Above- and below-ground mechanisms enhance competitiveness of invasive Phytolacca americana in heavy metal-rich soils

Increasing evidence suggests that some invasive species can sustain high growth rates in heavy metal-enriched environments, while native species often lack such adaptive traits, which may facilitate the invasion success of alien species. However, the underlying mechanisms are poorly understood. This study examined the invasive plant Phytolacca americana and native Phytolacca acinosa in cadmium (Cd)-contaminated soils in China. The results showed that, while native plant biomass decreased at higher soil Cd concentrations, invasive plant biomass remained unaffected. Despite decreasing overall with increasing Cd concentration, soil enzyme activities remained higher in soils where invasive plants grew compared to native plants. Phytolacca americana accumulated more Cd, particularly in the leaves and under higher soil Cd concentrations. Native seed germination was inhibited with increasing litter-Cd concentration, whereas invasive seed germination was unaffected. These findings suggest that the ability of the invasive species to mitigate the negative effects of Cd pollution on soil enzyme activities contributed to their higher Cd-tolerance by sustaining soil nutrient availability. Additionally, higher leaf-Cd levels in invasive plants may constitute an elemental defense. Overall, heavy metal pollution not only favored invasive plants by suppressing native plant growth but may have also promoted invasion through the effects of Cd accumulation in invasive litter on native seed germination.