CO2 enrichment reduces reproductive dominance in competing stands of Ambrosia artemisiifolia (common ragweed)

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Plants growing in dense stands may not equally acquire or utilize extra carbon gained in elevated CO(2). As a result, reproductive differences between dominant and subordinate plants may be altered under rising CO(2) conditions. We hypothesized that elevated CO(2) would enhance the reproductive allocation of shaded, subordinate Ambrosia artemisiifolia L. (Asteraceae) individuals more than that of light-saturated dominants. We grew stands of A. artemisiifolia at either 360 or 720 muL L(-1) CO(2) levels and measured the growth and reproductive responses of competing individuals. To test whether elevated CO(2) altered size and reproductive inequalities within stands, we compared stand-level coefficients of variation (CV) in height growth and final shoot, root, and reproductive organ biomasses. Elevated CO(2) enhanced biomass and reduced the CV for all aspects of plant growth, especially reproductive biomass. Allocation to reproduction was higher in the elevated CO(2) than in the ambient treatment, and this difference was more pronounced in small, rather than large plant positive relationships between the CV and total stand productivity declined under elevated CO(2), indicating that growth enhancements to smaller plants diminished the relative biomass advantages of larger plants in increasingly crowded conditions. We conclude that elevated CO(2) stimulates stand-level reproduction while CO(2)-induced growth gains of subordinate A. artemisiifolia plants minimize differences in the reproductive output of small and large plants. Thus, more individuals are likely to produce greater amounts of seeds and pollen in future populations of this allergenic weed.