Celtis sinensis Pers. (Ulmaceae) naturalised in northern South Africa and keys to distinguish between Celtis species commonly cultivated in urban environments

Background: Alien Celtis species are commonly cultivated in South Africa. They are easily confused with indigenous C. africana Burm.f. and are often erroneously traded as such. Celtis australis L. is a declared alien invasive tree. Celtis sinensis Pers. is not, but has become conspicuous in urban open spaces.Objectives: This study investigates the extent to which C. sinensis has become naturalised, constructs keys to distinguish between indigenous and cultivated Celtis species, and provides a descriptive treatment of C. sinensis.Methods: Land-cover types colonised by C. sinensis were randomly sampled with 16 belt transects. Woody species were identified, counted and height measured to determine the population structure. C. africana and the three alien Celtis species were cultivated for 2 years and compared morphologically.Results: Celtis sinensis, Ligustrum lucidum and Melia azedarach were found to be alien species, most abundant in urban areas. The population structure of C. sinensis corresponds to that of the declared invasive alien, M. azedarach. Although C. africana occurs naturally, it is not regularly cultivated. This is ascribed to erroneous plantings because of its resemblance to juvenile plants of C. sinensis. Keys are provided to identify Celtis species based on leaf shape and margin, and drupe characters.Conclusion: Celtis sinensis is naturalised in central South Africa, especially in urban open spaces and beginning to colonise natural areas. It is flagged as a species with invasive potential. Characteristics of the leaves and fruits allow for accurate identification of indigenous and alien Celtis species, both as juvenile or adult trees.