Spatiotemporal Differences in 15N Uptake and the Organization of an Old‐Field Plant Community

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Abstract
We investigated the hypothesis that spatial and temporal differences in nitrogen uptake facilitate species' coexistence in a nitrogen—limited, old—field plant community at Cedar Creek Natural History Area, Minnesota. Differences among the six most abundant species were assessed by measuring aboveground uptake of 15N injected at two soil depths at each of three times during the growing season. As described by 15N uptake, species were spatially and, to a greater extent, temporally differentiated into three groups. Differences in species' abundance between and within these groups suggest that spatiotemporal partitioning of nitrogen in a major determinant of community organization. Dominant species (Schizachyrium scoparium and Poa pratensis) were well differentiated and the abundances of subordinate species (Artemisia ludoviciana, Solidago nemoralis, Ambrosia coronopifolia, and Panicum oligosanthes) were positively related to the degree of differentiation from dominant species. A discriminant analysis of the data showed that subordinate species occupy "peripheral" spatiotemporal niches relative to Schizachyrium and Poa. This may promote coexistence by reducing diffuse competition. These results suggest that spatiotemporal resource partitioning slows the rate of competitive displacement among co—occurring plant species. Other research at Cedar Creek corroborates that this is an effective mechanism for coexistence on a successional time scale.