Effect of propagule source on competitive ability of pasture grasses: spatial dynamics of six grasses in simulated swards

Abstract

Simulated swards were constructed to investigate the potential effect of propagule source on competitive ability of pasture grasses, to monitor the spatial dynamics of pasture communities, and to assess the importance of species interactions and disturbances in community structure. The design used six species of grasses, Agrostis alba, Dactylis glomerata, Holcus lanatus, Lolium perenne, Phleum pratense, and Poa compressa, arranged in a mosaic of hexagonal patches with each hexagon being sown with one species of grass or left as bare ground and being surrounded by each of the other species. Three swards were constructed, one using plant material collected from an 11-year-old pasture, one with material from an adjacent 49-year-old pasture, and the third from material grown from seed. After 20 months of interaction across the interfaces between the patches, the swards were destructively harvested above ground and the contents of each hexagon separated into species. Grasses from four different-aged origins, including the three above, were also grown in plots without any neighbours for 16 months. Each species had its own individual behavior: Agrostis and Phleum from the older pastures were the better invaders, Holcus and Poa were the poorest, and Phleum from the oldest community was the most resistant to lateral expansion of the other species. In a number of cases the grasses from the older pastures, but never from the seeded bed, invaded other patches of grass as readily as they expanded into bare ground. The total aboveground community biomass showed a nonsignificant decline with age, and the community as a whole became more open to lateral expansion by individual species. No results were obtained to support the cyclic regeneration hypothesis of Watt; the functional equivalence arguments of Aarssen may be more appropriate in this fine-grained pasture environment. Key words: community dynamics, invasion, pasture, competitive effect and response, disturbance, patches.