Convergence and Divergence of Old‐Field Plant Communities Along Experimental Nitrogen Gradients

Data from experiments in three old fields were analyzed using similarity indices and population trajectories to determine if similarity of resource supply rates influenced the similarity of plant community composition. In two of three fields, similarity indices indicated that experimental plots having different resource supply rates diverged in species composition with time. There was some evidence of convergence both within and between fields on plots with similar resource supply rates; however, divergence was also common, especially at high rates of nutrient supply. Much of the divergence indicated by similarity indices can be explained by plot—to—plot differences in the initial abundances of three dominant species: Agropyron repens, Poa pratensis, and Schizachyrium scoparium. High nitrogen treatments favored Agropyron repens if it was present, but led to major increases in P. pratensis or S. scoparium if A. repens was absent. The trajectories through time of these three species suggest that plots will converge at high nitrogen levels.