Temperature and ontogeny mediate growth response to elevated CO₂ in seedlings of five boreal tree species

We tested the extent to which growth responses to elevated carbon dioxide (CO 2 ) 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 CO 2 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 CO 2 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 CO 2 treatments, rgr among species and temperatures correlated positively with leaf area ratio ( lar ) ( r ⩾0·90, P <0·0001). However, at a given lar , rgr was higher in elevated CO 2 , owing to enhanced whole‐plant net assimilation rate. On average in all species and temperatures at a common plant mass, CO 2 enrichment increased rgr (9%) through higher whole‐plant net assimilation rate (22%), despite declines in lar in high CO 2 (11%). Reductions in lar are thus an important feedback mechanism reducing positive plant growth responses to CO 2 . Proportional allocation of dry mass to roots did not vary between CO 2 treatments. Early in the experiment, proportional increases in plant dry mass in elevated CO 2 were larger in faster‐growing Populus tremuloides and B. papyrifera than in the slower‐growing conifers. However, growth increases in response to CO 2 enrichment fell with time for broadleaved species and increased for the conifers. With increasing plant size over time, compensatory adjustments to CO 2 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 CO 2 was not supported, given contrasting patterns of growth response to CO 2 with increasing plant size and age.