Competition between Two Annual Herbs, Atriplex gmelini C. A. Mey and Chenopodium album L., in Mixed Cultures Irrigated with Seawater of Various Concentrations

Atriplex gmelini and Chenopodium album were grown in mixed stands with various combinations of plant density and mixing ratio, and irrigated with seawater of different concentrations (f) to formulate the effect of changing concentration on the competitive relationship between the species. In single-species stands, the mean plant weight (w) ∼ plant density (ρ) relation for each level of seawater concentration could well be described by Shinozaki-Kira's reciprocal equation of crowding effect. On the other hand, the response of w to f followed Hozumi-Shinozaki's formulation for an optimum growth factor at respective levels of ρ. By introducing the density conversion factor (q) that enabled the conversion of the density of one species to that of the other species on the basis of their effects on growth of respective species, the results of mixed culture experiments could be successfully formulated by similar reciprocal equations. The dependence of q and coefficient values of the equations on seawater concentration was also formulated in a way similar to the case of pure stands. Based on all these quantitative relations, a comprehensive formulation was developed to describe the effects of plant density and seawater concentration on the growth of two species in mixed stands. The behavior of species biomass in mixed stands was then examined by means of the formulation. It was thereby demonstrated that the relative dominance of two species in a mixed stand was strongly affected not only by total plant density and density ratio between the two species but also by concentration of irrigated seawater. Even the optimum seawater concentration that resulted in the maximum species biomass differed between pure and mixed cultures.