Biological traits of tropical trees suitable for restoration of copper-polluted lands

Extensive areas around the world have soils polluted with heavy metals due to anthropogenic activities and are in need of restoration. Natural regeneration or active restoration of vegetation on mine wastelands, such as tailings dams, is difficult due to the harsh environment resulting from poor nutrient availability, low pH and elevated levels of heavy metals. Thus, selection of native trees that thrive in in such a harsh environment and can stabilize or remove heavy metals from the soil is a key step in phytoremediation of these areas. In this study, we examined the floristic composition and biological traits of species that colonized copper mine tailings dams in Zambia, an environment with elevated copper concentrations, low nutrient content and low pH, with the aim of identifying species suitable for phytoremediation. We found relatively little overlap (16%-26%) between species pools occurring on tailings dams and nearby native forests although the overall species richness was similar (10-12 species per 15 stems). The species colonizing the tailings dams were disproportionately light-demanding (93%), moderately tolerant to elevated copper concentration (87%), suitable for erosion control (75%) and had endomycorrhizal symbiosis (47%). Nitrogen-fixing species tended to be more abundant on tailings dams (29%) than in natural forest stands (23%). None of the species were copper accumulators, but rather facultative metalophytes, or possibly even excluders as evidenced by their bioaccumulation factors below one. Restoration of degraded lands would be aided by field trials targeting species with these beneficial traits to identify other suitable plants to accelerate the revegetation process.