Combination effects of heavy metal and inter-specific competition on the invasiveness of Alternanthera philoxeroides

Inter-specific competition is an essential factor affecting the invasiveness of alien plants. Although many exotic invasive plants are characterised by a high tolerance to many pollutants, e.g. heavy metals, little is currently known regarding the impact of heavy metals on the interaction between native and invasive plants. In this study, we used the invasive plant Alternanthera philoxeroides (Mart.) Griseb. and its native congener Alternanthera sessilis (L.) R.Br. ex DC to assess their competitive interaction in response to treatments with copper, cadmium, and zinc applied at different concentrations (10, 30, 60, and 100 mg kg⁻¹ for Cd; 200, 400, and 1000 mg kg⁻¹ for Cu; and 500, 1000, and 2000 mg kg⁻¹ for Zn), along with a control treatment. We accordingly found that both invasive and native plants show high tolerance to heavy metals; however, the tolerance index of A. philoxeroides increased significantly from monoculture to mixed culture in response to Cu and Cd treatments. In addition, A. philoxeroides showed greater phenotypic plasticity to culture type than its native congener. Furthermore, the specific leaf area of A. philoxeroides in the mixed culture was found to be higher than that in monoculture. Additionally, in response to Cu and Cd treatments, the biomass of A. philoxeroides in mixed culture was notably shifted from the roots to shoots, in contrast to the control group, the variances in A. philoxeroides responses in monoculture were less perceptible. We speculate that such high phenotypic plasticity may represent an important tolerance strategy for invasive plants that can enhance their competitive ability against native plants under conditions of heavy metal stress, and consequently a combination of heavy metal stress and inter-specific competition would be conducive to the invasiveness of A. philoxeroides, particularly in habitats characterised by high levels of heavy metal pollution.