Acacia crassicarpa
(northern wattle)

A. crassicarpa, native to Queensland, Australia and southern New Guinea, has proved to be one of the fastest-growing acacias for planting on degraded sites in seasonally dry tropical areas, especially in Indonesia and South-East Asia. It is known to have been widely introduced for trials throughout the world, though available records do not indicate that it proved successful everywhere. However, that it is widely promoted as a tropical timber tree, and as it clearly has the potential to become invasive as have so many other exotic Acacia species, special care should be taken when considering A. crassicarpa for further introduction, and further work is required to identify where it currently exists and may be spreading. It is reported as being invasive only on the Cook Islands, and A. crassicarpa was evaluated as a high risk species for the Pacific region, and as such it could be considered as a potential invasive species in the future.

The native range of A. crassicarpa is centered in northern Queensland, extending south along the north-eastern coast, on islands in Torres Strait, and on the south coast of New Guinea extending (Australia’s Virtual Herbarium, 2014). In New Guinea it occurs in the southern lowlands from south-eastern Irian Jaya, Indonesia and east to the Oriomo River region of Papua New Guinea. In north Queensland it extends from the Cape York Peninsula south to Townsville with a southern small population near Mackay. In the Torres Strait it occurs on Badu, Banks, Horn and Prince of Wales Islands which are continental islands (McDonald and Maslin, 1998). Single records are also reported from Northern Australia and an island close to Java (Australia’s Virtual Herbarium, 2014) but such isolated outliers may have been introduced.

A. crassicarpa is known to be grown as a successful plantation tree throughout much of Indonesia, South-East Asia, and also in South Asia and in some Pacific islands, though is reported to be invasive only in the Cook Islands. However, it is very likely that the species is present in more countries than indicated in the distribution table, for example in Africa and South America where it is currently not recorded.

A. crassicarpa has been widely introduced throughout the world for testing in species trials, and there continues to be significant demand for commercial quantities of seed for the establishment timber plantations in Indonesia and South-East Asia. Thus it is likely to be established in many more countries that it is recorded, and may be introduced further. However, in an Australian/New Zealand weed risk assessment adapted for Hawaii, information on A. crassicarpa resulted in a conclusion that it posed a high risk, with a score of 7 (PIER, 2014).

In Queensland A. crassicarpa is a primarily coastal lowland species found on coastal plains and foothills, at the rear of coastal foredunes and on slopes of stabilised sand dunes (McDonald and Maslin, 1998). It occurs on a variety of mainly sandy soil types which include calcareous beach sands, yellow earths derived from granite, red earths on basic volcanics, red-yellow podzolics over schists, and alluvial and colluvial soils. By contrast, New Guinea populations occur on the gently undulating terrain of the Oriomo Plateau which is a relict alluvial plain (Loffler, 1977). Soils are mainly well-drained, strongly acidic clays (infertile gleyed red and yellow earths, or red and yellow latosols) but it also occurs on imperfectly drained soils subject to flooding in the wet season and rapid drying-out in the dry season.

In Queensland, A. crassicarpa occurs in the understorey of open-forests or open-woodlands associated with Acacia leptocarpa, A. flavescens, A. mangium, A. auriculiformis, A. polystachya, Melaleuca spp., Eucalyptus platyphylla, E. tessellaris, and Lophostemon grandiflorus. On coastal foredunes it occurs in low woodland associated with Casuarina equisetifolia. On Cape York Peninsula A. crassicarpa is also a component of open-forest, low open-forest and woodland and low-woodland associated with E. tetrodonta, Allocasuarina littoralis and Melaleuca spp. (Pedley and Isbell, 1970).

