The stable carbon isotope composition (δ13C) of foliage integrates signals resulting from environmental and hydraulic constraints on water movement and photosynthesis. We used branch length as a simple predictor of hydraulic constraints to water fluxes and determined the response of δ13C to varying water availability. Foliage up to 6 years old was taken from Pinus pinaster Ait. trees growing at four sites differing in precipitation (P; 414–984 mm year−1) and potential evaporation (ET; 1091–1750 mm year−1). Branch length was the principal determinant of temporal trends in δ13C. The strong relationship between δ13C and branch length was a function of hydraulic conductance, which was negatively correlated with branch length (r2 = 0.84). Variation in P and ET among sites was reflected in δ13C, which was negatively correlated with P/ET (r2 = 0.66). However, this analysis was confounded by differences in branch length. If the effects of branch length on δ13C were first removed, then the ‘residual’ δ13C was more closely related to P/ET (r2 = 0.99), highlighting the importance of accounting for variation in hydraulic constraints to water flux between sites and years. For plant species that exhibit considerable phenotypic plasticity in response to changes in environment (e.g., variation in leaf area, branch length and number, or stem form), the environmental effects on δ13C in foliage can only be reliably assessed if deconvoluted from hydraulic constraints.