Vegetation establishment and functionality on kimberlite tailings in the afro-alpine zone of Lesotho
Mining activities can have severe negative impacts on the natural environment. It is therefore important for mines to develop and implement mitigation measures through a rehabilitation strategy. As such, Letšeng Diamond Mine in Lesotho has implemented a rehabilitation strategy that will attempt to meet their legal and contractual obligations for post-mining land-use, as well as adhering to Good International Industry Practice and International Finance Corporation guidelines. Frequent temperature drops below freezing, snow, a high wind-chill factor and high altitudes make rehabilitation in the afro-alpine zone of Lesotho difficult. These unique conditions present in the mountains of Lesotho have necessitated that Letšeng implement rehabilitation trials of various scales to improve the accuracy of the closure liability, and to explore alternative sustainable and cost-effective rehabilitation methods. The aims of these trials were to determine the most cost-effective growth medium that can sustain indigenous vegetation and minimise the risk of erosion, determine the extent and type of amelioration required to support vegetation establishment and growth, and assess the ability of indigenous vegetation to not only establish on these growth-mediums but also to persist and propagate. The aim of this study was to assess and compare vegetation establishment and functionality on different types of tailings and topsoil mixes (treatments), and compare these with reference sites within the mine lease area, to determine which treatments would be the most suitable for optimal rehabilitation. To achieve this aim, the plant species richness, abundance and diversity of the various treatments were compared between treatments and the reference sites, as well as within treatments over time. The functionality of the various treatments was assessed using the Landscape Function Analysis (LFA) methodology. The functionality was also compared between treatments and the reference sites and within treatments over time. All treatments at all sites received the same type and extent of amelioration and seed mixture. It was found that treatments containing topsoil had significantly higher plant diversity and functionality than treatments that did not contain any topsoil. LFA data revealed that there was a positive correlation between patch percentage and total Soil Surface Assessment (SSA) functionality, while there was a weak negative correlation between interpatch length and total SSA functionality. This suggests that vegetation cover contributes directly to the functionality of the treatments. Floristic data revealed that treatments containing topsoil had higher species diversity than treatments without topsoil in almost all cases. NMDS analyses and PCA ordinations of the floristic data revealed that treatments containing topsoil were more similar to the reference sites with regards to species composition than treatments that did not contain topsoil. PCA ordinations also revealed that the species composition of treatments with higher amounts of topsoil were even more similar to the reference sites than treatments that did not contain topsoil. Treatments containing a mixture of coarse tailings and waste rock, regardless of the presence of topsoil, were found to have lower species diversity and functionality than treatments containing one of the two materials mixed with topsoil. The cause of the poor vegetation establishment is unknown; however, it could be due to chemical or physical factors related to mixing waste rock and coarse tailings, or external factors such as rainfall. Based on the results of this study, it is recommended that the vegetation be monitored annually, while the functionality is monitored biennially. The decrease in functionality and diversity on treatments containing a mixture of coarse tailings and waste rock should be investigated. The effect that the method of ripping may have on the diversity and functionality of different growth mediums may also be a worthwhile avenue of further study. The use of traditional rehabilitation species such as Digitaria eriantha, Chloris gayana, Eragrostis tef and Cynodon dactylon in the seed mixture should be reconsidered. The establishment of these species on the rehabilitation trials was minimal and they did not seem to provide much biomass or basal cover compared to Triticum and the two native species, Sisymbrium turczaninowii and Merxmuellera disticha. This study provides scientific insight into the rehabilitation of kimberlite tailings in high-altitude alpine zones. The data on the vegetation establishment, composition and functionality of various potential rehabilitation treatments of kimberlite tailings provides invaluable guidance for cost-effective future management decisions.