Trait convergence and niche differentiation of two exotic invasive free-floating plant species in China under shifted water nutrient stoichiometric regimes

The effects of eutrophication on the growth and phenotypic performance of macrophytes have been widely studied. Experimental evidence suggests that an increase in the water nutrient level would promote the performance of several invasive free-floating macrophytes. However, few studies have focused on how a shift in water nutrient (nitrogen and phosphorus) stoichiometric regimes may influence the performance of invasive free-floating macrophytes. In the present study, two exotic invasive plant species, free-floating Eichhornia crassipes and Pistia stratiotes, were subjected to different water nutrient stoichiometric regimes, and their phenotypic performance was studied. We found that the two species converged in several resource use traits and diverged in lateral root length. This implied that their similarities in fitness-correlated traits and their underwater niche differentiation probably contribute to their stable coexistence in the field. Additionally, the eutrophic conditions in the different N:P regimes scarcely altered the performance of both species compared to their performance in the oligotrophic condition. Based on previous studies, we predicted that moderate eutrophication with slight overloading of nitrogen and phosphorus would not improve the performance of several invasive free-floating plants and thus would scarcely alter the invasive status of these species. However, moderate eutrophication may cause other problems, such as the growth of phytoplankton and algae and increased pollution in the water.