ts ofelevated atmospheric CO. concentrations on the ecophysiological responses (gas exchange, chlorophyll a fluorescence, Rubisco activity, lear area development) as well as on the growth and biomass production of two poplar clones (i.e. Populus Irichoearpa×P deltoides clone Beaupre and P. ×euramericana clone Robusta) were examined under open top chamber conditions. The elevated CO2 treatment (ambient+350μmol mol-1) stimulated above-ground biomass of clones Rohusta and Beaupr6 after the first growing season by 55 and 38%, respectively-This lncreased biomass production under elevated CO2 was associated with a significant increase in plant height. the latter beingthe result of enhancedInternode elongation ratherthan alllncreased production of leaves or internodes Both an Increased leararea index (LAI) and a stimulated net Dhotosvnthesls per unit leaf contributed to a significantly higher stem biomassperunitleararea. andthustothelncreased above-ground biomassproduction underthe elevated CO2 concentratlons 1n both clones The larger LAl was caused by a larger individuaI leaf size and lear growth rate: the number ofleaveswas not altered bythe elevatedCO2 treatment The highernetlearphotosynthesiswasthe result of an 1ncrease 1n the hotochemical fmaximal chlorophyll fluorescence Fm and photochemical efficiency Fv/Fm)as well as m the biochemical (increased Rubisco activity) process capacities No significant difierences were found in dark respiration rate, neither between dongs nor between treatments, but specific leaf area significantly decreased under elevated CO2 conditions.