Many Australian acacia species have been introduced to South Africa, mostly during the 1850s. These species were introduced
for many purposes including stabilizing sand dunes, and to produce bark for tanning. Black wattle (Acacia mearnsii) is an important
forestry crop, blackwood (A. melanoxylon) produces high-quality timber for furniture, and rooikrans (A. cyclops) produces
good wood for barbeques and pizza ovens. Despite their usefulness in some parts of the country, these and at least nine other Australian acacia
species are listed as either major or emerging invaders in South Africa. Black wattle is one of the most widespread invasive plants in South
Africa, and thousands of kilometres of riparian vegetation have been invaded by this species. Dense stands cause destablization of river banks
and a large increase in water use. Huge tracts of vegetation in the fynbos biome have been invaded by A. cyclops and A. saligna
(Port Jackson willow). These species fix atmospheric nitrogen and dense stands of these trees lead to marked nutrient enrichment of fynbos
soils which causes severe ecosystem-level impacts that can be difficult to reverse.
A very large part of the budget allocated to the management of invasive alien plants in South Africa is spent on clearing
Australia acacias. Impressive results have been achieved in some areas, but huge invaded areas remain. Biological control agents have been
released to assist in the management of many of the invasive species, some of which have reduced seed production substantially. However, the
massive store of persistent, long-lived seeds in the soil hangs over control efforts like the sword of Damocles. Overall success of the
multi-million dollar management initiatives against these species relies on the reduction of seed banks.
Part of the C·I·B’s new focus on the ecology of invasive Australian acacias involved a detailed review of
the role of seeds in the reproduction and persistence of species in this genus. A paper published in the journal Perspectives in Plant
Ecology, Evolution and Systematics proposed a model that recognises five stages at which intervention to reduce seed numbers is possible.
Elucidation of the stages and options will guide the development of more efficient strategies to manage the seed banks of these species. A
detailed review was undertaken of natural mortality and dispersal factors that affect the incorporation of seeds into the seed bank and factors
that potentially influence the size and level of persistence of seed banks.
The study suggests that only three of the potential stages of intervention are practical for implementing meaningful control
measures in the habitats invaded by these species. Seed reduction through biological control is seen as the only option for achieving levels of
reduction that could significantly reduce the level of the problem. The results of this review strongly support the need for further research
to identify and release additional bio-control agents – to supplement those already released and on species for which none are currently
available. Limited options exist for reactive integrated control in the leaf litter and upper seed bank using burning, and management of the
lower seed bank through containment, rehabilitation and restoration. In all cases, environmental features, vegetation type, and the proposed
landuse following control are critical considerations in the selection of appropriate management options. Further research is required to improve
our knowledge of key aspects of the reproduction and persistence of invasive acacias in South Africa.
Richardson, D.M. & Kluge, R.L. 2008. Seed banks of invasive Australian Acacia species in South Africa: role in invasiveness
and options for management. Perspectives in Plant Ecology, Evolution and Systematics 10, 161–177.
Prolific regeneration of seedlings from huge seed stores in the soil is a key reason for the persistence of invasive
populations of Australian acacias. The picture shows a dense stand of rooikrans (Acacia cyclops) at De Hoop Nature Reserve in
the Western Cape (Photo: D.M. Richardson)
Acacia pycnantha (golden wattle) is one of the species of Australian acacias that is classified as a major
invader in South Africa. Biological control agents have been released against this species and are probably
stabilizing seed banks of these species, but data to support this contention are lacking.
The introduced rust fungus Uromycladium uromyces has greatly reduced seed production of Port Jackson
willow (Acacia saligna). Masses of seeds as shown in this photograph taken the 1980s are no longer seen
in South Africa. Although this biological control agent has stabilized the size of seed banks, huge stores of seeds
produced before the bio-control agent was effective and will continue to hamper control efforts for several decades.