The invasive Melia azedarach in Durban (South Africa): impacts on tree community structure

Melia azedarach L. (Meliaceae) is an invasive, rapidly growing tree, considered one of the most important transforming invasive alien plants in South Africa. Here we survey several localities across the city of Durban (eThekwini Municipality in the province of KwaZulu-Natal), with and without adult Melia trees, including indigenous trees, as well as other alien species. Notwithstanding significant differences between localities, we find that localities with adult Melia trees have substantially reduced tree species richness and marginally smaller tree diameter at breast height (values combining indigenous trees, Melia, and other alien trees). The composition of adult tree assemblages differs considerably from the composition of tree seedling assemblages at most localities, with Melia seedlings present at all localities, irrespective of the presence of adult trees. The differences between adult and seedling communities are more pronounced where adult Melia trees are absent, indicating rapid transformation due to incipient invasion. This also suggests that Melia dominance may cause long-lasting vegetation uniformity and that the transformation process caused by the alien trees at these localities is already somewhat stabilized. We discuss the implications of Melia presence and local dominance on the local ecosystems and suggest controlling this species at target localities of conservation relevance.