The response of plants to elevated CO2 : I. Competition among an assemblage of annuals at two levels of soil moisture

Abstract

Four coexisting annual plant species were grown in competition at three levels of CO₂ (300, 600, and 1,200 ppm) and two levels of soil moisture (moist and dry). Plant height was higher at high CO₂ concentrations for the three C₃ species but not for the C₄ species (Amaranthus retroflexus). Total community biomass increased with increasing CO₂ at both soil moisture levels. The contribution of each species to total community biomass was influenced by CO₂ concentration. The effects were especially pronounced for Polygonum pensylvanicum which contributed more to community production as CO₂ and soil moisture increased. Amaranthus behaved in exactly the reverse way; it did best under ambient CO₂ and dry soil moisture conditions. The results suggest that changes in competitive interactions and community structure will occur with the anticipated rise in global CO₂ concentration.