Mining practices often expose parent materials and waste rock laden with heavy metals, which then need to be revegetated with plant species that resist metal toxicity.  Zn is one such heavy metal that is often liberated from parent material associated with mining efforts for gold, silver, and copper, and is a metal of concern in mining lands in the western United States.  Understanding toxicity thresholds for semi-arid shrub species will help restoration ecologists select species most conducive to revegetating and reclaiming metal-contaminated mining lands.  Our study focuses on determining the Zn toxicity thresholds for three species of semi-arid shrubs, big sagebrush (Artemisia tridentata Nutt.), skunkbush sumac (Rhus trilobata Nutt.), and fourwing saltbush (Atriplex canescens [Pursh] Nutt.), commonly used in reclamation of degraded mine sites.  We grew these shrubs in a sand medium in the greenhouse and exposed them to six different concentrations of Zn solution (0, 100, 200, 300, 400, 500, 600 mg Zn L^-1).  We harvested plants after 22 weeks of growth and recorded mortality as well as root and shoot biomass.  In addition, we analyzed both above and below ground plant tissue for Zn content.  Using these measures, we determined lethal concentrations (LC50 = substrate Zn concentration resulting in 50% mortality), effective concentrations (EC50 = substrate Zn concentration resulting in 50% biomass reduction) and phytotoxicity (PT50 = tissue Zn concentration resulting in 50% biomass reduction) of Zn for each shrub species tested. 

Results/Conclusions

After 8 weeks of treatment with Zn solutions, big sagebrush’s survival rates never fell below 67 percent for any treatment level, while survival rates of skunkbush sumac and fourwing saltbush fell to 14 percent and 0 percent, respectively, when treated with 600 mg Zn L^-1.  Our LC50 calculations indicate that half of big sagebrush plants die at a concentration of 1,018 mg Zn L^-1, half of skunkbush sumac plants die at 228 mg Zn L^-1, and half of fourwing saltbush plants die at 134 mg Zn L^-1. Based on LC50 values as well as other metrics of Zn tolerance measured here, we recommend using big sagebrush for the reclamation of soils containing high levels of Zn.  The range of big sagebrush, however, will limit its use as a species for revegetation of metal-impacted, high alpine sites.