Australian acacias as invasive species: Lessons to be learnt from regions with long planting histories §

Abstract

Problems associated with invasiveness of non-native tree species used in forestry are increasing rapidly worldwide and are most severe in areas with a long history of plantings. Lessons learnt in areas with long histories of plantings and invasions may be applicable to areas with shorter planting histories. Most research towards understanding such tree invasions has focused on Pinus species, though all groups of trees that have been widely used in forestry are invasive to some extent. This paper explores the experience of Australian Acacia species (wattles). Unlike some other groups of trees, no particular set of traits clearly separates highly invasive from less- or non-invasive wattles. All species that have been widely planted over a long period have become invasive; the extent of invasions is largely a function of human usage. These findings imply that propagule pressure in concert with residence times are the main drivers of invasiveness in wattles (many factors  mediate these drivers, including fire, forest clearance and soil disturbance). The massive extent of recent plantings of Australian Acacia species in South-east Asia is therefore likely to result in large-scale invasions unless proactive management is implemented. The history of wattles in South Africa highlights the need for such  proactive management. Wattles were of considerable net value to the South African economy immediately after introduction. However, the costs of wattle invasions increased over time to such an extent that (certainly over the last few decades) these costs exceed the benefits derived from the forestry industry. Wattles now dominate many natural ecosystems. We recommend several interventions to prevent a similar pattern in South-east Asia and to ensure the sustainability of plantation forestry based on wattles in the region. A spatially explicit assessment of invasion risk is required, and a monitoring system should be implemented.  Cost–benefit analyses (that consider the full suite of perspectives relating to costs and benefits) need to be applied to determine the need for sustainable mitigation methods. Options for reducing potential invasiveness should be implemented; these include biological control targeting seed production (very good success has been achieved in South Africa) and the use of sterile cultivars.

Keywords: biological invasions, forestry, sustainable forestry, tree invasions, wattles