The Response of Annuals in Competitive Neighborhoods: Effects of Elevated CO2

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Abstract
Four members of an annual community were used to investigate the effects of changing neighborhood complexity and increased CO2 concentration on competitive outcome. Plants were grown in monoculture and in all possible combinations of two, three, and four species in CO2—controlled growth chambers at CO2 concentrations of 350, 500, and 700 μL/L with ample moisture and high light. Species responded differently to enhanced CO2 level. Some species (e.g., Abutilon theophrasti) had increased biomass with increasing CO2, while others (e.g., Amaranthus retroflexus) had decreased biomass with increasing CO2 concentration. In mixtures, species tended to interact strongly, and, in some cases, the interaction canceled out the effects of CO2. Furthermore, there were cleared differences in species behavior in different competitive mixtures as assessed by total biomass and seed biomass, and by an index of response to neighbors. In general, competitive arrays that had C3 species depressed the response of C4 species, especially Amaranthus. Ambrosia artemisiifolia was the strongest competitor in this assemblage. Strong statistical interactions between CO2 and the identity of the competing species in mixtures were found to be primarily due to the as yet unexplained response of plants with CO2 at 500 μL/L. The potential effects of CO2 on community structure could be profound, particularly at the intermediate levels of CO2 that are predicted to be reached during the first half of the next century.