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 (P_max ) 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 .