Competitive abilities of introduced and native grasses

Differences in competitive ability may explain the maintenance of existing plant populations and the invasion of new areas by plant species. We used field experiments to examine the competitive responses of Agropyron cristatum (L.) Gaertn., an introduced C-3 grass, and Bouteloua gracilis (HBK.) Lag., a native C-4 grass, and the competitive effects of Agropyron-dominated vegetation and successional prairie. We also tested whether the outcome of competitive interactions varied with water availability. In each vegetation type, transplants of each species were grown under two levels of competition (presence or absence of neighboring vegetation) and three levels of water availability (high, medium, or low). Transplant survival, growth, and biomass allocation patterns were measured. Water availability had no effect on the measured variables, suggesting that both species were limited by another resource. Growth rates were affected more by competition, while survival and root: shoot ratio were affected more by transplant species identity. In the successional prairie, neighboring vegetation suppressed the growth of Agropyron transplants less than that of Bouteloua transplants, suggesting that Agropyron has a stronger ability to resist competitive suppression in that vegetation type. The spread of Agropyron into surrounding vegetation may relate to its ability to resist competitive suppression. In the Agropyron-dominated vegetation, neighboring vegetation suppressed the growth of both species by the same extent. However, competition accounted for more variation in transplant growth in Agropyron-dominated vegetation than in successional prairie, suggesting that Agropyron has strong competitive effects which hinder plant growth and prevent other species from establishing in Agropyron fields.