaches for using the isotope composition of tree‐ring subdivisions to study seasonal dynamics in tree–climate relations. Across a 30‐year time series, the δ13C and δ18O values of the earlywood (EW) cellulose in the annual rings of Pinus ponderosa reflected relatively high intrinsic water‐use efficiencies and high evaporative fractionation of 18O/16O, respectively, compared with the false latewood (FLW), summerwood (SW), and latewood (LW) subdivisions. This result is counterintuitive, given the spring origins of the EW source water and midsummer origins of the FLW, SW, and LW. With the use of the Craig–Gordon (CG), isotope‐climate model revealed that the isotope ratios in all of the ring subdivision are explained by the existence of seasonal lags, lasting several weeks, between the initial formation of tracheids and the production of cellulosic secondary cell walls during maturation. In contrast to some past studies, modification of the CG model according to conventional methods to account for mixing of needle water between fractionated and nonfractionated sources did not improve the accuracy of predictions. Our results reveal new potential in the use of tree‐ring isotopes to reconstruct past intra‐annual tree–climate relations if lags in cambial phenology are reconciled with isotope ratio observations and included in theoretical treatments.