Acacia mearnsii
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Overview:
Acacia mearnsii, commonly known as black wattle, late black wattle or green wattle, is a species of flowering plant in the family Fabaceae and is endemic to south-eastern Australia
It is usually an erect tree with smooth bark, bipinnate leaves and spherical heads of fragrant pale yellow or cream-coloured flowers followed by black to reddish brown pods
In some other parts of the world, it is regarded as an invasive species
Common name: ...
Conservation status: ...
Etymology:
Borrowed from Latin acacia, from Ancient Greek ἀκακία (akakía) "a thorny Egyptian tree", from ἀκή (akḗ) "point, thorn
Flowers:
The scented flowers are arranged in spherical heads of 20 to 40, pale yellow or cream-coloured, with the heads on hairy peduncles 2–8 mm long
Flowering mainly occurs from October to December
Fruit:
Black to reddish-brown pods, 30–150 mm long and 4.5–8 mm wide develop from October to February
Leaves:
The leaves are bipinnate with 7 to 31 pairs of pinnae, each with 25 to 78 pairs of pinnules
There is a spherical gland up to 8 mm below the lowest pair of pinnae
Stem & branches:
Smooth bark, sometimes corrugated at the base of old specimens
Roots:
Habit:
A spreading shrub or erect tree that typically grows to a height of 10 m
Habitat:
Distribution:
A. mearnsii is native to south-eastern Australia and Tasmania, but has been introduced to North America, South America, Asia, Europe, Pacific and Indian Ocean islands, Africa, and New Zealand
In these areas it is often used as a commercial source of tannin or a source of firewood for local communities
In some regions, introduced plants of this species are considered a weed
This is because they threaten native habitats by competing with indigenous vegetation, replacing grass communities, reducing native biodiversity and increasing water loss from riparian zones
In Africa, A. mearnsii competes with local vegetation for nitrogen and water resources, which are particularly scarce in certain regions, endangering the livelihoods of millions of people
In its native range A. mearnsii is a tree of tall woodland and forests in subtropical and warm temperate regions
In Africa the species grows in disturbed areas, range/grasslands, riparian zones, urban areas, water courses, and mesic habitats at an altitude of between 600 and 1,700 m
In Africa it grows in a range of climates including warm temperate dry climates and moist tropical climates. A. mearnsii is reported to tolerate an annual precipitation of between 66 and 228 cm, an annual mean temperature of 14.7 to 27.8 °C, and a pH of 5.0–7.2
It does not grow well on very dry and poor soils
Additional notes:
Taxonomy
Belgian naturalist Émile Auguste Joseph De Wildeman described the black wattle in 1925 in his book Plantae Bequaertianae
The species is named after American naturalist Edgar Alexander Mearns, who collected the type from a cultivated specimen in East Africa
Along with other bipinnate wattles, it is classified in the section Botrycephalae within the subgenus Phyllodineae in the genus Acacia
An analysis of genomic and chloroplast DNA along with morphological characters found that the section is polyphyletic, though the close relationships of many species were unable to be resolved
Acacia mearnsii appears to be most closely related to A. dealbata, A. nanodealbata and A. baileyana
Ecology in Australia
A. mearnsii plays an important role in the native ecosystem of Australia. As a pioneer plant it quickly binds the erosion-prone soil following the bushfires that are common in its Australian habitats
Like other leguminous plants, it fixes atmospheric nitrogen in the soil
Other woodland species can rapidly use these increased nitrogen levels provided by the nodules of bacteria present in their expansive root systems
Hence they play a critical part in the natural regeneration of Australian bushland after fires
Consumption and excretion: The seeds are potentially distributed by rodents or birds
For ornamental purposes (local)
By animals: The dispersal of the seeds is believed to be aided by cattle and birds
By people: Local people collecting branches and logs for firewood may spread seeds
Transportation of soil: The seeds may also be spread by the movement of seed-contaminated soil
By water: The hard-coated seeds are spread readily down water courses
Mycorrhizal fungi attach to the roots to produce food for marsupial animals, and these animals in turn disperse the spores in their droppings to perpetuate the symbiotic relationship between the wattle's roots and mycorrhizal fungi
The cracks and crevices in the wattle's bark are home for many insects and invertebrates. The rare Tasmanian hairstreak butterfly lays her eggs in these cracks, which hatch to produce caterpillar larva attended by ants (Iridomyrmex sp.) that feed off the sweet exudates from the larva
A. mearnsii is used similarly as a larval host plant and food source by the imperial hairstreak, Jalmenus evagoras
The tree is home to various grubs, such as wood moths, which provide a food source to black cockatoos, which strip the bark for access to these borers
During winter insects, birds and marsupials are hosted by the black wattle with the aid of their supplies of nectar in their leaf axials
These creatures provide an important predatory role to deal with tree dieback caused by scarab beetles and pasture pests
Black wattles, along with gums, native box and native hop form the framework vegetation on so-called "hill-topping" sites
They are often constitute isolated remnant pockets of native vegetation amongst a lower sea of exotic pasture
These "hill-topping" sites are critical habitat for male butterflies to attract females for mating, which then lay their eggs under the wattle's bark elsewhere but still within close proximity
It's the only acceptable mating site in the area for these butterflies
Black wattle flowers provide very nitrogen-rich pollen with no nectar
They attract pollen-feeding birds, such as wattlebirds, yellow-throated honeyeaters and New Holland honeyeaters
The protein-rich nectar in the leaf axials is very sustaining for nurturing the growth of juvenile nestlings and young invertebrates, e.g. ants
Ants harvest the seed, attracted by the fleshy, oil-rich elaiosome (or seed stalk), which they bury and store in widely dispersed locations
These seeds are buried ready for germinating with the next soaking rains
However, a "wattle seed-eating insect" which enjoys liquid meals using its proboscis-like injector to pierce the testa and suck out the embryo, often reduces the seed's viability
Status as an invasive species
In some parts of the world, A. mearnsii is considered to be an invasive species. Its invasiveness is due to its production of large numbers of seeds each year and to its large crown that shades other species.[22]
In South Africa it is listed as a Category 2 invader in the National Environmental Biodiversity Management Act
This means a permit is required to handle a species and ensure it does not spread beyond the area of the permit
Uses
The Ngunnawal people of the Australian Capital Territory use the gum as food and to make cement (when mixed with ash), and to ensure a supply of sap, the bark was cut in the autumn
The bark was also used to make coarse rope and string, and used to be infused in water to make a medicine for indigestion
Reproduction
A. mearnsii produces copious numbers of small seeds that are not dispersed actively. The species may resprout from basal shoots following a fire
It also generates numerous suckers that result in thickets consisting of clones
Seeds may remain viable for up to 50 years
Chemistry
Leuco-fisetinidin, a flavan-3,4-diol (leucoanthocyanidin) and a monomer of the condensed tannins called profisetinidins, can be extracted from the heartwood of A. mearnsii.
Sources of information: