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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.