Invasive Eupatorium catarium and Ageratum conyzoides benefit more than does a common native plant from nutrient addition in both competitive and non-competitive environments

We explored the roles of phenotypic plasticity and competition in contributing to the success of Eupatorium catarium, a recently invasive noxious annual forb in South China. We compared the growth and functional traits of E. catarium with those of a common native plant, Vernonia cinerea, and a long-term historic invader, Ageratum conyzoides, along a nutrient gradient under both competitive and non-competitive conditions. Under non-competitive conditions, biomass differed little between species under low-nutrient conditions. However, nutrient addition resulted in a greater increase in biomass of the two invasive species than that of V. cinerea. The greater increase in biomass in the two invasive plants may be partly explained by their greater plasticity in specific leaf area. The competition experiments involved different combinations of species: plants of E. catarium were grown with either a plant of A. conyzoides or with a plant of V. cinerea, or individual plants of all three species were grown in competition with a grassland sward grown from 1.5 g seeds of Digitaria radicosa. Relative to non-competitive conditions, competition further increased the biomass difference between the two invasive plants and V. cinerea under high nutrient conditions. A. conyzoides outcompeted E. catarium in terms of biomass production under high nutrient conditions. Overall, these results indicate that A. conyzoides and E. catarium tend to outperform V. cinerea under high nutrient conditions and in both competitive and non-competitive environments. However, E. catarium cannot outperform A. conyzoides in terms of biomass production under each of the nutrient and competitive conditions.