Do above-ground growth dynamics of poplar change with time under CO2 enrichment?

In a free-air CO2 enrichment (FACE) study, above-ground growth of Populus alba, Populus nigra and Populus x euramericana was followed continuously during the first rotation cycle of a short rotation culture (SRC) plantation to test possible changes in the response to elevated CO2 occurring from planting until canopy closure. Height, stem basal area, stem volume index, branch production, and bud phenology were monitored for 3 yr. Moreover the coefficient of variation and a competition index were calculated to analyse the onset and the typology of competition. Volume index was higher under elevated CO2 by 77%, 24% and 22%, as mean value for the three species, in the first, second and third years, respectively. The stimulating response, although univocal, differed in extent among species. Branch production was stimulated only in the first year, whereas bud phenology was unaffected. The analysis of these results show that growth was stimulated by elevated CO2 only in the first year, although differences in volume index remained significant even in the second and third years. in the third year, under canopy closure, only competitively advantaged individuals profited by the FACE treatment.