CHEATGRASS AND YELLOW STARTHISTLE GROWTH AT 3 SOIL DEPTHS

Community dynamics and dominance on cheatgrass (Bromus tectorum L.) an d yellow starthistle (Centaurea solstitialis L.) infested rangeland ap pear to be influenced by resource acquisition rates and duration of gr owth, Objectives were to determine the effects of densities, proportio ns, and soil depth on the growth rate and duration of growth of these species, In 6 field experiments isolated individuals, monocultures (10 0, 1,000, 10,000 plants m(-2)), and mixtures (same densities arranged factorially) were grown with unrestricted and restricted (0.2- and 0.5 -m) soil depths, Shoot weights were determined on 12-day intervals beg inning on day 24 and ending on day 72 for plants grown with restricted soil depth and day 96 (cheatgrass) and day 108 (yellow starthistle) f or plants grown in unrestricted soil, Quadratic growth curves were fit for each replication for plants grown in isolation, Linear and quadra tic models were developed for plants grown in monocultures and mixture s, Simple linear regression coefficients were used as growth rates and regressed over plant density, Time of inflection (an indicator of the duration of growth) for plants growing in monocultures and mixtures w as calculated from quadratic models, Growth rate of yellow starthistle was about 7 times faster and duration of growth 25 days longer, than those of cheatgrass when grown in isolation without soil depth restric tion, As densities were increased and/or soil depth decreased, growth rate and duration of growth were lowered and the difference between sp ecies were masked, Growth rates and duration of growth of cheatgrass a nd yellow starthistle appear to depend on plant density, soil depth, a nd available soil moisture.