In New Guinea it occurs in woodlands and open-forests most commonly along the ecotone between areas with impeded drainage that support low open savanna woodlands and areas of well-drained alluvium which support closed vine forests (McDonald and Maslin, 1998). Commonly associated species include Acacia mangium, A. leptocarpa, A. "peregrina" ms, A. simsii, Melaleuca cajuputi, M. leucadendra, Lophostemon suaveolens, Asteromyrtus brassii, A. symphyocarpa and Eucalyptus brassiana. Unique acacia forests with an even canopy dominated by either A. crassicarpa or A. mangium are present along the Oriomo River, Papua New Guinea (Skelton, 1987); other genera present include Flindersia, Grevillea, Syzygium and Planchonella. In Papua New Guinea A. crassicarpa has been noted as a vigorous colonizer of degraded soils following slash-and-burn cultivation (Gunn et al., 1991).

Reproductive Biology

A. crassicarpa is usually propagated from seed (direct sowing or in the nursery) but stem cuttings, marcotting or air layering have also been successful (Ryan et al., 1987; Thomson, 1994). The seeds have a hard seed coat, low moisture content and are well-suited to storage in an air-tight container at room temperature (24°C), and pre-treatment such as boiling seeds for 1 minute promotes germination, then allowed to cool and imbibe water for 24 hours, with only swollen sunken seeds being viable and floating seeds are not viable, though other pretreatments have been used. A germination rate of about 70% is usual after pre-treatment, with optimum germination at 25-30°C. Seedlings germinate after five days and all viable seeds have germinated after 25 days (ATSC, 1998; Doran and Gunn, 1987).

Physiology and Phenology

A. crassicarpa is a fast-growing and light-demanding tree adapted to the seasonally dry tropics. It is a nitrogen-fixing tree that produces abundant natural root nodulation. It is fire resistant after ten years of age, and competes favourably against weed species such as Imperata cylindrica. It is reported to be tolerate salt wind, though is susceptible to damage by cyclones and strong winds. Coppicing ability varies with cutting height and provenance (Ryan and Bell, 1989) and is not a suitable regeneration method for the species.

Where introduced in Thailand, active shoot growth is maintained even during the dry season (Puriyakorn and Luanviriyasaeng, 1988). In the native range in Queensland, flower buds were first observed when plants were at 27 months of age, with the main flowering period beginning in April to May and continuing to June and July, during the first dry and relatively cool months of the year, with pods maturing from October to November toward the end of the dry season (McDonald and Maslin, 1998; Ryan and Bell, 1989).

Associations

In open forests in Queensland, Australia, A. crassicarpa is associated with Acacia leptocarpa, A. flavescens, A. mangium, A. auriculiformis, A. polystachya, Melaleuca spp., Eucalyptus platyphylla, E. tessellaris, and Lophostemon grandiflorus, also with Allocasuarina littoralis, E. tetrodonta, and Melaleuca spp., and with Casuarina equisetifolia on low woodlands on coastal dunes (Pedley and Isbell, 1970).

In New Guinea, commonly associated species include Acacia mangium, A. leptocarpa, A. peregrina, A. simsii, Melaleuca cajuputi, M. leucadendra, Lophostemon suaveolens, Asteromyrtus brassii, Asteromyrtus. symphyocarpa and Eucalyptus brassiana. Unique acacia forests with an even canopy dominated by either A. crassicarpa or A. mangium are present along the Oriomo River, Papua New Guinea (Skelton, 1987). Other genera present include Flindersia, Grevillea, Syzygium and Planchonella.

The species readily forms natural nodulation so artificial inoculation of nursery seedlings is not essential (Harwood et al., 1994; Thomson, 1994; Doran et al., 1997).

Environmental Requirements

A. crassicarpa is a fast-growing, light-demanding tree suitable for planting in the seasonally dry tropics, and in its natural habitat the species is found in warm to hot, humid and sub-humid climates in the lowland tropics (Doran et al., 1997). A. crassicarpa is most suited to regions which receive between 1200 and 2800 mm mean annual rainfall with a pronounced dry season of 4-6 months, extending to 1000-3500 mm, though it will also tolerate rainfall as low as 500 mm per year under certain conditions in Australia, and up to 3500 mm in New Guinea. Rainfall often follows a monsoonal pattern with December-March being the wettest period. Length of dry season ranges from around 6 months at the southern limit of the distribution in Australia to 3 months in New Guinea.

