Three poplar (Populus) clones were grown in outdoor, sunlit and closed-top chambers under two CO2 treatments (i.e., an ambient atmospheric CO2 treatment of ca. 350 ppmV and an elevated CO2 treatment of 700 ppmV), and one outside control treatment. In general, the enclosure or chamber effect on growth and development was significantly more pronounced than the CO2 enrichment effect, but significant clonal variation was observed in the response to elevated CO2. Near the end of the growing season, an acclimation effect of the response curves of net photosynthesis to CO2 internal partial pressure (A-C, curves) was observed with lower values of maximum net photosynthesis and carboxylation efficiency in the 700 ppmV treatment than in the 350 ppmV CO2. No significant effects of elevated CO2 on leaf growth rate and mean internode length were observed; less leaves were produced in the 700 ppmV treatment as compared to the 350 ppmV treatment. Leaves of clones Beaupre and Columbia River grown at elevated CO2 concentrations were only slightly (but not significantly) thicker than those of the control treatment of 350 ppmV, while their nitrogen content remained unchanged (between 2 and 2.5% dry mass). In all clones less and smaller branches (both in size and biomass) were produced under the elevated CO2 levels than under the control CO2 conditions. Height growth was significantly higher in the closed-top chambers as compared to the outside treatment, but was not increased by the elevated CO2 treatment. Total biomass at the end of the growing season was increased by 25% in the elevated CO2 treatment, but only in clone Becaupre.
