TEMPERATURE AND ONTOGENY MEDIATE GROWTH-RESPONSE TO ELEVATED CO2 IN SEEDLINGS OF 5 BOREAL TREE SPECIES

We tested the extent to which growth responses to elevated carbon diox ide (CO2) are temperature-dependent and change through early seedling ontogeny among boreal tree species of contrasting relative growth rate s (RGR). Populus tremuloides Michx, Betula papyrifera Marsh, Larix lar icina (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 mu mol mol-L) and fiv e temperature regimes of 18/12, 21/15, 24/18, 27/21 and 30/24 degrees C (light/dark). Growth increases in response to CO2 enrichment were mi nimal at the lowest temperature and maximal at 21/15 OC for the three conifers and at 24/18 degrees C or higher for the two broadleaved spec ies, corresponding with differences in optimal temperatures for growth . In both CO2 treatments, RGR among species and temperatures correlate d positively with leaf area ratio (LAR) (r greater than or equal to 0. 90, P < 0.0001). However, at a given LAR, RGR was higher in elevated C O2, owing to enhanced whole-plant net assimilation rate. On average in all species and temperatures at a common plant mass, CO2 enrichment i ncreased RGR (9 %) through higher whole-plant net assimilation rate (2 2 %), despite declines in LAR in high CO2 (11 %). Reductions in LAR ar e 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 in creases in plant dry mass in elevated CO2 were larger in faster-growin g Populus tremuloides and B. papyrifera than in the slower-growing con ifers. However, growth increases in response to CO2 enrichment fell wi th time for broadleaved species and increased for the conifers. With i ncreasing plant size over time, compensatory adjustments to CO2 enrich ment in the factors that determine RGR, such as LAR, were much larger in broadleaves than in conifers. Thus, the hypothesis that faster-grow ing species are more responsive to elevated CO2 was not supported, giv en contrasting patterns of growth response to CO2 with increasing plan t size and age.