Acacia mangium (brown salwood)

Acacia mangium is perhaps the most widely planted and commercially available of tropical acacia trees. Because the species is also known to be a threat to indigenous flora in various Pacific Islands, invasive in the Dominican Republic and a potential invader and transformer species in Cuba, its invasive traits warrant further research and caution when cultivating. These traits include its extremely vigorous growth (wood volumes can grow at over 30 m³/ha/year), tolerance of acidic and nutrient-depleted soils and its ability to compete with other vegetation. The species is listed in the Global Compendium of Weeds as ‘environmental weed, naturalized weed’ and received a high PIER risk score of 8, where any score over 6 should be rejected for import. Risk of introduction for this species is therefore high.

Acacia (family Fabaceae, subfamily Mimosoideae) represents a cosmopolitan genus of 1030 species previously contained in three subgenera: subgenus Acacia, subgenus Aculeiferum and subgenus Phyllodinae (Maslin, 1995). In the newly IBC adopted circumscription of Acacia, the type of Acacia changes from the African/Asian species, A. scorpioides (=A. nilotica), to the Australian species, A. penninervis. The nomenclatural consequences at the infrageneric level that flow from this are: (1) the name subgenus Acacia now applies to the ‘Australian group’ formerly known as Acacia subgenus Phyllodineae (Maslin, 2008) and (2) species in Africa and the Americas previously referred as Acacia are now treated under Senegalia [Acacia], Vachellia, Acaciella and Mariosousa. For further information: see Jawad et al. (2000), Maslin et al. (2003), Orchard and Maslin (2003), Seigler and Ebinger (2005), Kodela and Wilson (2006), Seigler et al. (2006).

The species A.mangium is in the subgenus Acacia, the ‘Australian group’ formerly known as subgenus Phyllodinae. Within this subgenus the species are grouped into seven sections, with A.mangium assigned to section Juliflorae (235 species), a group characterized by having flowers in elongated spikes and phyllodes with numerous, often anastomosing, longitudinal nerves. Pedley (1986) proposed a classification in which Acacia was formally subdivided into three genera, namely Acacia, Senegalia and Racosperma. Most botanists concerned with this group did not adopt Pedley's 1986 classification; however, there are citations for Racospermamangium [A. mangium] (Khasa et al., 1995).

Acacia mangium was originally described by the German botanist Georgius Everhardus Rumphius as Mangium montanum Rumph. in Herbarium Amboinense 3:123, t.81 (1750), based on his studies on the Indonesian island of Ambon. The species name mangium is an allusion to Rumphius' observation that the tree resembled 'mangge' or mangroves in Indonesia. In 1806, the species was moved to the genus Acacia by C.L. Willdenow in Sp. Plant 4: 1053 (1806).

Acacia mangium may be confused with A. holosericea and A. neurocarpa but can be most readily distinguished by its arborescent habit, glabrous phyllodes and branchlets, white to cream flower spikes and seed with an orange aril (Maslin and McDonald, 1996). A. holosericea and A. neurocarpa occur naturally as shrubs or small trees on drier sites.

General

A. mangium is a large tree, to 30 m tall, with a straight bole, which may be over half the total tree height. Trees with a diameter over 50 cm are rare. It may be reduced to a small tree or large shrub of 7-10 m on adverse sites. The bark surface is rough, furrowed longitudinally, and varies in colour from pale grey-brown to brown. The lower bole is sometimes fluted.
A detailed botanical description is provided by Pedley (1975).

Foliage

Borne on very acutely angled, glabrous and stout branchlets, the mature phyllodes of A. mangium are very large, normally 11-27 cm long and 3-10 cm broad. They are dark green, glabrous on a glabrous pulvinus 0.6-1 cm long. The phyllodes are characterized by four (rarely three or five) main longitudinal nerves, basally confluent but distinct from lower margin, minor nerves strongly anastomosing to form a prominent reticulum (Maslin and McDonald, 1996).

