Competitive responses of eight old-field plant species in four environments

Variation in relative competitive abilities among environments has been proposed to control the species composition of plant communities. We tested whether the competitive responses of eight species varied with fertility and disturbance in an old field. We also tested whether the competitive effects of vegetation varied with fertility and disturbance in an old field. We also tested whether the competitive effects of vegetation varied with fertility and disturbance. Plots received one of four combinations of nitrogen (added or not) and disturbance (tilled or not) in each of five years. There were 10 replicates of each treatment. In the fifth summer, transplants of eight species comprising annual and perennial forbs and grasses were grown in all four combinations of the N and disturbance treatments. Transplants were grown in subplots with all neighbors, with only roots of neighbors, or with no neighbors. Transplant roots and shoots were harvested after one summer and individual growth rates were determined. Growth rate varied significantly with the interaction among species, competition, and N, suggesting that species differed in their competitive responses and that these differences varied with N availability. Comparisons among competition treatments at each level of N showed six distinct responses in the eight species examined. The competitive effect of neighboring vegetation shifted from roots to shoots as N increased, but decreased in disturbed plots. The total effect of competition increased significantly with neighbor mass in 4 of the 32 examined (eight species in four combinations of N and disturbance). All four cases involved annuals in annual neighborhoods. The results for eight species confirmed earlier single—species diverged in their responses to competition as soil N availability varied.