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

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.