Mean maximum of the hottest month is 32-34°C and mean minimum of the coolest month is 12-21°C. Daily maximum temperature exceeds 32°C on 20-40 days each year but rarely goes above 38°C. The mean minimum temperature of the coolest month is 15-22°C. Although the entire natural range of A. crassicarpa is frost-free it has demonstrated tolerance of light frosts in trails at Gympie, Queensland (Ryan and Bell, 1989).

In its native range, it occurs on a variety of mainly sandy soil types which include calcareous beach sands, yellow earths, red earths, red-yellow podzolics, and alluvial and colluvial soils in Australia, whereas in New Guinea, it also occurs on the gently undulating terrain of the Oriomo Plateau which is a relict alluvial plain (Loffler, 1977). Soils are mainly well-drained, strongly acidic clays (infertile gleyed red and yellow earths, or red and yellow latosols) but it also occurs on imperfectly drained soils subject to flooding in the wet season and rapid drying-out in the dry season. Where introduced, it tolerates a range of soil types, particularly those of low fertility. It has been grown successfully on mildly alkaline soils, sandy loams, peaty, highly acidic soils (pH 3.3), shallow, sandy soils and on a site with a shallow water table. Survival after planting out is usually high and trees with a bole over 10 cm in diameter at breast height are fire tolerant, producing vegetative regrowth following fires (Arentz et al., 1995).

A. crassicarpa is mainly a lowland species, with the altitudinal range in New Guinea commonly 20-80 m, whereas in north Queensland it is from sea level to 150 m with minor occurrences at 700 m (McDonald and Maslin, 1998) and even to 900 m where introduced.

In Sabah, Malaysia, A. crassicarpa was preferred for planting on rocky, shallow and sandy soils infested with Imperata cylindrica where it grew to 15-23 m tall and 10-16 cm diameter after 4 years, outperforming A. auriculiformis and A. mangium (Sim and Gan, 1991). A. crassicarpa was the best performing of five Acacia species after 2 years in South Kalimantan (Otsamo and Adjers, 1995), and was one of the better performing Acacia species tested in Nepal, reaching a height of 4.5 m in 1.5 years (Karki and Karki, 1993).

A. crassicarpa is reported to be susceptible to attack by stem borers.

Animal feed, fodder, forage

• Fodder/animal feed
• Forage

Environmental

• Agroforestry
• Erosion control or dune stabilization
• Land reclamation
• Revegetation
• Shade and shelter
• Soil conservation
• Soil improvement
• Windbreak

Fuels

• Charcoal
• Fuelwood

Human food and beverage

• Root crop

Boats

Charcoal

Furniture

Roundwood

• Building poles

Sawn or hewn building timbers

• Carpentry/joinery (exterior/interior)
• Flooring
• For light construction
• Wall panelling

Wood wool

A. crassicarpa has morphological affinities to the closely related A. aulacocarpa, the Papua New Guinea species A. peregrina M. W. McDonald & Maslin, and A. midgleyi M.W. McDonald & Maslin. However, A. crassicarpa differs primarily from these species in having larger lanceolate-falcate phyllodes, 2-6 flower spikes per axil and larger, thicker pods (McDonald and Maslin, 1998). 

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.

Control

Hand pulling of seedlings and small saplings are likely to prove effective in small patches. Trees coppice so cutting of larger trees would also require removal of the roots or herbicide stump treatments. No information of herbicides specific to A. crassicarpa are available, however, it is likely that those that have proved effective on closely related Acacia species may also be effective on this species. 

Noting earlier problems regarding the definition of taxonomical limits to the species and its relationships to other species in the A. aulacocarpa group, previous misidentifications, and its high risk assessment as an invasive species, it is recommended that further work is conducted on this species. Specifically, a detailed assessment of the current distribution of the species is required, alongside that of other species in the group, and an updated assessment of invasions in the Cook Islands and the status of naturalised populations elsewhere where it has been recorded.

04/03/16 Updated by:

Nick Pasiecznik, Agroforestry Enterprises, Villebeuf, Cussy en Morvan, France

03/06/98 Original text by: 

Duncan McDonald, Consultant, UK