Browsing by Author "Mulenga, Charles"

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    Growth Response of Brachystegia longifolia to Copper Mining Pollution-Induced Heavy Metal Toxicity
    (Stellenbosch : Stellenbosch University, 2022-10) Mulenga, Charles; Meincken, Martina; Clarke, Catherine E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: Mining-induced heavy metal dispersion often contaminates the soil in forests surrounding copper mines. Heavy metals are highly toxic to plant health at elevated levels depending on many factors, including species. However, the impact of copper mining-induced metal toxicities on the productivity of miombo forests is not well known. This study investigated the degree of forest soil contamination, bioavailable proportions of heavy metals, uptake and their subsequent effect on the growth and wood quality of Brachystegia longifolia, one of the commercially important miombo woodland species growing in a mining environment. The effect of copper mining activities on B. longifolia was studied as a function of distance and wind direction around an active copper mine in Mufulira, Zambia. Four sampling plots were demarcated along each transect, stretching 19 km upwind and 12 km downwind from the mine and trees were sampled from 50 m radius sample plots. Wood discs and crown leaves free of visible growth abnormalities were collected to form the biomass samples, as well as topsoil. Portable X-ray fluorescence was used to analyze the total concentration of Mn Fe, Cu and Zn in sieved soil and ashed leaves. The degree of soil contamination was analyzed using the geoaccumulation and enrichment factors, while the bioaccumulation potential of the studied elements in B. longifolia was assessed by the bioaccumulation factor. Furthermore, the bioavailable proportions of trace elements were analyzed by ICP-AES. The annual ring widths were measured and the relative tree growth was determined using the mean annual increment (MAI) across the sampling plots. The growth-limiting trace elements and stress conditions were then identified using the growth-response curve with the relative tree growth as a dependent variable. Furthermore, wood cell dimensions, wood density and elemental composition were analyzed from selected annual rings that included dry and wet years based on the mean annual temperature and mean annual precipitation, respectively. The degree of soil contamination illustrated that the forest soils around Mufulira are strongly polluted by heavy metals dispersed from the copper mine. Soil contamination extended up to 7 km upwind and the entire sampled 12 km stretch downwind. The sample plot at 2 km downwind was the most polluted site accounting for 296 mg/kg, 2337 mg/kg, 1101 mg/kg and 109 mg/kg accumulation of Mn, Zn, Fe and Cu in B. longifolia leaves, respectively. In contrast, only 3196 mg/kg, 154 mg/kg, 516 mg/kg and 55 mg/kg of Mn, Zn, Fe and Cu, respectively, were reported on the furthest unpolluted site upwind 19 km from the mine. Significant decreases in the concentration of Fe and Cu were observed with increasing distance from the mine, whereas the Zn and Mn content increased significantly. Furthermore, it was also observed that B. longifolia leaves accumulated Zn (x15) and Mn (x38) more than the soil throughout all sample plots. The radial growth of B. longifolia on all contaminated sites was significantly reduced compared to the control site. A multivariate analysis attributed the reduction in tree growth to Fe and Cu toxicities and it confirmed that B. longifolia trees accumulate and tolerate Zn and Mn. The decrease in radial expansion results from a significant reduction in wood cell dimensions and proportions on polluted sites. It was further observed that Cu and Fe toxicity increases the basic wood density of B. longifolia and compromises the capacity of wood tracheary elements to transport water and nutrients. This study demonstrates that copper mining-induced heavy metal forest soil pollution and the subsequent metal stress affects the growth and productivity of B. longifolia. It shows that Cu and Fe are highly toxic to this tree species, although it can accumulate and tolerate elevated levels of Zn and Mn. Fe and Cu toxicities alter the anatomical characteristics of B. longifolia wood, affecting the sustainable utilization of this tree species and possibly other miombo species naturally growing in heavy metal polluted environments.