Two hybrid poplar (Populus) clones (i.e. fast growing clone Beaupre and slower growing clone Robusta) were grown from cuttings at close spacings in four open top chambers (OTCs) on the Campus of the University of Antwerpen, Belgium. The four OTCs represented two atmospheric CO2 treatments, i.e. ambient and elevated (= ambient + 350 mu mol mol(-1)). Treatments lasted for a full growing season (April-November 1993) and results of the first growing season are being reported. In both clones the elevated CO2 treatment resulted in a significant increase in plant height and in biomass production, both of stems and branches. Plants of both clones produced significantly more, but shorter, side branches under the elevated CO2 treatment. In terms of biomass accumulation the slower growing clone Robusta benefited relatively more (+ 37%) from the elevated CO2 concentrations than the fast growing clone Beaupre (+ 24%). In terms of leaf weight ratio, the slower growing clone became relatively more efficient under elevated CO2 than the fast growing clone. The elevated atmospheric CO2 treatment significantly increased the total leaf area per plant and leaf area index per OTC; maximum LAI increased by 18% in clone Beaupre and by only 8% in the slower growing clone Robusta. In the fast growing elope the increase in leaf area index was entirely caused by an increase in individual leaf area, while in the slower growing clone also a 5% higher leaf production was observed under the elevated CO2. The total length of the growing season was on average reduced by the elevated CO2 treatment; in the slower growing clone mainly by an advancement of bud set and in the faster growing clone by a slight delay of bud break in early spring. In both clones elevated CO2 decreased nitrogen concentration and increased C/N ratio in all plant organs, but no data for the below-ground compartment were available. Therefore, although similar trends in the responses to elevated atmospheric CO2 were observed in both clones, the relative efficiency of these responses differed between the fast and the slower growing poplar clones, suggesting interactions between growth rate, growth strategy and response to elevated atmospheric CO2.
