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<br />The addition of nitrogen (N) has been shown to be a major <br />factor modifying successional dynamics in a number of ecosystems <br />following various types of disturbances (Robertson and Vitousek <br />1981, Westman 1981, Parrish and Bazzaz 1982, Robertson 1982, <br />Titlyanova 1982, Heil and Diemont 1983, Smith and Rice 1983, Tilman <br />1984 and 1986, Thorne and Hamburg 1985, Inouye et al. 1987, Carson <br />and Barrett 1988, Zak and Pregitzer 1989, Carpenter et al. 1990, <br />Carson and Pickett 1990, McLendon and Redente 1991). This ePlact <br />has been attributed to changes in competitive relationships among <br />species on sites with higher levels of available nitrogen compared <br />to sites without supplemental nitrogen (Raynal and Bazzaz 1975, <br />Peterson and Bazzaz 1978, Bazzaz 1979, Tilman 1982 and 1986, Miller <br />and Werner 1987, Carson and Barrett 1988). High growth rates of <br />early-successional species (Bazzaz 1979) require high levels of <br />available nutrients to reach their potential and under such <br />conditions can dominate sites by rapidly utilizing the available <br />resource or causing other resources (e.g. light) to become limited <br />to slower-growing species. However, as resources become more <br />limited, species with lower nutrient requirements should have a <br />competitive advantage (Grime 1979, Leps et al. 1982, McGraw and <br />Chapin 1989). Central to this explanation is the concept that <br />secondary succession occurs, at least in part, because of <br />competitive displacement of species (Grace 1987). ', Since. <br />competition among species occurs only when resources are limited <br />and the degree of competition increases as resources become more <br />limited, the rate of succession should increase along a resource <br />limitation gradient. <br />