Acacia auriculiformis
(northern black wattle)

A. auriculiformis is a tree from the legume family that has been introduced into tropical and subtropical areas as an ornamental, for reforestation, soil improvement and for its wood and pulp (PROTA, 2016). ). In Florida, USA, A. auriculiformis is a category 1 alien plant (Langeland and Burks, 1998), and it is prohibited in Miami-Dade County (PROTA, 2016). It is listed as invasive in Asia (Bangladesh, Singapore), Africa (Comoros, Mayotte, Tanzania), North America (Florida, USA), the Caribbean (Bahamas) and Oceania (Cook Islands, Federated States of Micronesia, Guam) (Islam, 2002; SE-EEPC, 2002; Tan and Tan, 2002; Kotiluoto et al., 2009; PIER, 2016. Space and Flynn (2000) list it among species that are invasive elsewhere and are invasive or potentially invasive on the Pacific island of Chuuk. A. auriculiformis is presently rare or uncommon in American Samoa but was listed among those naturalized species considered invasive elsewhere and classed as common or weedy (Space and Flynn, 2000). The species is also listed as a category 2 invasive plant species in the Bahamas (BEST Commission, 2003). Islam (2002) reports that following recent introduction of this species to Bangladesh, A. auriculiformis germinates naturally in plantation forests and prevents the germination of native species. It is one of 17 plant species named on a preliminary list of invasive alien species for Singapore (Tan and Tan, 2002). Starr et al. (2003) recommended the eradication of the species in Hawaii, USA, to prevent its invasion.

Natural stands of A. auriculiformis are found in Australia, Papua New Guinea and Indonesia (PROTA, 2016). In Australia it occurs on Cape York Peninsula and in Torres Strait, Queensland, and in the north of the Northern Territory including several off-shore islands (Boland et al., 1990). In Papua New Guinea it occurs in the Central and Western Provinces, and extends into Irian Jaya (Papua Barat) and the Kai Islands of Indonesia. It is naturalised in Asia, Africa, North America, Central America, the Caribbean, South America and Oceania; see Distribution Table for details (World Agroforestry Centre, 2002; Acevedo-Rodríguez and Strong, 2012; PIER, 2016; PROTA, 2016; USDA-ARS, 2016; WorldWideWattle, 2016).

A. auriculiformis is a  tree that has been extensively introduced across the tropics. Several countries state that it is being monitored in anticipation that it may become invasive following news of its invasiveness in Florida, USA. Its desirability as a wood/pulp tree, and its use for reforestation and as an ornamental make it a tree with a high risk of introduction. Although is reported as not invading closed canopy forest, its rapid growth in open areas can hinder native revegetation (Encyclopedia of Life, 2016).

A. auriculiformis occurs in the lowland tropics, growing naturally in narrow belts along river banks, where it may be dominant or one of the principal species. It also occurs in small pockets in depressions and in open-forest dominated by various eucalypts and acacias. It is also found in littoral rain forest behind either mangroves or coastal dunes. On the Oriomo Plateau of Papua New Guinea this species is common on the floodplains and levees of Bensbach and Morehead Rivers. Elsewhere it occurs as scattered trees in the riparian habitats, tall savannah woodland and in tall open-forest (monsoon forest). It is a component of swamp forest dominated by Melaleuca species, usually on the better drained sites. It is also common in littoral forest. Regular associates in these forests include Acacia mangium, A. aulacocarpa and Melaleuca cajuputi (Paijmans et al., 1971; Skelton, 1987; Boland et al., 1990). It is also planted for its pulp and wood, and as an ornamental, from where it has escaped into nearby areas, being also found in disturbed sites and roadsides (Encyclopedia of Life, 2016; PIER, 2016). In Florida, it invades pinelands, hammocks and scrub habitats.

Genetics

A. auriculiformis is predominantly outcrossing (Moran et al., 1989; Ibrahim, 1991; Khasa et al., 1993) and exhibits marked genetic variation. Isoenzyme studies revealed three distinct clusters of populations corresponding to the geographic distribution of the species in the Northern Territory, Queensland and Papua New Guinea; Queensland populations are more closely related to populations from Papua New Guinea than populations from the Northern Territory. These studies showed about 73% of the isoenzyme variation was among progenies within populations and indicated that weight should be given to both intra- and inter-population genetic variability in initial selections in domestication programmes of this species (Wickneswari and Norwati, 1991, 1993). These regional groupings were also apparent in differences in seedling morphology (Pinyopusarerk et al., 1991).

