Inter- and Intraspecific Competition in Invasive Lactuca serriola and Co-Occurring Weedy Plant Species

While invasive success of alien plant species is often attributed to their superior competitive abilities, it is also suggested that competitive ability depends on the target species of competition and resource availability. In addition, it remains unclear whether invaders and co-occurring plants in the introduced area exhibit distinctive inter- and intraspecific competitive intensities. This study aimed to evaluate the competitive ability of a successful invader, Lactuca serriola, through a combination of field surveys and a growth chamber experiment. First, we assessed biodiversity and the biomass of co-occurring plants in both L. serriola-invaded and uninvaded plots across nine sites in South Korea. Subsequently, a pairwise competition experiment was conducted between L. serriola and three weedy plant species commonly found in the invaded plots, Chenopodium album, Erigeron canadensis, and Oenothera biennis, under differential nutrient levels. Diversity indices of plant communities and the biomass of most co-occurring plants showed no significant difference between invaded and uninvaded plots. L. serriola and testing weedy plants exhibited mutually negative effects on biomass when grown together in the same pot, with the intensity of interspecific competition being comparable across nutrient treatments. Notably, intraspecific competition of L. serriola was weaker than testing weedy plants, particularly manifest in the high-nutrient treatment. The results of both field and growth-chamber studies demonstrated that L. serriola was not a particularly strong competitor compared to its neighboring weedy plants. Its successful invasion can be partially attributed to its weak intraspecific competition intensity, which potentially facilitate successful establishment with high density.