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~.~! <br />ABSTRACT <br />A soil nitrogen (N) availability gradient was induced on a <br />disturbed sagebrush site in northwestern Colorado by fertilizing <br />with nitrogen (high available Nj, applying sucrose (low available <br />N), and applying neither nitrogen nor sucrose (control). <br />Successional dynamics were studied for 3 years. At the tertaination <br />of the study, N concentration of aboveground tissue of 5 major <br />seral species were determined. The rate of secondary succession, <br />as measured by change in species composition, was most rapid on <br />plots receiving the sucrose treatment and was slowest on plots <br />receiving the N treatment. Early-seral dominants were more <br />effective in N utilization (greater tissue N concentration) when <br />availability of the resource was high but were poor competitors <br />(lower tissue N concentration) when available soil N became <br />limited. Mid-seral dominants displayed the opposite pattern, i.e., <br />they were less able to rapidly utilize high levels of available <br />soil N but became increasingly effective competitors as soil N <br />levels decreased.. These results suggest that the supply of <br />available soil N, and therefore the dynamics of N incorporation in <br />perennial plant tissue, is a primary mechanism in controlling the <br />rate of secondary succession within the semiarid system of our <br />study. <br />Key words: succession, nitrogen, competition, disturbance, <br />semiarid, shrubland, Colorado plateau <br />Key phrases: effect of nutrient availability on early succession, <br />control of early successional replacement <br />