Significations et usages de Acacia_mearnsii

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Acacia mearnsii is a fast-growing leguminous tree native to Australia. Common names for it include Black Wattle, Acácia-negra (Portuguese), Australian Acacia, Australische Akazie (German), Swartwattel (Afrikaans), Uwatela (Zulu). This plant is now known as one of the worst invasive species in the world.^[2]

Taxonomy

The trees are unarmed, evergreen and grow six to 20 meters high. The branchlets are shallowly ridged; all parts finely hairy; growth tips golden-hairy. Leaves dark olive-green, finely hairy, bipinnate; leaflets short (1.5 - 4 mm) and crowded; raised glands occur at and between the junctions of pinnae pairs. Flowers pale yellow or cream, globular flower heads in large, fragrant sprays. Fruits dark brown pods, finely hairy, usually markedly constricted.^[3][4][5]

The species is named after American naturalist Edgar Alexander Mearns, who collected the type from a cultivated specimen in East Africa.^[6]

Geographical range

A. mearnsii is native to Southeastern Australia and Tasmania, but has been introduced to North America, South America, Asia, Europe, Pacific and Indian Ocean islands, Africa, Europe, and New Zealand.^{[4][7][8][9][10]}

It has been introduced to numerous parts of the world, and in those areas is often used as a commercial source of tannin or a source of firewood for local communities. In areas where it has been introduced, it is often considered a weed, and is seen as threatening native habitats by competing with indigenous vegetation, replacing grass communities, reducing native biodiversity and increasing water loss from riparian zones. Found in tropical rainforests.

Habitat description

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 600-1700m. 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 6.6 – 22.8 dm (mean of 6 cases = 12.6), an annual mean temperature of 14.7 – 27.8°C (mean of 6 cases = 2.6°C), and a pH of 5.0 – 7.2 (mean of 5 cases = 0.5).^[11] A. mearnsii does not grow well on very dry and poor soils.^[12]

A. mearnsii plays an important role in the ecosystem in its native Australia. As a pioneer plant it quickly binds the erosion-prone soil following the bushfires that are common in the Australian wilderness. Like other leguminous plants, it fixes the atmospheric nitrogen in the soil. Other woodland species can rapidly utilise 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.

Local dispersal methods

• Consumption and excretion: The seeds are potentially distributed by rodents or birds.^[13]
• For ornamental purposes (local)
• By animals: The dispersal of the seeds of A . mearnsii is believed to be aided by cattle and birds.^[14]
• By people: Local people collecting branches and logs for firewood may spread seeds.^[15]
  • Transportation of soil: The seeds may also be spread by the movement of seed-contaminated soil.^[5]
• By water: The hard-coated seeds are spread readily down water courses.^[5]

Mycorrhizal fungi attach to the roots to produce food for marsupial animals, and these animals in turn disperse the spores in their droppings to perpetuates the symbiotic relationship between wattle's roots and the mycorrhizal fungi.

The cracks and crevices in the wattle's bark are home for many insects and invertebrates. The rare Tasmanian Hair Streak Butterfly lays her eggs in these cracks, which hatch to produce caterpillar larva attended by ants (Indomyrmex sp.) that feed off the sweet exudates from the larva.

The tree is home to various grubs, such as wood moths, which provide a food source to the Australian Black cockatoos, who 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 die back caused by scarab beetles and pasture pests.

Black wattles, along with gums, native box, native hop form the framework vegetation on so-called "Hill-topping" sites. They are often 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 our Wattle Birds, Yellow Throated Honey Eaters and New Holland Honey Eaters. 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 elaisome (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 seeds viability.^[16]

Uses

The species is grown commercially in many areas of the world for a variety of uses, in addition to use as an ornamental.^[8] Commercial stands have been established in Africa, South America and Europe. The tannin compounds extracted from the bark of A. mearnsii are commonly used in the production of soft leather. A range of other products, such as resins, thinners and adhesives, can also be made from bark extracts. The timber is used for building materials, the charcoal is used for fuel and the pulp and wood chips are used to produce paper. In rural communities in South Africa the trees are important as a source of building material and fuel. Growers of the tree in that country banded together to form the South African Wattle Growers Union.^{[5][11][8][9][10][10]} A. mearnsii has some known medical applications, such as its use as a styptic or astringent. The planting of wattles has also been used as a soil stabiliser to decrease erosion (preferably far from river courses to minimise the water loss caused by the tree's high rate of transpiration). The agroforestry industry promotes the use of the species (among other similar species) as a potential "soil improver".^{[5][11][8][9][10][17]}

Acacia mearnsii has been shown to contain less than 0.02% alkaloids.^[18]

