Spatial Competition between Grasses -- Rates of Mutual Invasion between Four Species and the Interaction with Grazing

Monoculture turves of four grasses, Festuca rubra, F. arundinacea, Lolium perenne and Poa pratensis, were grown in adjacent plots in all permutations of two species. This experiment was nested within the treatments of a grazing experiment with sheep that factorially combined three grazing treatments in a complete block design. The invasion of each grass species into its neighbour was measured by counting tillers that crossed the interface between species. The invasion of the experimental turves by Agrostis stolonifera from the native sward was also measured. 2. Grazing treatments affected the invasion rate of experimental species as a whole. Species differed in their invasion rates, and there was a significant interaction of grazing treatments X species. Invasion of turves by A. stolonifera was also affected by an interaction between grazing treatments and species. 3. Although the ranking of species was affected by grazing, Festuca rubra and Lolium perenne were always in the top two species by invasiveness. This was unrelated to size of plant, though relationships between species were all transitive. No clear correlations between invasiveness and invasibility occurred. 4. There was some preliminary evidence that, after applying grazing treatments for four years, the composition of the native sward was consistent with the competitive relationships of Festuca rubra and Lolium perenne as measured by their net invasion rates.