Inflorescences, flowers and fruits

The whitish (or cream) flowers are in rather loose spikes 5-12 cm long on peduncles 0.6-1 cm long, singly or in pairs in the upper axils. The seed pods are linear, tightly coiled when ripe, sometimes tightly spirally coiled, slightly woody, 7-8 cm long and 0.3-0.5 cm wide. The seeds are black and shiny, longitudinal, ovate to oblong 3-5 x 2-3 mm with a yellow or bright orange (rarely red) funicle folded to form an oily, fleshy aril beneath the seed.

Phenology

A. mangium is a fast-growing, evergreen species. It is able to grow throughout the year if conditions are suitable. In Thailand, it has been observed that growth appears to slow down or cease in response to the combination of low rainfall and cool temperatures in January-February. Trees start to grow actively again in April before the start of the wet season (Atipanumpai, 1989).

Flowering phenology differs throughout its natural and planted range. In its natural habitat, flowers are present during February to May in Australia and the seed matures in October-December (Sedgley et al., 1992). Farther north the fruits mature earlier with seed available from July in Indonesia, and late September in Papua New Guinea (Skelton, 1987; Turnbull et al., 1983).

As an exotic, the normal flowering cycle may be disrupted and flowering can occur throughout the year; however, a distinct peak is usually discernible (Awang and Taylor, 1993). The peak is reported to be June-July in Peninsular Malaysia (Zakaria and Awang, 1991), January in Sabah (Sedgley et al., 1992), October-November in Taiwan (Kiang et al., 1989) and September in Thailand (Kijkar, 1992). Mature fruits occur 3-4 months after flowering.

Acacia mangium is native to the humid tropical forests of northeastern Australia, particularly the coastal tropical lowlands of northern Queensland, Papua New Guinea and into Irian Jaya and the Maluku Islands of Indonesia (Doran and Turnbull, 1997; Krisnawati et al., 2011). It has spread across tropical southeast Asia and has become the most widely planted and commercially important of the tropical acacias (Doran and Turnbull, 1997; Orchard and Wilson, 2001; Arisman and Hardiyanto, 2006). In Brazil, plantations of Acacia spp. rank third among all cultivated forest species, with the species A. mangium being the most widely planted within the state of Roraima (Parreira et al., 2014). A. mangium is also cultivated in the Caribbean, including Puerto Rico where it is now naturalized (Kairo et al., 2003; Acevedo-Rodríguez and Strong, 2012). It has become invasive in the Dominican Republic (Kairo et al., 2003) and a potential invader and transformer species in Cuba (Oviedo Prieto et al., 2012). The species was not included in Wagner and Lorence (2016)’s work on the Marquesas Islands, although the species is known to be grown in other parts of French Polynesia (PIER, 2016).

Acacia mangium is native to Australia and parts of Papua New Guinea and Indonesia, but has become a major commercial wood source across Southeast Asia and Brazil (Doran and Turnbull, 1997; Orchard and Wilson, 2001; Krisnawati et al., 2011; Aguiar et al., 2014; FAO, 2016). Although it had already been in cultivation in the Philippines by 1923 (Merrill, 1923), the species was reportedly first introduced beyond its native range as a plantation species to Sabah, Malaysia in 1966; after its success there, the species was then planted across Southeast Asia and to parts of the Americas (Pinyopusarerk et al., 1993; Arisman and Hardiyanto, 2006; Krisnawati et al., 2011).

Various reports have listed A. mangium as now weedy or invasive to Hawaii, the Dominican Republic and Guam. Six of the seven specimens held at the US National Herbarium, all collected in Guam, Hawaii, the Dominican Republic, Western Samoa and Atiu (Cook Islands), were taken from plants in cultivation. Similarly, most of the 21 specimens listed in Missouri Botanical Garden (Missouri Botanical Garden, 2016) from the Caribbean and Central America were collected in the 1990s and 2000s, with none prior to 1989. Although it was reported to be invasive in French Guiana by Aguiar et al (2014), the species was not listed in Funk et al (2007)’s flora on the Guiana Shield. This suggests that the species, a relatively recent introduction, may not yet be as high of an invasive concern in the Americas (excepting Brazil) as it is in Southeast Asia, where it has been established as a plantation species since the 1960s, but more research is needed.

