Establishment and spread of founding populations of an invasive thistle: the role of competition and seed limitation

Successful plant invasions require both the founding and local spread of new populations. High plant densities occur only when founding plants are able to disperse their seeds well locally to quickly colonize and fill the new patch. We test this ability in a 7-year field experiment with Carduus acanthoides, an invasive weed in several North American ecosystems. Founder plants were planted in the center of 64 m(2) plots and we monitored the recruitment, distribution pattern, mortality, and seed production of the seedlings that originated from these founding plants. Competing vegetation was clipped not at all, once, or twice each year to evaluate the importance of interspecific competition. More seedlings recruited in the intermediate once-clipped plots, and these seedlings also survived better. The control plots had fewer microsites for seedling recruitment; clipping a second time in September stimulated grasses to fill up the gaps. The number of C. acanthoides recruits and their median distances from the founder plants were also explained by the initial seed production of the founding plants. Overall, the experiment shows that the success of founder plants can fluctuate strongly, as 55% of the plots were empty by the sixth year. Our study suggests that the local invasion speed following initial establishment depends strongly on both the propagule pressure and availability of suitable microsites for seedling recruitment and growth.