We report an analysis of both the long- and short-term drivers of the carbon (C) isotope composition (δ13C) values of current year needles of Pinus sylvestris L. linked to changing atmospheric carbon dioxide (CO2) concentrations (ca) and climate using data from a uniquely long-term nitrogen (N) fertilization experiment in the north of Sweden (consisting of three N dosage levels and a control treatment) from 1970 until 2002. N loading produced trees with less negative δ13C of foliage, by around 0.45‰ on average, with the difference in δ13C between control and N treatments not dependant upon N dosage. The average δ13C values decreased at a rate of around 0.03‰ yr−1, even after accounting for the Suess effect (the decrease in the atmospheric CO2δ13C due to anthropogenic emissions of isotopically light CO2). This decrease is large enough to cause a significant, progressive change in the δ13C down through a soil profile. Modelled values of plant intrinsic water use efficiency (WUEi) and the ratio of leaf internal to external [CO2] (ci/ca) showed that this was the result of ci increasing in parallel with ca (while ci/ca increased), thus causing little change in WUEi over the 32 years of study. The residuals from the relationships between year and δ13C were used to examine the impact of climate on the interannual variation of C isotope composition of needles. This included the use of a fire hazard index (FHI) model, which integrates climatic factors known to influence plant stomatal conductance and hence δ13C. The FHI produced the best fit with δ13C values when climate data were averaged over the whole growth season (for control plots) and for July for all the N treatments, explaining ca. 60% of the total interannual variation in δ13C. Further, trees from the N treatments appeared more susceptible to air-humidity-based climate parameters, as seen from higher correlation coefficients, than were control trees. Thus, our data suggest the possibility of increased susceptibility to drought conditions in ecosystems with moderate to high N deposition rates. Also, there is the possibility that, because there was no apparent change in WUEi of P. sylvestris in this ecosystem over the last 32 years, the rate of sequestration of C into boreal ecosystems may not increase with ca, as has been predicted.