Summary Elevated CO2 increases intrinsic water use efficiency (WUEi) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood. We combined tree ring analysis with isotope measurements at three Free Air CO2 Enrichment (FACE, POP‐EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ13C to assess carbon isotope discrimination and changes in water‐use efficiency, while direct CO2 effects on stomatal conductance were explored using δ18O as a proxy. Across all the sites, elevated CO2 increased 13C‐derived water‐use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water‐use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species‐specific relationships that can help modelling elevated CO2 and climate impacts on forest productivity, carbon and water balances.