Based on the current evidence, risk of introduction for A. mangium is high. The species received a high PIER risk score of 8, where any plant with a score over 6 is recommended to be rejected from import and is likely to be invasive (PIER, 2016). A. mangium grows rapidly, achieving up to 23 m in 9 years (FAO, 2016) and has a short generative time of 1 year (PIER, 2016). The species benefits from fire and disturbances and is a pioneering species (FAO, 2016; PIER, 2016). It has a history of intentional, repeated introductions beyond its native range, which is likely to continue as it is a commercially important species in Southeast Asia (Doran and Turnbull, 1997; Orchard and Wilson, 2001; Krisnawati et al., 2011; FAO, 2016). It is known to be an environmental and congeneric weed (Randall, 2012; PIER, 2016) and can tolerate a wide range of environmental conditions and soil types, including nutrient depleted soil (Doran and Turnbull, 1997; Krisnawati et al., 2011). Considering these invasive traits and the fact that other members of the Acacia genus are vigorous invaders with the ability to transform and replace native ecosystems, risk of introduction for A. mangium is high and further research is needed.

Acacia mangium thrives in coastal tropical lowland forests, as in its native Australian habitat (Doran and Turnbull, 1997; Orchard and Wilson, 2001). It occurs naturally at the edges of mangrove stands, in the transition area between lowland primary forests and rivers and grasslands and in areas recently disturbed, especially by fire (FAO, 2016); Doran and Turnbull (1997) report it ‘may be regarded as [a component] of closed-forest but [is] more often found in marginal communities or in more extensive disturbances within the closed-forest, usually where the soils are of low fertility’. Duke (1983) reported that the species may be capable of ranging from tropical very dry to moist through subtropical dry to wet forest life zones, as is the case in Colombia and Costa Rica. In Antioquia, Colombia, the species is found at altitudes between 0-1500 m in Humid Forest Montano Under (BMH-MB), Rainforest Premontane (bh-PM), tropical wet forest (BMH-T) (Vascular Plants of Antioquia, 2016). Similarly, in Costa Rica, it reportedly grows in ‘Bosque húmedo y muy húmedo, cult. (a veces en plantaciones)’, (‘Humid and very humid forest, cultivated (sometimes in plantations)) at altitudes of 0-1100 m (Manual de Plantas de Costa Rica, 2016).

A status report on diseases of A. mangium where the species is currently being grown in plantations in tropical areas of South East Asia, the Indian subcontinent and northern Australia has been prepared by Old et al. (1997). Earlier reviews are provided by Lenné (1992) and Lee (1993).

Mehrotra et al. (1996) describe root and heart rots and their causal agents and potential control measures in plantations of A. mangium in West Bengal, India.

Root rot of A. mangium, caused by Ganoderma sp. and Phellinus spp., affects young stands in most places where A. mangium is now grown. Management options include the physical removal of stumps and woody debris and possible use of fungicides. A yellow, mottled, soft, light heart rot involving a range of wood decay fungi is a potentially serious source of wood degrade throughout all areas where the species is grown, with 50% of trees affected in some stands.
Cankers associated with decayed branch stubs and pruning wounds are good indicators of heart rot. Infected trees can continue to grow vigorously to maturity. Management options include adopting silvicultural practices that limit wounds to the stem, including early singling of multistemmed trees, short rotations and selecting provenances for slender branches and single stems.

Other significant diseases include stem cankers involving a range of pathogens, pink disease (Corticium salmonicolor) and phyllode rust (Atelocauda digitata).

Hutacharern (1992, 1993) lists a wide range of insect pests affecting A. mangium and identified a stem borer (Zeuzera coffeae), root pests (Coptotermes curvignathus and Sternocera aequisignata) and a shoot and stem girdler (Sinoxylon sp.) as serious pests. Insect damage to the wood by carpenter ants (Camporatus sp.), termites (Coptotermes sp.) and a Cerambycid wood borer (Xystocera sp.) has been recorded in localized areas in Sabah (National Academy of Sciences, 1983).

