DYNAMICS OF NITROGEN COMPETITION BETWEEN SUCCESSIONAL GRASSES

Pairwise competition experiments were performed for 3 yr on an experimental nitrogen gradient at Cedar Creek Natural History Area, Minnesota, where N is the major limiting resource during early succession. Agrostis scabra, an early successional grass, competed against another early (Agropyron repens), a later (Schizachyrium scoparium), and an even later (Andropogon gerardi) successional species. On low N soils, Agrostis was competitively displaced by each of the later successional species, but persisted with Agropyron. On high N soils, Agrostis was displaced by all three of the other species. The inferior competitive ability for N of the early successional species refutes the resource ratio hypothesis of succession. Rather, the high allocation of Agrostis to seed and its rapid colonization of fields support a colonization-competition hypothesis of succession. For two of three cases, the outcome of competition on low nitrogen soils was predicted by R*, the nitrogen concentration to which monocultures of each species reduced extractable soil nitrate and ammonium on N-limited soils. In these cases, the species with the significantly lower R* for nitrogen displaced the other species. In the third case, the species had more similar R* values, and the species with the lower R* had not displaced the other species within 3 yr (but it had done so after 5 yr; see Note added in proof).