Variation was examined at 12 months for 28 provenances of A. auriculiformis in a trial in Malaysia. All provenances had a survival rate of greater than 92%, but differed significantly in their growth performance (Nor Aini et al., 1994a, b). At 5 years from planting in Sabah, eight provenances including three from Papua New Guinea and five from Queensland were identified as superior for height and diameter growth (Bernard, 1996). Provenance trials on four low fertility test sites in the Democratic Republic of Congo (Khasa et al., 1995) showed variation in growth and morphological characters when assessed at ages 3, 9, 15 and 21 months. The provenances with the greatest volume production were from Papua New Guinea.

International provenance trials were established in 1989 to examine the extent of genotype/environment interactions. Results from Australia and Thailand showed that provenances from Queensland have a higher proportion of straight stems (Awang et al., 1994; Puangchit et al., 1996; Turnbull and Awang, 1997). In a trial on an Imperata grassland site in South Kalimantan, variation in growth and form at 69 months after planting showed that the most highly productive A. auriculiformis provenances in this environment were from Papua New Guinea (MAI up to 35.6 m³/ha), Queensland (MAI up to 30.3 m³/ha) and Northern Territory (MAI up to 30.2 m³/ha) (Otsamo et al., 1996). There were also differences in tree quality with Queensland sources generally having the lowest occurrence of multi-stemmed trees. Similar results were obtained 8 months after the planting of a seedling seed orchard of A. auriculiformis in south Sumatra, where the best height and diameter growth, and lowest occurrence of multi-stemmed trees were shown by the Wenlock River provenance from the far north of Queensland (Susanto, 1996). The relative performance of provenances of A. auriculiformis in provenance trials on several sites in Vietnam has been reported by Nguyen Hoang Nghia and Le Dinh Kha (1996). Provenance variation in tolerance to salt and waterlogging has been noted in pot trials (Marcar et al., 1991b).

Several countries have genetic improvement programmes that aim to produce better quality seed for future planting programmes. Seed orchards established on Melville Island in the Northern Territory of Australia (Harwood et al., 1994) have failed to produce worthwhile amounts of seed (Harwood, 1996). The best clones are being relocated to environments where better seed production can be obtained.

The use of A. auriculiformis as a parent of hybrids, particularly in combination with A. mangium, is of great potential. Many hybrids show desirable commercial characteristics such as fast growth, fine branching and straight boles. Sedgley et al. (1992) found that the A. auriculiformis x A. mangium hybrid was more successful than the reciprocal, but fertile seeds were produced following interspecific pollination in both directions. Vacuum drying of pollen and storage in a deep freeze is recommended for the medium length storage (3 years) of pollen used in crossing programmes of these species (Harbard et al., 1994). Experimental A. mangium x A. auriculiformis hybrid seed orchards have been established in Indonesia to build up a base for a clonal forestry programme (Arisman et al., 1994). Outstanding hybrid clones have been selected and mass propagated for clonal forestry in Vietnam (Le Dinh Kha, 1996).

The Australian Tree Seed Centre of CSIRO Forestry and Forest Products, Canberra, Australia maintains seed stocks of representative provenances from throughout the native range of the species.

DNA barcode information for the species is available at the Barcode of Life Data Systems (BOLDS, 2016). Germplasm is stored at various institutions (Kew Royal Botanic Gardens, 2016; USDA-ARS, 2016). The chromosome number reported for A. auriculiformis is 2n=26 (PROTA, 2016).

Reproductive Biology

A. auriculiformis reproduces by seeds and vegetatively by cuttings (PROTA, 2016). Pollination is by insects (PROTA, 2016). A. auriculiformis produces large quantities of seed at an early age. Germination is rapid after suitable treatment and typically exceeds 70%. There is an average of 71,600 viable seeds/kg (Doran and Turnbull, 1997). Seedlings grow quickly and reach a height of 25-30 cm in 3-4 months, 6 m in 2 years, and 6-12 m in 3 years under favourable conditions (Turnbull and Awang, 1997). Young seedlings produce 2-3 bipinnate leaves, soon followed by phyllodes. Phyllodes are retained during the dry season; their average life is about 1 year in west Java, Indonesia.

Although A. auriculiformis has the ability to coppice, it is not a vigorous or prolific sprouter and careful management is required to obtain good results from coppicing.

