The boreal forest located in high northern latitudes stores about a third of the world’s carbon and covers almost a quarter of the Earth’s land surface. This region is experiencing one of the fastest temperatures increases on the planet. Yet it is unclear how global warming affects carbon sequestration and storage in this biome. Here, we explore how white spruce (Picea glauca [Moench] Voss) growing in North America responded to climate change during the 20th century using tree-ring width and stable carbon (δ13C) and oxygen (δ18O) chronologies from 10 sites between 60-69ºN and 104-162ºW. We found that carbon isotope discrimination (∆13C) tended to increase over time and especially since 1982, but that δ13C and δ18O were not significantly or only weakly related to each other. These results suggest that stomatal regulation is not the most important driver of the observed increase in water-use efficiency and that photosynthetic CO2 uptake increased. However, radial growth did not increase concomitantly at all the study sites, suggesting that distinct environmental drivers affect leaf gas exchange processes and wood formation or xylogenesis. This agrees with different sensitivity of stable isotopes and tree-ring growth to spring and summer environmental variables. A strong spring temperature signal and the same large-scale atmospheric patterns were recorded by tree-ring δ18O and modelled precipitation δ18O (i.e. source water signal). This may have attenuated the physiological signal in tree-ring δ18O related to summer temperature (i.e. leaf evaporative isotopic enrichment). Overall, white spruce in high-latitude forests seems to benefit from warming, but how the carbon is distributed within the plant is still unknown.