A range of tree species have been introduced into southern Africa (considered here to be South Africa, Zambia and Zimbabwe) for a range of purposes, such as timber, poles, pulp, paper, bark extraction for leather tanning, soil improvement, livestock fodder, windbreaks, shelter, shade, ornamental uses, stabilizing sand dunes and for fruit. There are also cases of accidental introduction, i.e. the trees were introduced for no defined purpose, while other cases of introduction were a result of taxonomic confusion. While most of trees have achieved the original objective for which they were introduced, some of them have found the new environment favourable and in the absence of threat from their natural enemies (pests) left behind in their native environments, the introduced species have flourished and invaded new areas, threatening the survival of native species and sometimes whole ecosystems.
Invasive tree species in southern Africa are capable of invading every habitat. They replace native flora with a monotonous growth that depresses scientific interest, diversity and the beauty of landscapes. The threat to biological diversity as a result of invasive alien species is considered second only to that of habitat loss (CBD News, 2001). Invasive alien species can cause significant and often irreversible environmental and socio-economic impacts, from gene to ecosystem levels. The cost of managing invasive alien tree species includes costs of prevention, control and mitigation (where already introduced), and indirect costs due to their impact on ecological services, such as water yields in catchment areas.
