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 (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 ≥ 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.