Flood-induced recruitment of the invasive perennial herb 'Phyla canescens' (lippia)

Abstract

'Phyla canescens' (Kunth) Greene, lippia, (Verbenaceae) is an important invasive species in the Murray-Darling Basin, Australia. The general lack of quantitative information on aspects of the life-history of 'P. canescens' is a substantial impediment to the sustainable management of this species and the communities it invades. Complementary laboratory and field experiments investigated 'P. canescens' germination. A thermogradient plate was used to examine its germination response to a variety of temperature regimes. Recruitment in the field was investigated at four sites across two catchments following four season disturbances. In the laboratory trials, seeds required temperatures that alternated by at least 5°C, light, and to be covered by a thin film of water. Field germination occurred only at the one site that experienced a flood, despite periods of high rainfall that stimulated germination of other species. In this site seedling density and survivorship were reduced in the presence of existing vegetation. A survey of 'P. canescens' following flooding of a temporary billabong revealed recruitment from both seed and vegetative fragments. This recruitment was almost exclusively restricted to the area that had been flooded. The results suggest that 'P. canescens' requires inundation for successful recruitment and that these germination requirements are typical of species from disturbance-prone environments.