Pinhole beetles of 3 species (Xyleborus perforans, X. semiopacus [Xylosandrus crassiusculus] and an unidentified Xyleborus sp.) were observed on logs of A. mangium during pruning studies in an industrial plantation at Surigao del Sur, Philippines (Braza, 1995). Ho and Maznah (1995) found a total of 21 species of beetles from the family Scolytidae attacking both unseasoned and seasoned A. mangium timber.

Where introduced, researchers consider that exotic Acacia species are likely to face an ever-increasing threat from new pests and diseases in the future. One example is a new defoliator (Pelidnota filippiniae) reported from Brazil in 2011.

The species is relatively free from serious pests and diseases and main threats include fungal infections of heartwood, roots and leaves (Mead and Miller, 1991; Krisnawati et al., 2011). Main insects that attack nursery seedlings include grasshoppers and bagworms, as well as termites that feed on roots and stems (Krisnawati et al., 2011).

Duke (1983) reports: “There are problems with leaf insects. Mangium has symbioses with the bacterium Rhizobium and the fungus Thelephora. Specimens (ca. 12%) in Sabah suffer from a heart rot and a ‘pink disease’ (Corticium salmonicolor [Phanerochaete salmonicolor]). Seedlings in Hawaiian nurseries are attacked by a powdery mildew (Oidium sp.). Three pinhole borers attack the tree in Sabah, especially on poorer sites. Carpenter ants (Camponotus sp.) form galleries in the heartwood of young trees. Wood borers of the genus Xystrocera may be a problem. Seedlings may be defoliated by Hypomecessquamosus. Scale insects and mealy bugs may also be problematic with young plants”.

Although the species is commercially important, it has been recognized as a pest that outcompetes native species; in French Guiana, for example, farmers have attested to the expense, lack of tools and difficulty of physically removing trees (Aguiar et al., 2014; IABIN, 2016).

Acacia mangium can negatively alter native habitats by changing soil composition through nitrogen-fixation, outcompeting native species for water and light resources and crowding other vegetation by casting shade with its dense canopy. The species is also allelopathic and may prevent the germination of other species attempting to grow (IABIN, 2016).

The species is known to be a threat to native biodiversity, both in its native Australia and in introduced places such as Brazil, Indonesia and the Dominican Republic (Oviedo Prieto et al., 2012; IABIN, 2016; PIER, 2016). Its destruction of native habitats, alteration of soil composition and changes to the fire regime have been linked to the decline of vulnerable and near-threatened Australian species on the IUCN Red List (2016). These include the marsupial Sminthopsis butleri, found only on the Tiwi Islands where A. mangium had ‘recently been established’ as of 2001 (Orchard and Wilson, 2001), the black-footed tree rat Mesembriomys gouldii, which has had a marked population decline in Queensland and Western Australia over the last 10 years and the marsupial Phascogale pirata, which as of 2008 was estimated to have as low was 2500 mature individuals left in the wild (IUCN Red List, 2016).

Awang and Taylor (1993) provide a comprehensive monograph on the silviculture and utilization of A. mangium.This species is a fast-growing, relatively short-lived (30-50 years) tree, adapted to a wide range of acidic soils (pH 4.5-6.5) in moist tropical lowlands. It is intolerant of shade (Guzman et al., 1997). In China, A. mangium grows slowly when mean monthly temperatures fall below 17ºC, and the species is sensitive to frost. While it grows better on fertile sites with good drainage (but not excessively well drained), it will tolerate soils of low fertility and impeded drainage. It is killed by fire only if the stem diameter is less than about 10 cm. The root system is shallow and vigorous. Branches are persistent as the species does not naturally self-prune. Stem heart rot sometimes develops from dead branch stubs. Fluting of the bole is often a problem.In some locations, A. mangium has a tendency to form multiple stems. The cause of this is not fully understood although it does appear to be partly related to soil fertility, competition, and use of oversized spindly seedlings. Higher phosphate levels in the trees and less competition appear to encourage it. Turvey (1995) showed that the number of dominant stems increased as mean tree volume increased, suggesting a relation to conditions favouring faster growth rates. Stem straightness also varies with site, being poorer on sites with higher fertility where growth is fast (Mead and Miller, 1991). A. mangium is very susceptible to typhoon damage in areas prone to high winds such as Hainan Island (China), the Philippines and Vietnam.

