Growth performance of Populus exposed to "Free Air Carbon dioxide Enrichment" during the first growing season in the POPFACE experiment

Stem diameter, total plant height and number of sylleptic branches of three poplar (Populus) genotypes were followed during the first growing season o f a high density intensively cultured plantation (in Central Italy) both un der ambient CO2 (Control) and under elevated atmospheric CO2 (550 ppm) usin g the FACE technique. The three poplar genotypes belonged to different spec ies of Populus alba L., Populus nigra L. and Populus x euramericana Dode (G uinier). All three genotypes responded by an enhanced growth performance bu t the extent of their response to the FACE treatment was different. A stem volume index was calculated considering the stem composed by a truncated co ne in the lower part and by a cone in the upper part. At the end of the fir st growing season, stem volume index was increased in the FACE treatment by 54% to 79% as compared to Control treatment, depending on the genotype. Th is increased stem volume index was caused by an increase of basal stem diam eter rather than by an enhancement of plant height. Number of sylleptic bra nches was stimulated by more than 35% in the P. nigra genotype. The results confirm the optimal performance of this new POPFACE experiment and show th e positive response of this fast-growing tree species to elevated CO2 condi tions at an ecosystem scale even if considering the genotypic differences.