Opportunities and constraints in the restoration of riparian ecosystems invaded by alien trees: insights from the Western Cape, South Africa

Invasive alien species are widely considered to be the second most significant threat to
biodiversity globally following direct habitat destruction. The invasion of riparian systems
worldwide by alien plants has contributed to profound changes in biodiversity and ecosystem
functioning. In South Africa, river banks and river beds are amongst the most severely
invaded landscapes, with the most damaging invaders, especially in the Fynbos Biome,
being trees and shrubs of the Australian genera Acacia and Eucalyptus. Although large-scale
management operations are underway to clear invasive trees and restore ecosystems, little
is known regarding opportunities and constraints of native species recovery after alien
clearing. The core aim of this thesis is to consider whether key aspects of two widely cited
restoration models (successional and alternative-state models) are useful for guiding
effective management of severely-invaded riparian vegetation. As a study system, I used the
Berg River in the Western Cape, South Africa which is severely impacted by invasive trees,
especially Eucalyptus camaldulensis. By linking the studies of constraints for restoration and
opportunities for native species recovery, the aim was to provide new possibilities for
restoration in riparian zones.