Competition among grasses along a nitrogen gradient: Initial conditions and mechanisms of competition

We grew four perennial grass species (Poa pratensis, Agropyron repens, Agrostis scabra, and Schizachyrium scoparium) for 5 yr in monocultures and in pairwise competition plots on an experimental nitrogen gradient. The gradient contained plots ranging from 100% sand to 100% black soil, plus plots that received additional N fertilizer. To examine the impact of initial conditions on the long-term outcome of interspecific competition, three competitive situations were created: seed vs. seed competition (both species planted simultaneously), seed invasions (each species added as seed to year-old monocultures of the other), and vegetative invasions (dividers separating adjacent monocultures of two species removed after 1 yr). Extractable soil NO₃, and NH₄ were measured to test if species differences in the concentration of available soil N in monoculture (i.e., R * for N, Tilman 1982) could predict the long-term outcome of competition. By year 5, Schizachyrium displaced or greatly reduced the biomass of both Poa and Agropyron on the soil mixture gradient (the mixed soils but not the added-N plots) independent of the wide range of starting conditions. On these soils, Schizachyrium monocultures had significantly lower soil concentrations of both NO₃ ^-, and NH₄ ⁺ than either Poa or Agropyron monocultures. Similarly, Agropyron displaced or greatly reduced the biomass of Agrostis by year 5. Agropyron monocultures had significantly lower concentrations of NO₃ ^- and NO₃ ^- + NH₄ ⁺, but not NH₄ ⁺, than Agrostis monocultures. In contrast, no competitive displacement occurred in competition between Poa and Agropyron, and initial differences persisted over 5 yr. Monocultures of these two species did not differ in NO₃ ^- concentration, but did differ for NH₄ ⁺ and NO₃ + NH₄ ⁺. Thus, species differences in ability to deplete soil NO₃ ^- successfully predicted the outcome of competition for all four species pairs on the soil mixture gradient. If resource preemption or asymmetric competition had been the mechanism of competition, initial conditions would have affected the long-term outcome of competition. Rather, these results support the R* (i.e., resource reduction) model for competition for soil N. In the added-N fertilizer plots, Schizachyrium had decreased biomass in competition with both Poa and Agropyron. However, neither Agropyron nor Poa appeared to have an advantage when they competed with each other in the added-N plots. For these three species pairs, the 5-yr results of competition in the added-N plots, which had greatly reduced light availability because of increased production and litter accumulation, depended on initial conditions. In the fourth pair, Agrostis was displaced by Agropyron in all competition treatments in the added-N plots. Thus, we cannot reject the hypothesis that resource preemption (i.e., asymmetric competition) is important in light competition.