Impacts of invasive alien trees on threatened lowland vegetation types in the Cape Floristic Region, South Africa

• Indigenous cover recovered poorly in areas cleared of dense invasive alien tree stands.
• Recovery was affected to a greater extent by acacias than by pines.
• Stands cleared of invasive alien trees were dominated by alien herbaceous species.
• The characteristic proteoid overstorey should be introduced after clearing.
• Impacts of invasive alien trees increase with duration of invasion.

This study investigated the autogenic recovery potential of native vegetation after clearing of dense stands of invasive alien trees in two critically endangered vegetation types in South Africa's Cape Floristic Region: Cape Flats Lowland Fynbos and Swartland Alluvium Fynbos. Sampling was done in areas previously occupied by the invasive tree Acacia saligna and plantations of Pinus radiata and in a fynbos reference site. Treatments varied in terms of the length of invasion and management histories.Plots previously under pines recovered well in terms of indigenous perennial species richness, but indigenous species cover decreased with increasing number of planting rotations. Areas cleared of acacia recovered poorly in terms of indigenous species cover (after one cycle of invasion), and indigenous species richness exhibited a declining trend with increasing cycles of invasion. Proteoid overstorey was lost in all previously invaded/planted plots and this element will need to be re-introduced to areas after one cycle of invasion regardless of the invasive species. Acacias changed some abiotic variables after two cycles of invasion.Follow-up clearing generally promoted better vegetation recovery in terms of overall species richness and structure but care should be taken not to damage indigenous ericoid shrubs. Overall, acacia invasion caused a greater change in biodiversity and vegetation structure than pine plantations.