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SEEDS, SEEDS, SEEDS! THE PERSISTENT AND TROUBLESOME LEGACY OF 150 YEARS OF AUSTRALIAN ACACIAS IN SOUTH AFRICA

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·Bs 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, 161177.

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.