The Growth Performance of Exotic and Indigenous Tree Species in Rehabilitating Active Gold Mine Tailings Dump at Shamva mine in Zimbabwe

The objective of the study was to compare the performance of exotic (Eucalyptus grandis and Acacia saligna) and indigenous (Acacia polyacantha and Bauhinia thonningii) tree species in rehabilitating active gold mine tailings dump. The seedlings were produced in a nursery and transplanted onto the tailings dumps during the rainy season in the years 2000, 2001 and 2002. The saplings were monitored for survival rates one year after transplanting and growth performances by measuring root collar diameters and heights. There was no significant difference among the species in survival. The mean height increment of the exotics significantly surpassed indigenous tree species. E. grandis performed best followed by A. saligna. Bauhinia thoningii showed the lowest height increment. Root collar diameters were highest for A. polyacantha, followed by the exotics. There was no significant difference for the mean root collar diameter and height between the exotics. It was concluded that both exotic and indigenous tree species could be used for the rehabilitation of mine tailings. @JASEM Mining tailings dumps destroy wildlife habitats and leach out toxic minerals into streams and groundwater if not properly rehabilitated. In many instances, these dumps have been abandoned, with limited or no rehabilitation treatment (Gardner, 2001). If the mining industry is to contribute effectively to sustainable development, it must develop and consistently apply sound environmental management practices to minimize on and off site environmental impacts. There are cases in which tailings dumps have been effectively rehabilitated to agriculture, forestry, nature conservation, urban or industrial land uses (Laurence, 2001; Moffat, 2001). Of all the methods, revegetation is the most documented and widely used because it is most practical and economic though it is often slow due to the hostile conditions such as toxicity of heavy metals (Troung, 1999). In the process of vegetative rehabilitation, the normal ecological succession using low-cost techniques should be initiated, so that the environment can enrich itself naturally in terms of biodiversity (Mudorch-Eaton et al., 1997). Those involved in rehabilitation projects have recognized the importance of using indigenous species not only for environmental reasons, but also to meet the livelihood and cultural needs of local communities who may depend on forest products and services (Sarrailh and Ayrault, 2001). Although the idea of using indigenous species in vegetation of slime dams is now widely accepted there is need for continued investigation into the establishment of indigenous species, which perform comparably to exotic species on tailings dams. The overall objective of this study was to compare the performance of exotic tree species namely, Eucalyptus grandis and Acacia saligna and indigenous tree species, Acacia polyacantha, and Bauhinia thonningii in rehabilitating active mine tailings dump. MATERIALS AND METHODS Description of the study area: The project was carried out on the active Shamva Mine slime dump, in Zimbabwe, which was established in 1982. The dump is terraced and rises at 0.121 m/year at a slope of 26 %. Each year four terraces are built and vegetated. The total area revegetated by 2004 was 7.15 ha. Common tree species growing in the area are Terminalia sericea, Acacia. karroo, Acacia. polyacantha, Uapaca kirkiana and Combretum. The grasses are made up mostly of Hyparrhenia filipendula, Hyperthelia dissoluta, Brachiaria brizantha, Pogonathria squarrosa, Craspedorachis rhodesiana and Eragrostis species (Shoko and Tobani, 2002). Materials: Two indigenous (Acacia polyacantha and Bauhinia thonningii) and two exotic (Acacia saligna and Eucalyptus grandis) tree species were tested for their potential use in the rehabilitation of a mine dump. Seedlings of the four tree species were raised in a nursery in planting sleeves, 100 x 150 mm. The seedlings were planted on terraces with a spacing of 2 m x 2 m during 2000, 2001 and 2002. Measurements: Ten percent sampling intensity was used for each species from each planting year. Trees were selected randomly for measurement of height JASEM ISSN 1119-8362 All rights reserved J. Appl. Sci. Environ. Mgt. 2005 Vol. 9 (2) 57 59 Full-text Available Online at www.bioline.org.br/ja The Growth Performance of Exotic and Indigenous... Mulizane, M; Katsvanga, CAT; Nyakudya, IW; Mupangwa, JF 58 using height rods and root collar diameter using callipers. Survival percentages of the four species within the same year of establishment were compared. Statistical Analysis: The data was analysed separately for height, root collar diameter and survival. Analysis of variance (ANOVA) was conducted using the Statistical Package for Social Sciences (SPSS Version 10) with the following model of analysis: Yijk = u + Si + Yj + eijk Where: Yijk is the dependent variable (e.g. height), u is the overall mean, Si is the effect of species, Yj is the effect of year and eijk is the random error. Statistical differences between treatment means were assessed using the Tukey Studentised Range test. RESULTS Pair-wise comparisons for species survival performance showed that there were no significant differences (P > 0.05) among all species (F value 0.400) for the all the plantings at one year of age. The mean survival percentage of E. grandis was 77.33±15.5, for A. saligna, 74.67±25.9, A. polyacantha had 83.00±15. and finally B.thoningii had 66.67±14.9. A. polyacantha depicted the highest variation in survival rates as shown by the relatively high standard deviation. The results for mean height and mean root collar diameter are shown in Table 1. There was no significant difference (P > 0.05) in mean height between exotic species (E. grandis and A saligna) within the same age group, however, the exotic species were significantly (P < 0.05) taller than indigenous species. Between the indigenous species, A. polyacantha had a greater mean height than B. thonningii in all the three age groups. Table 1. The mean height (cm) and mean root collar diameter (cm) of exotic and indigenous tree species.