Density-dependent responses of reproductive allocation to elevated atmospheric CO2 in Phytolacca americana.

• This study was conducted to determine whether elevated CO 2 alters patterns of plant reproduction, and whether density affects population- and individual-level responses to elevated CO 2 . • Phytolacca americana was grown in a glasshouse at three population densities under ambient and elevated CO 2 environments, and harvested at both vegetative and seed mature stages. • CO2 did not affect the observed or estimated minimum size required for reproduction. At the population-level, elevated CO2 increased the total and above-ground biomass at both harvests. Density decreased both measurements at the second harvest. At the individual-level, elevated CO2 increased reproductive mass but decreased seed size, and the responses of reproductive allocation were density-dependent. Net photosynthesis at saturating light (Pmax ) increased under elevated CO2 , but decreased with density, with a CO2 × density interaction. • hese results indicate that CO 2 advances timing of flowering by changing growth rate rather than modifying minimum size required for reproduction, while density modifies the responses of reproductive allocations to elevated CO 2 in P. americana .