Effects of density on above and below ground biomass of the native alpine grass Poa fawcettiae and the environmental weed Achillea millefolium

Competition between Poa Jawcettiae Vickery, the dominant native snowgrass, and the invasive herbaceous Achillea millefolium L., was examined in three glasshouse experiments. The first experiment investigated the potential for intraspecific competition in plants by growing them in pots with low and high density. The second experiment examined the potential for inter-specific competition at low, medium and high density. In the third experiment plants in pots where either roots or shoots of the species could not compete were compared to those where root and shoot competition was possible. Achillea millefolium plants produced more than four times the biomass of P. fawcettiae plants. As a result the two species responded differently. In the A. millefolium monocultures both root and shoot biomass per plant declined at high density. By contrast, P. fawcettiae biomass was not affected. In mixed species pots, P. fawcettiae had no effect on the biomass of A. millefolium plants, while P. fawcettiae shoot and root biomass per plant decreased when grown with A. millefolium at all densities tested. Root competition from A. millefolium appears to be the main cause of the decrease in biomass of P. fawcettiae. The results imply that A. millefolium may have a competitive advantage over P. Jawcettiae in the Australian Alps.