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

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.