INTERFERENCE BETWEEN CHEATGRASS AND YELLOW STARTHISTLE AT 3 SOIL DEPTHS

Cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea sols titialis L.) have invaded over 250 thousand hectares throughout the Pa cific Northwest, Future management of rangelands dominated by these sp ecies will require an understanding of the plant-plant interactions wh ich contribute to the regulation of community dynamics and the establi shment of desirable plants. Addition series experiments, with total st and densities ranging from 20-20,000 plants m(-2), were used to quanti fy the interference between cheatgrass and yellow starthistie in unres tricted soil depths on 12-day intervals throughout the growing season and in soil depths restricted to 0.2- and 0.5-m. Intraspecific interfe rence was nearly twice as important as interspecific interference when plants were grown in unrestricted soil, Resource partitioning by chea tgrass and yellow starthistle was associated with rooting depth, When restricted to a 0.5-m depth, resource partitioning did not occur and i ntra- and interspecific interference were similar for both species, Re striction to a 0.(2)-m depth resulted in cheatgrass density being twic e as important as yellow starthistle density for predicting yellow sta rthistle shoot weight, Yellow starthistle density was not important fo r predicting cheatgrass shoot weight, Cheatgrass appeared to have a co mpetitive advantage over yellow starthistle in shallow soils, but the rooting depth and seed production capacity of yellow starthistle was g reater than cheatgrass in deep soil.