COMPETITION OF 5 NATIVE PRAIRIE GRASSES WITH BROMUS-INERMIS UNDER 3 MOISTURE REGIMES

We examined the effect of moisture on the competitive ability of five native grasses (Agropyron and Stipa) in competition with the non-nativ e grass Bromus inermis Leyss. The plants were grown in Large pots in g rowth rooms, and the moisture treatment was imposed by the frequency o f watering. The competitive ability of the five native grasses, as mea sured by shoot biomass ratio, increased with reduced water availabilit y, but even under the driest conditions, they competed poorly with B. inermis. The order of competitive ability found in this experiment did not match the order predicted from field observations based on the hy pothesis of a strict reciprocal relationship between stress tolerance and competitive ability. Stipa viridula Trin. was the best competitor of the five, and Stipa curtiseta (A.S. Hitchc.) Barkworth was consiste ntly the least competitive. The order of competitive ability was more variable when measured by the effect on Bromus than when measured by t he effect on the native grasses themselves. The results suggest that t olerance of abiotic stress is less important than competitive exclusio n in determining species distributions on environmental gradients.