Most of the invasive tree species reported in southern Africa (Republic of South Africa, Zambia and Zimbabwe) have been introduced. Only one indigenous species, Acacia ataxacantha, has been reported as an aggressive invader of stream banks, dry watercourses and forest margins in the eastern part of South Africa. The most invasive exotic tree genera are Pinus and Acacia, which have invaded a combined area of over 8 100 000 ha in South Africa, and an area estimated to exceed 200 000 ha in Zimbabwe. These two genera are however not considered serious invaders in Zambia.
Plantation species are blamed for 38 percent of the invasions in South Africa, while the remainder comprise other species introduced for functions such as hedges, ornamental use, shelter, shade and fixing or stabilizing sand dunes. Thirteen Australian Acacia species (A. 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 all declared invader species in South Africa, while five of these same species (A. decurrens, A. dealbata, A. elata, A. mearnsii, A. melanoxylon and A. podalyriifolia) are believed to be invaders in Zimbabwe. Among the pines, 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) of these have been identified as invaders to varying degrees in Zimbabwe.
The 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 didymobotrya, Caesalpinea decapetala and Toona ciliata, all in Zimbabwe and South Africa. Ziziphus mauritiana, a species believed to have been introduced in the Zambezi valley centuries ago, is an aggressive invader species in both Zimbabwe and Zambia, where the fruit is now widely consumed.
Documented environmental and economic impacts of alien invasive tree species in South Africa include:
a reduction in species richness of some ecosystems by between 45 and 67 percent,
reduction in stream flow of 6.7 percent,
altering of nutrient status of soils, and
increased biomass of other ecosystems.
The cost of controlling alien plant invasions in South Africa, using the current mechanical, fire and chemical methods, is about US$ 1 200 million for an estimated period of 20 years, but it could be reduced to US$ 400 million by using biological control methods. The Government of South Africa committed US$ 100 million between 1995 and 2000 to the Working for Water Programme, which is spearheading the control of invasive species. The current control of invasive tree species in Zimbabwe is estimated to be costing over US$ 100 000 annually.
Although a rough survey indicates that Zambia may still be free of major alien tree invasions, it will be important that this is confirmed by a well-designed and thorough survey base on methods used in South Africa and Zimbabwe
Adverse impacts of exotic tree species, and associated costs to control them when they turn invasive, should be viewed together with the benefits they generate for local people, national development and the environment. The forestry sector makes intensive use of exotic species, and provides a significant contribution to the GDP of the countries. A holistic, ecosystem approach is necessary, as some species may provide useful environmental benefits in some areas and generate problems as invasive species in other areas.
Awareness of the side effects of exotic tree species appears to be quite high in both South Africa and Zimbabwe, particularly among forestry companies, some NGOs, environmentalists and some private citizens, while the average person appears to value them aesthetically in their gardens or for fuelwood and construction purposes in rural areas. Zimbabwe probably needs to implement a multisectoral, comprehensive review to identify the issues related to invasiveness by exotic forest trees, quantify the environmental and socio-economic impacts, and draw up relevant conclusions and regulations.