Weed impacts outside of native range

The invasiveness of this species is partly due to its ability to produce large numbers of long-lived seeds (which may be triggered to germinate en masse following bush fires), and the development of a large crown which shades other vegetation. Its leaves and branches may have allelopathic properties. A. mearnsii competes with and replaces indigenous vegetation. It may replace grass communities to the detriment of the grazing industry and grazing wildlife. By causing an increase in the height and biomass of vegetation A. mearnsii infestations increase rainfall interception and transpiration, which causes a decrease in streamflow. Soil under A. mearnsii becomes desiccated more quickly (than it does under grass). A. mearnsii stands also destabilise stream banks and support a lower diversity of species^{[15][7][19][20]}

Commercial plantations and invasive stands of A . mearnsii in South Africa reduce surface runoff and decrease water ability, causing an estimated annual economic loss of $US 2.8 million. According to KwaZulu-Natal Wildlife (the governmental agency responsible for managing protected areas in KwaZulu-Natal Province, South Africa) the advance of alien plants (particularly Chromolaena odorata, Lantana camara, Acacia dealbata, and Acacia mearnsii) is the most significant past and future threat to conservation in these areas.^[5][21]

Management in non-native regions where its is considered a weed

Preventative measures

A Risk Assessment of Acacia mearnsii for Hawai'i and other Pacific islands was prepared by Dr. Curtis Daehler (UH Botany) with funding from the Kaulunani Urban Forestry Program and US Forest Service. The alien plant screening system is derived from Pheloung et al. (1999) with minor modifications for use in Pacific islands (Daehler et al. 2004). The result is a score of 15 and a recommendation of: "Likely to cause significant ecological or economic harm in Hawai'i and on other Pacific Islands as determined by a high WRA score, which is based on published sources describing species biology and behavior in Hawai'i and/or other parts of the world."

Chemical

Saplings sensitive to foliar applications of triclopyr. Dicamba, glyphosate and picloram applied cut-surface effective, triclopyr probably effective, although applications to drilled holes in larger trees probably necessary. Cut-surface (notching) applications of picloram provided complete control, glyphosate and dicamba caused 80% control, and 2,4-D was inadequate at Kala'e, Molokai. Alton Arakaki (Univ. Hawai'i) and Ed Misaki of The Nature Conservancy (TNC) confirmed the efficacy of picloram but got much better results with glyphosate and dicamba, each resulting in over 90% control at Kamakou Preserve. Basal bark and stump bark treatments with 2,4-D or triclopyr effective. Pat Bily (TNC) reported that basal bark applications with triclopyr ester at 20% in oil was effective, as was cut stump application of triclopyr amine at 50% in water. Hawaiʻi Volcanoes National Park (HAVO) staff got good control with triclopyramine at 10% in water applied to cut stumps (Chris Zimmer, HAVO). Anecdotes indicate that wattle is sensitive to basal bark treatment with diesel alone and to girdling.^[4]

Biological

The use of Cecidomyiidae gall midges to inhibit reproduction of Acacia species is being researched for use in South Africa. A Dasineura sp. has been identified as a promising control agent as it prevents fruit formation (and thus reproduction) without affecting vegetative growth (which may be a concern for industries or commerce that rely on the species). Dasineura is also known to have a narrow host range. Melanterius maculatus, a seed-eating weevil (native to Australia), was introduced into South Africa in 1993 and caused reductions in A. mearnsii seed numbers in some areas. Larvae feed on developing seeds inside the pods and adults feed on the green pods and pinnules. In South Africa a local stump-colonising fungus has been isolated and added to cut trees to prevent regrowth. Finally, a wasp (Bruchophagus acaciae) has been described that attacks the seeds of some Acacia species but not those of A. mearnsii (Adair et al., 2000; Adair, 2002; ARC-PPR, 2003; Hill, Gordon and Neser, 1999).^[7]

Integrated management

The Working for Water programme implemented by the South African government is a collaborative program that aims to ameliorate the problems caused by Acacia species and other invasive plants. The program consists of more than 300 sub-projects in all nine provinces in the country and consists of the clearing of weeds from water courses (by mechanical and chemical methods). Between 1995 and 2000 over $100 million of poverty-relief funds on the program which was labor intensive and provided job opportunities for local communities. After seven years of implementation of the project it became clear that rehabilitation of sites (following the removal of alien plant species) would sometimes be needed in order to prevent or reduce the soil erosion stimulated by the clearing of plants (Van Wilgen et al., 2002, Milton, Dean and Richardson, 2003).

Reproduction

A. mearnsii produces copious numbers of small seeds that are not dispersed actively. The species may resprout from basal shoots following a fire.^[4] It also generates numerous suckers that result in thickets consisting of clones.^[4] Seeds may remain viable for up to 50 years.^[22]

References

External links

• Australian Plants online

• The IUCN/SSC Invasive Species Specialist Group