Physiology and Phenology

Flowering usually starts within 2 years after sowing (Pinyopusarerk, 1990). The yellow flower spikes can be found on individual trees throughout the year but there is usually a distinct peak flowering season that may vary considerably with location. In the Northern Territory of Australia, flowering occurs from April to July with ripe seed available some 4-5 months later in August to October (Brock, 1988). Sedgley et al. (1992) found that peak flowering occurred in February to May at Atherton in Queensland, near Kuala Lumpur in Peninsular Malaysia, and Tawau in Sabah, with ripe seed pods available between October and April. In Java, peak flowering occurs in March to June (Turnbull and Awang, 1997). Mature seed can be collected between August and February in Thailand (Pukittayacamee et al., 1993).

It is fire adapted (EDDMapS, 2016). Profuse natural regeneration may appear after fire or on disturbed sites in the absence of severe weed competition

Associations

A. auriculiformis can fix nitrogen after nodulating with a range of Rhizobium and Bradyrhizobium strains in many tropical soils. In the Philippines, 52-66% of nitrogen uptake was shown to be derived from nitrogen fixation (Dart et al., 1991). This nitrogen-fixing potential may only be realized in many soils if adequate fertilizer, especially phosphorus, is applied. A. auriculiformis has associations with both ecto- and endo-mycorrhizal fungi. The ecto-mycorrhizal fungus, Thelephora spp., forms a beneficial association, and several species of vesicular arbuscular mycorrhizas, including Glomus etunicatum and Gigaspora margarita, are effective (Dart et al., 1991; de la Cruz and Umali-Garcia, 1992).

Environmental Requirements

A. auriculiformis occurs naturally in hot humid and hot subhumid climatic zones. The data in the climate table pertain to the native range, whereas Nguyen Hoang Nghia (1996) provides a climatic profile of the species combining information from both native and planted ranges. For the natural distribution of this species, the mean maximum temperature of the hottest month (November-December) is within the range 32-34°C, and the mean minimum temperature of the coolest month (May-September) is 17-22°C. Outside the natural distribution, a wider range of temperatures is tolerated, indicating the adaptability of A. auriculiformis. Frost does not occur in its natural range, but elsewhere, light frosts are tolerated. Mean annual rainfall ranges from 760 mm in the Northern Territory of Australia to 3400 mm in Papua New Guinea (Doran and Turnbull, 1997). However, for most of the planted and natural distribution, rainfall is generally much lower (up to 2500 mm), with a summer monsoonal pattern and most rain falling from December to March.

In Australia, A. auriculiformis grows on dissected lateritic lowlands and alluvial coastal plains. Occurrences in the Northern Territory are along drainage channels just above the tidal range, on the edges of sand dunes, behind mangrove swamps, and along river levees. In Queensland it is mainly restricted to river banks and drainage lines. The soils are frequently yellow earths, but vary from dune sands and sandy loams to alluvial soils with a high clay and humus content. The pH usually ranges from 4.5-6.5, but in the Northern Territory it grows on alkaline beach sands with a pH of 8-9. In West Timor it is one of the best species for cultivation on highly alkaline soils (McKinnell and Harisetijono, 1991). A. auriculiformis is also highly tolerant of acidic conditions. In Australia, Malaysia and the Philippines it has grown on acid mine spoils of pH 3 (NAS, 1983), while A. auriculiformis is one of the few tree species to become widely planted on the acid sulphate soils (pH 3) of the Mekong Delta of Vietnam (Nguyen Hoang Nghia, 1996). It can also tolerate saline soils. In an experiment in Thailand, it continued growing under saline conditions ranging from 0.15 to 7.25 dS/m, in both wet and dry soils (Turnbull and Awang, 1997). A. auriculiformis was also amongst the best performing Acacia spp. on slightly to moderately saline seasonally waterlogged soils in south-eastern Queensland (Marcar et al., 1991a).

The natural occurrences in western Papua New Guinea and Irian Jaya are mainly on the relict alluvial plain known as the Oriomo Plateau. They are on shallow well-drained sandy loam overlying heavy clay or imperfectly drained soils subject to temporary or prolonged flooding in the wet season. These soils are strongly acid and of poor fertility with low values for nitrogen, exchangeable potassium and available phosphorus (Bleeker, 1983). It is generally a lowland species though has been found up to 1000 m altitude.