Seed production areas and seed orchards have been established in Australia, Indonesia, Malaysia and Vietnam and these will eventually become the major source of seed for plantation establishment. However, at present most seed for planting comes from regions-of-provenance selected for superior productivity in far northern Queensland and Papua New Guinea. Flowering and seeding commence at about 2-3 years old under plantation conditions, with commercial quantities of seed available after about 4 years old. The methods described by Doran et al. (1983), including use of a mechanical flailing thresher and winnowing with a specially designed blower, are used for extracting and cleaning A. mangium seed in bulk.The seed has a very hard seed coat when fully ripened and retains viability well in storage. Seed of good initial physiological quality should give adequate germination rate after several years of storage in airtight containers in a dark, cool room. Mature seeds require a pregermination treatment, such as mechanical scarification of the seed coat or immersion in boiling water to break dormancy; immersion for 30 seconds or 1 minute with the heat source removed followed by a 24-hour soak in tap water is recommended (Bowen and Eusebio, 1981; Doran and Gunn, 1987; Sim, 1987). Germination is rapid after suitable treatment and typically exceeds 75%. There is an average of 63,600 viable seeds/kg (Doran and Turnbull, 1997). Nursery techniques are described in detail in Awang and Taylor (1993). Newly emerged seedlings should receive 50% shade; once established full sunlight is optimal. Use of a potting medium of topsoil mixed with compost or a mixture of tropical peat and rice husk in containers of up to 300 cc capacity is recommended. A twice weekly application of NPK fertilizer or use of a slow release fertilizer may be necessary if the potting medium is low in nutrients. Damping off, the most serious disease during the nursery stage, can be controlled by use of fungicide and cultural practices.In general, 3-4 months are needed to raise seedlings to plantable size (min. 25 cm in height). Inoculation with appropriate rhizobial and mycorrhizal strains is rarely necessary but may be beneficial, especially when seedlings are raised in sterilized media or planted on highly degraded soils. Research into vegetative propagation of A. mangium is continuing and progress to date is encouraging. Grafting (Monteuuis, 1995) and marcotting are possible but they are not suitable for large-scale production of clonal material. Stem cuttings of young A. mangium seedlings are easily rooted when they are treated with hormones and planted in suitable rooting medium under the correct environmental conditions in a misting chamber. However, the rooting percentage of stem cuttings decreases significantly with older stock plants (Wong and Haines, 1992; Poupard et al., 1994). Trees over four years old are very hard to strike from basal cuttings (Arisman, 1996). Micropropagation techniques can also be used, such as nodal bud culture and shoot apex micrografting (e.g. see Bhaskar and Subhash, 1996; Bon and Monteuuis, 1996).Although A. mangium is planted in industrial plantations it is also used for site amelioration and in agroforestry systems. The silvicultural practices employed vary depending on the objective of the planting. While A. mangium stumps coppice profusely, the coppice shoots lack vigour and are an unsuitable base for the second rotation. The second rotation may utilize the abundant natural regeneration that appears after logging debris is burned, but generally logged areas are replanted.Plantations for chipwood, pulpwood or firewood are usually harvested at 6-7 years, or about the point of maximum mean annual increment. Sawlog plantations with rotations of 15-20 years were envisaged in Malaysia (Mead and Miller, 1991) but these have become less likely due to incidence of heart rot.Plantations are generally established using containerized seedlings. Direct seeding has been tried but results are variable. Initial spacing is governed by the need for rapid canopy closure and the end products required. For high quality logs, sufficient numbers need to be planted to enable the selection of crop trees with excellent form. Practices vary: in Peninsular Malaysia, trees are planted at 900 stems/ha (3 