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 vr in monoculture s and in pairwise competition plots on an experimental nitrogen gradie nt. 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 interspecifi c competition, three competitive situations were created: seed vs. see d competition (both species planted simultaneously), seed invasions (e ach 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 meas ured 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 o r greatly reduced the biomass of both Poa and Agropyron on the soil mi xture gradient (the mixed soils but not the added-N plots) independent of the wide range of starting conditions. On these soils, Schizachyri um monocultures had significantly lower soil concentrations of both NO 3- and NH4+ than either Poa or Agropyron monocultures. Similarly, Agro pyron 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 o f these two species did not differ in NO3- concentration, but did diff er for NH4+ and NO3- + NH4+. Thus, species differences in ability to d eplete soil NO3- successfully predicted the outcome of competition for all four species pairs on the soil mixture gradient. If resource pree mption or asymmetric competition had been the mechanism of competition , initial conditions would have affected the long-term outcome of comp etition. Rather, these results support the R (i.e., resource reductio n) model for competition for soil N. In the added-N fertilizer plots, Schizachyrium had decreased biomass in competition with both Poa and A gropyron. However, neither Agropyron nor Poa appeared to have an advan tage when they competed with each other in the added-N plots. For thes e three species pairs, the 5-yr results of competition in the added-N plots, which had greatly reduced light availability because of increas ed production and litter accumulation, depended on initial conditions. In the fourth pair, Agrostis was displaced by Agropyron in all compet ition treatments in the added-N plots. Thus, we cannot reject the hypo thesis that resource preemption (i.e., asymmetric competition) is impo rtant in light competition.