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

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