There are several diseases and insect pests of A. auriculiformis, but none are limiting to establishment on appropriate sites at present (Day et al., 1994). During a recent workshop on diseases of tropical acacias (Old et al., 1997), a number of diseases were identified as potential threats to the future productivity of industrial plantations. These included stem cankers caused by a range of pathogens (Botryodiplodia theobromae [Lasiodiplodia theobromae], Botryosphaeria spp. and Hendersonula sp.) and most often associated with stem borer damage, pink disease (Erythricium salmonicolor) which is most prevalent in high rainfall areas, and phyllode rust (Endoraecium digitatum) which has impaired the growth of A. auriculiformis in Australia and Indonesia. It is not susceptible to heart rot which affects A. mangium (Ito and Nanis, 1997). A root rot fungus, Ganoderma sp., was observed to cause crown dieback and defoliation in A. auriculiformis plantations in parts of West Bengal, India (Barari, 1993). Ganoderma lucidum causes root rot in India (World Agroforestry Centre, 2002) and a species of Cuscuta (Convolvulaceae) has also been recorded as a parasite of A. auriculiformis in West Bengal (Banerjee et al., 1993). Seedlings in the nursery can be infected by powdery mildew (Oidium) especially where there is heavy shading (Harsh et al., 1992).

Stressed trees are particularly susceptible to attacks by insect pests. A beetle, Sinoxylon sp., can girdle small stems or branches. In Australia, the wood is attacked by borers and termites, and scale insects are prevalent on young trees (Hearne, 1975). Experimental results suggest that A. auriculiformis shows some resistance to termites (Turnbull and Awang, 1997). A. auriculiformis was recorded as host to Xystrocera festiva at two locations in south Sumatra, Indonesia (Suharti et al., 1994).

Animal feed, fodder, forage

• Invertebrate food for lac/wax insects

Environmental

• Agroforestry
• Erosion control or dune stabilization
• Revegetation
• Shade and shelter
• Soil improvement

Fuels

• Charcoal
• Fuelwood

Human food and beverage

• Honey/honey flora

Materials

• Bark products
• Carved material
• Dye/tanning
• Fibre
• Gums
• Miscellaneous materials
• Mulches
• Wood/timber

Medicinal, pharmaceutical

• Traditional/folklore

Ornamental

• Christmas tree
• Cut flower
• garden plant
• Potted plant
• Propagation material
• Seed trade

Charcoal

Furniture

Roundwood

• Building poles
• Posts
• Roundwood structures
• Stakes

Sawn or hewn building timbers

• Flooring
• For light construction

Veneers

Wood-based materials

• Composite boards
• Wood cement

Woodware

• Industrial and domestic woodware
• Tool handles
• Toys
• Turnery
• Wood carvings

A. auriculiformis is related to A. polystachya , A. cincinnata. and A. spirorbis subsp. solandri and more distantly to A. aulacocarpa and A. crassicarpa (Pedley, 1975). It hybridizes readily with A. leptocarpa and A. mangium in nature and in cultivation (Pinyopusarerk, 1993). The hybrids with A. mangium are intermediate between the two parents in morphology and wood properties. They inherit the straighter stem form of A. mangium and the self-pruning ability and the stem circularity of A. auriculiformis (Turnbull and Awang, 1997). Hybrids tend to have more vigorous growth and are more resistant to heart rot. There is much interest in the domestication of this hybrid as a result of this combination of commercially desirable characteristics. Aspects of seed production and vegetative propagation of the hybrids are covered in Carron and Aken (1992).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

Public awareness

A. auriculiformis is listed as invasive in Florida, USA and its planting is prohibited in some counties (Langeland and Burks, 1998; PROTA, 2016). It is also listed as invasive in the Bahamas (BEST Commission, 2003).

Eradication

Starr et al. (2003) advised that plantings should be discontinued in Hawaii, USA due to its invasive characteristics.

Control

Physical/mechanical control

Hand pulling of the seedlings and tree girdling are recommended control measurements (PIER. 2016), but might disturb soils and increase other non-natives (NatureServe, 2016).

Biological control

No known biological control has been attempted.

Chemical control

The use of triclopyr herbicide mixed with oil is recommended (PIER, 2016). In Florida, triclopyr applications to basal bark and cut stumps are listed as permitted treatments (Langeland et al., 2011).

25/09/2016 Updated by:

Jeanine Vélez-Gavilán, University of Puerto Rico at Mayagüez