The genus Quercus (Fagaceae) in South Africa: Introduction history, current status, and invasion ecology

Hundreds of species of alien trees have been introduced and widely cultivated in South Africa, and many species have become invasive. Despite their long residence time and widespread cultivation, little is known about the introduction history, current status, and invasion ecology of oaks (genus Quercus) in South Africa. This study reviews various aspects of the genus Quercus in South Africa. We determine the number of oak species present, assess their introduction status, map their distribution, highlight areas of putative naturalisation, and evaluate the usefulness of the community science platform iNaturalist for assessing these factors. We also conducted a field study to elucidate environmental factors that can mediate invasibility in a riparian zone. A literature search was conducted to review events associated with the introduction, cultivation and naturalisation of oaks and associated pests and diseases in South Africa. Records of Quercus species in South Africa were collated from herbaria, arboreta, the Southern African Plant Invader Atlas, the Plants of Southern Africa database and iNaturalist. A field study was conducted along the Eerste River in Stellenbosch to elucidate the role of environmental factors in mediating naturalisation. We found records of 47 Quercus taxa in South Africa which were refined to a list of 22 to 34 species likely present. The earliest record was for Quercus robur in 1656 but first records for most species date from between the late 1800s to the early 1900s. More than 99 % of presence and naturalisation records were of the ten commonly cultivated species. Quercus palustris, Q. robur, and Q. suber were categorised as invasive, and putative naturalisation was documented for Q. acutissima, Q. canariensis, Q. cerris, Q. ilex, and Q. nigra. Most naturalisation occurred at the urban-wildland interface and in riparian vegetation in Cape Town and Stellenbosch. Steepness of the riverbank was found to affect seedling recruitment in riparian areas. iNaturalist records shed new light on the presence, distribution, and introduction status of Quercus species in South Africa. However, taxonomic complexity and data quality complicated attempts to develop a robust inventory of Quercus in South Africa. Molecular studies are needed to achieve better resolution. Nonetheless, this study has greatly improved our understanding of the distribution and status of oaks in South Africa, with implications for their management.