Shifts in growth and competitive dominance of the invasive plant Alternanthera philoxeroides under different nitrogen and phosphorus supply

The availability of nitrogen and phosphorus is an important factor determining the growth and competitive dominance of invasive plants. To clarify the influence of nutrient availability on the performance of alien species, we conducted a greenhouse experiment in which the invasive plant Alternanthera philoxeroides was planted as a monoculture or in mixed culture with noninvasive species (Oenanthe javanica or Iris pseudacorus) under combined conditions of various nutrient level (low, medium, and high) and N/P ratio (1, 10, and 100). Nutrient level and N/P ratio both affected the growth of A. philoxeroides and the effects were similar in mixed culture and monoculture. Increased nutrient level significantly increased the total biomass and total stolon length of A. philoxeroides under all N/P ratios. At low nutrient level, N/P ratio had little influence on the total biomass and total stolon length of A. philoxeroides. At medium and high nutrient levels, however, increased N/P ratios inhibited the growth of A. philoxeroides. Nutrient level rather than N/P ratio affected the relative dominance index (RDI) of A. philoxeroides, with increased nutrient levels enhancing the competitive dominance of A. philoxeroides. Furthermore, the RDI of A. philoxeroides in mixed culture with I. pseudacorus was consistently higher than that of A. philoxeroides when grown with O. javanica. Therefore, our results indicate that nutrient enrichment (such as over-fertilization and eutrophication) would aggravate the already serious impact of biological invasions and that the identity of resident plant species might also affect the competitive dominance of exotic plants.