We tested the extent to which growth responses to elevated carbon dioxide (CO2) are temperature-dependent and change through early seedling ontogeny among boreal tree species of contrasting relative growth rates (rgr). Populus tremuloides Michx, Betula papyrifera Marsh, Larix laricina (Du Roi) K. Koch, Pinus banksiana Lamb., and Picea mariana (Mill.) B.S.P. were grown from seeds for 3 months in controlled-environment chambers at two CO2 concentrations (370 and 580 μmol mol−1) and five temperature regimes of 18/12, 21/15, 24/18, 27/21 and 30/24°C (light/dark). Growth increases in response to CO2 enrichment were minimal at the lowest temperature and maximal at 21/15°C for the three conifers and at 24/18°C or higher for the two broadleaved species, corresponding with differences in optimal temperatures for growth. In both CO2 treatments, rgr among species and temperatures correlated positively with leaf area ratio (lar) (r[ges ]0·90, P<0·0001). However, at a given lar, rgr was higher in elevated CO2, owing to enhanced whole-plant net assimilation rate. On average in all species and temperatures at a common plant mass, CO2 enrichment increased rgr (9%) through higher whole-plant net assimilation rate (22%), despite declines in lar in high CO2 (11%). Reductions in lar are thus an important feedback mechanism reducing positive plant growth responses to CO2. Proportional allocation of dry mass to roots did not vary between CO2 treatments. Early in the experiment, proportional increases in plant dry mass in elevated CO2 were larger in faster-growing Populus tremuloides and B. papyrifera than in the slower-growing conifers. However, growth increases in response to CO2 enrichment fell with time for broadleaved species and increased for the conifers. With increasing plant size over time, compensatory adjustments to CO2 enrichment in the factors that determine rgr, such as lar, were much larger in broadleaves than in conifers. Thus, the hypothesis that faster-growing species are more responsive to elevated CO2 was not supported, given contrasting patterns of growth response to CO2 with increasing plant size and age.