Invasive tree species are now considered to be a threat to the biological diversity of Southern Africa. Most of the invasive tree species in Southern Africa were either deliberate or accidental introductions. Most of the invasive tree species are exotic, with only one or two being indigenous. These invasive exotic tree species are causing significant environmental and economic damage to the ecosystems of the region. The documented environmental damage includes: (i) reduction of species richness; (ii) reduction of stream flow; (iii) alteration of the nutrient status of soil, (iv) alteration of the biomass of ecosystems, and (v) changes in fire intensity due to altered fuel load. In South Africa, an estimated 100 739 km2 (8.07 percent of national area) have been affected by invasive alien tree species. Figures are not readily available for Zimbabwe, but a conservative estimate is 350 000 ha of total area invaded. South Africa is now the worst affected by invasive tree species followed by Zimbabwe. Zambia is considered safe although lack of awareness could be contributing to this notion. This scenario may be attributable to the fact that South Africa has more than half a dozen climatic zones and is the most developed among the three countries. It also has had the highest number of species introduced over the longest period (over 300 years).
Thirteen Australian acacia species (Acacia baileyana, A. cyclops, A. dealbata, A. decurrens, A. elata, A. implexa, A. longifolia, A. mearnsii, A. melanoxylon, A. paradoxa, A. podalyriifolia, A. pycnantha and A. saligna) were declared invader species or weeds in South Africa, while five of them (A. decurrens, A. dealbata, A. elata, A. mearnsii, A. melanoxylon and A. podalyriifolia) are believed to be invaders in Zimbabwe. Among the pine species, nine (Pinus canariensis, P. elliottii, P. halapensis, P. patula, P. pinaster, P. pinea, P. radiata, P. roxburghii and P. taeda) have been declared invaders in South Africa and six (P. elliottii, P. kesiya, P. patula, P. radiata, P. roxburghii and P. taeda) have been identified as invaders to varying degrees in Zimbabwe. Other notable aggressive invader tree species occurring in at least two countries are Populus x canescens, Psidium spp., Melia azedarach, Jacaranda mimosifolia, Albizia procera, Grevillea robusta, Bauhinia spp., Senna spp., Caesalpinea decapetala and Toona ciliata, all in South Africa and Zimbabwe, and Ziziphus mauritiana in both Zimbabwe and Zambia.
Active control programmes against invasive tree species are now in place in Zimbabwe and South Africa, where the authorities annually spend US$ 100 000 and US$ 20 million respectively. Methods of control include mechanical (cutting, slashing, hand pulling), chemical (herbicides), controlled burning (fire) and biological control using seed-feeding weevils and fungi. In South Africa, biological control of acacias (specifically A. mearnsii) using seed-feeding weevils such as Melanterius maculatus is underway and is envisaged as the long-term strategy, together with policy and legislation controlling the introduction and growing of targeted species. Concerning the three countries, South Africa needs to continue with the control programme. Zimbabwe needs to quantify the extent of invasions through systematic studies, determine their environmental and economic effects, and then, if necessary, formulate policies and legislation to combat the spread of invasive tree species. For Zambia, an awareness programme is necessary, followed by a thorough survey to verify the presence or absence of invasive tree species. In all cases, an ecosystem approach is necessary, associating all concerned parties, and documenting the positive and adverse impacts of exotic tree introductions on the life of local people, national economies and the environment.
