Effects of drought and CO2 enrichment on competition between two old‐field perennials

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Abstract
We studied the effects of drought stress and CO2 enrichment on the competition between Aster pilosus Willd. (aster, C3) and Andropogon virginicus L. (broomsedge, C4) under two CO2 concentrations (350 and 650 μl l−1 CO2) and two water treatments (well‐watered and water‐limited). Although broomsedge is the more drought‐tolerant species, this did not increase its competitive ability against aster under drought conditions. With CO2 enrichment, aster was a stronger competitor than broomsedge and comprised 75% of above‐ground pot biomass in both water treatments. CO2 enrichment also increased aster survival when competing with broomsedge under extreme drought conditions. Although drought stress and CO2 enrichment interacted to affect the two species in different ways, there was no interaction of drought stress and competition; aster was a stronger competitor than broomsedge under CO2 enrichment in both well‐watered and water‐limited conditions. With future increases in the atmospheric CO2 concentration, aster may delay broomsedge dominance in old‐field communities.