A weed somewhere else

Acacia mearnsii (Black Wattle) is a large shrub or tree to 12 m tall with dark-green feathery leaves and clusters of pale yellow flowers.  It is widespread and abundant throughout southern Victoria and is absent only from alpine areas and open grasslands.  The species is native to Victoria and although its numbers here and in NSW may have diminished since European settlement it has become a serious weed in other countries.

Acacia mearnsii was a useful species for Koories before European settlement.  They extracted carbohydrate-rich gum from the bark which was chewed, mixed with water for drinks or used as a glue for spearheads.  The wood was made into weapons and the bark was sometimes soaked in water to produce a medication for indigestion.  The early European settlers quickly realised that the bark was a rich source of tannin and the bark of a large number of trees was harvested for the leather industry.  This industry grew rapidly and both leather products and the bark itself became important exports for the new colony.  Unfortunately a lack of regulation in the industry created the inevitable problem of over-harvesting and by the late 1890s bark exports declined sharply as did the availability of mature trees for the local industry.  In the mean time plantations established in South Africa from the 1860s had begun to mature and that country gradually increased its tannin production.  By the 1990s South Africa was the main source of tannin from Acacia mearnsii bark but it was also the county in which this species had become a significant environmental and agricultural weed.
       

The IUCN Global Invasive Species Database lists Acacia mearnsii as one of the top 30 most invasive land plants in the world.  It has been introduced to Africa, Asia, Europe, North and South America and a number of Pacific islands.  The high concentrations of tannin in the bark was undoubtedly the primary reason for its importation to these countries but it was also recognised as a rapidly growing species whose wood produced quality charcoal for fuel or could be used in the manufacture of paper.  Extracts from the bark could also be used for thinners and adhesives or even medication.  The trees were widely used for re-establishment on cleared and eroding soils and, as the species is a legume, its roots were capable of nitrogen enrichment of the soil.

Unfortunately these many benefits are sometimes outweighed by the ability of the species to spread from areas of cultivation to other less desirable localities.  The large numbers of seeds it produces may be moved long distances by ants and ground mammals, they may lay dormant for many years and then germinate into huge populations after fires and following rains.  The leaves of Acacia mearnsii release toxins and germination inhibitors into the soil around them to restrict potential competitors, as a consequence the species may invade disturbed vegetation and restrict the available ground cover for grazing animal and reduce an area's biodiversity.  Large, dense stands can increase water usage and hence reduce the available ground water for other species of plants.  

Despite these drawbacks there is still an enormous amount of interest in the cultivation of  Acacia mearnsii and there has recently been joint research programs between Australian and Chinese biologists to examine the efficacy of the expansion of plantations in China.

© Paul Gullan, Viridans Biological Databases