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

Increasing evidence suggests that the ability of invasive plants to maintain high growth in environments polluted with heavy metals when natives cannot contribute to their invasion success but the mechanisms underlying this pattern are poorly understood. For instance, heavy metal accumulation in invasive litter may facilitate invasion if native germination is metal-sensitive, though this remains unverified. We investigated heavy metal bioaccumulation and performance of the invasive alien plant Phytolacca americana and native plant Phytolacca acinosa along an experimental gradient of Cd-contaminated soils in China. We examined the effects of Cd contamination on soil enzyme activity to investigate their abilities to maintain soil nutrient availability. We tested the effects of leaf litter produced in this experiment on seed germination of these two species. We found that native biomass decreased with higher rates of Cd addition while invasive biomass was not impacted. Soil enzyme activities decreased with increasing Cd addition but were higher with the invasive vs. native plant. The invasive plant accumulated more Cd in its tissues than the native, especially in leaves and at higher Cd soil addition rates. Native seed germination was inhibited by higher litter Cd concentrations, but invasive germination was unaffected. Our results show that the invasive plant’s ability to alleviate the negative effects of Cd pollution on soil enzymes compared to the native plant may contribute to their higher Cd tolerance by maintaining soil nutrient availability. The higher levels of Cd in leaves of the invasive plant may contribute to greater levels of elemental defense. In addition, our results showed not only that heavy metal pollution favors invasive plants by suppressing the growth of native plants but may also increase invasions via effects of Cd accumulation in invasive litter that suppresses native plant germination.