Vegetative reproduction facilitates early expansion of Phyla canescens in a semi-arid floodplain

Abstract

Phyla canescens (Verbenaceae, hereafter lippia) is an invasive perennial forb from South America that is now widespread in Australia, threatening the biological values of internationally significant wetlands (Ramsar listed) and the productivity of the grazing industry (Crawford 2008; Earl 2003). Under certain conditions, lippia can displace native herbaceous vegetation and form mono-specific stands, but little is known about the process of colonization and early invasion. Lippia has a dual reproductive strategy, recruiting via seedlings and vegetative fragments in periods following inundation (Macdonald 2008). Determining the relative contribution of vegetative and sexual reproduction for lippia across environmental gradients is important for management action—if spread is primarily by vegetative fragments then control efforts should be focussed on managing vectors (e.g. movement of stock and vehicles), whereas if spread is mostly by seeds then management should focus on reducing the conditions favourable to seed production and seedling establishment. The predominant reproductive mode also has implications for the selection of biocontrol agents. A comparison of lippia in Australia with native range populations found that Australian populations had evolved to invest more energy in sexual reproduction than in the native range (Xu et al. 2009). Genetic analysis found that the importance of vegetative and sexual reproduction is highly variable among populations in Australia (Fatemi and Gross, unpublished data). In some cases, populations exhibited extremely low diversity, with single clones covering distances of up to 100 m and in others, genetic diversity was relatively high with samples 20 m apart representing genetically distinct individuals (Fatemi and Gross, unpublished data). It remains unknown what factors influence the mode of reproduction in different populations in Australia but it is likely that the type of flood event and propagule availability are important. Previous field studies have found that seedling recruitment was the predominant reproductive mode (approx. 70%) following winter flooding, in single site studies (Macdonald 2008; Price et al. 2010a). We examine the density of recruits, dispersal distance and direction from adult plants, and mode of reproduction in a semi-arid floodplain. The study area J. N. Price M. J. Macdonald C. L. Gross I. H. Simpson Ecosystem Management, University of New England, Armidale, NSW 2351, Australia