We examined the response of hybrid poplar to elevated CO 2 in contrasting growth environments: controlled environment chamber (CE), open-top chamber (OTC) and poplar free air CO 2 enrichment (POPFACE) in order to compare short versus long-term effects and to determine whether generalisations in response are possible for this fast growing tree. Leaf growth, which for poplar is an important determinant of stemwood productivity was followed in all environments, as were the determinants of leaf growth—cell expansion and cell production. Elevated CO 2 (550–700 μmol mol −1, depending on environment) resulted in an increase in final leaf size for Populus trichocarpa× Populus deltoides ( Populus× interamericana) and P. deltoides× Populus nigra ( Populus× euramericana), irrespective of whether plants were exposed during a short-term CE glasshouse study (90 days), a long-term OTC experiment (3 years) or during the first year of a POPFACE experiment. An exception was observed in the closed canopy POPFACE experiment, where final leaf size remained unaltered by CO 2. Increased leaf extension rate was observed in elevated CO 2 in all experiments, at some point during leaf development, as determined by leaf length. Again the exception were the POPFACE experiment, where effects were not statistically significant. Leaf production and specific leaf area (SLA) were increased and decreased, respectively, on five out of six occasions, although both were only statistically significant on two occasions and interestingly for SLA never in the FACE experiment. Although both cell expansion and cell production were sensitive to CO 2 concentration, effects appeared highly dependent on growth environment and genotype. However, increased leaf cell expansion in elevated CO 2 was often associated with changes in the biophysical properties of the cell wall, usually increased cell wall plasticity. This research has shown that enhanced leaf area development was a consistent response to elevated CO 2 but that the magnitude of this response is likely to decline, in long-term exposure to elevated CO 2. Effects on SLA and leaf production suggest that CE and OTC experiments may not always provide good predictors of the ‘qualitative’ effects of elevated CO 2 in long-term ecosystem experiments.