x 3.7 m), in Sabah, stockings of 1075, 1250 and 1680 stems/ha are used.Site preparation for planting on former forest sites may involve simply burning logging debris in cut-over forest areas but in some areas of Indonesia logging debris is spread but, not burned. On Imperata grassland sites slashing or complete ploughing of the grass followed by chemical control is recommended (Otsamo et al., 1995). Chemical weed control almost doubled total volume production of A. mangium at 30 months on Imperata grassland in South Kalimantan, Indonesia, indicating its importance to successful establishment on these sites (Turvey, 1995). Weed control is applied during the first year, and occasionally in the second year. The method and frequency will vary with site and weed problem.A. mangium can fix nitrogen after nodulating with a range of Rhizobium and Bradyrhizobium strains in many tropical soils. It is, however, much more specific in its rhizobial affinities than A. auriculiformis (Dart et al., 1991; Prin et al., 1993). Galiana et al. (1996) reported a highly significant Bradyrhizobium strain x provenance interaction in field trials in Côte d'Ivoire. The best strain (Aust 13c) increased height growth by 10% and diameter by 15% in comparison with uninoculated trees or trees inoculated with another strain (CB 756). Estimates of percentage nitrogen due to atmospheric nitrogen fixation varied from 20 to 90% depending on site and treatment, and appeared to be negatively correlated with soil fertility. This nitrogen-fixing potential may only be realized in many soils if adequate fertilizer is applied. Dart et al. (1991) reported large responses to P fertilizer and sometimes K fertilizer while dela Cruz and Umali-Garcia (1992) reviewed several papers where responses to N fertilizer applied at the rate of 30-300 kg N/ha were reported. Nodule development was suppressed or reduced at higher N levels. A. mangium has associations with both ecto- and endomycorrhizal fungi. The ectomycorrhizal Thelephora sp. forms a beneficial association and Glomus fasciculatus and Gigaspora margarita have been shown to be effective vesicular arbuscular mycorrhizas (dela Cruz and Umali-Garcia, 1992).Use of fertilizer at establishment varies. In Peninsular Malaysia, Mead and Miller (1991) recommended the application of 100 g of triple superphosphate (TSP)/tree at planting followed by a second application of 150 g TSP at 5-6 months on fertile sites and, on less fertile Imperata sites, N and trace elements in addition to TSP. On degraded soils deficient in N, P and K in South Kalimantan, Turvey (1995) found that the addition of NPK (180, 78 and 150 kg/ha was best) fertilizer was one of the keys to successful establishment of A. mangium. This species showed a strong response to inorganic P and N fertilizers when planted on moderately acid (pH 5.0) soils in Mindanao, Philippines: seedlings that had been fertilized with 30g P2O5 and 30g N grew to nearly four times the height of unfertilized seedlings after three years (Manubag et al., 1995). In Hawaii on a degraded tropical soil of high acidity, high aluminium saturation, and low fertility which had been treated with 8 t/ha lime prior to planting, A. mangium seedlings responded well to the application of a complete fertilizer (143 kg/ha 14-14-14 NPK plus micronutrients), followed 4 months later by 200 kg/ha P and 77 kg/ha K (Cole et al., 1996).On sites where A. mangium develops multiple stems from its base the trees need to be singled before they are 1.5 m tall. Later singling is more time-consuming and runs the risk of encouraging butt rot. Turvey (1995) found that the more the growth of A. mangium is increased by silvicultural treatments such as weed control, cultivation and fertilizer, the more important it is to single stems early in the growth of the tree (probably the first month). He stressed the importance of selecting provenances that produce trees of one well-formed stem as well as giving fast early growth in domestication of this species. Plantations for chipwood, pulpwood or firewood harvested at 6-7 years are not usually thinned or pruned. When growing A. mangium on longer rotations for sawn timber, pruning is required to overcome the degradation caused by branches and associated heart rot, and regular heavy thinning is needed to maintain large deep crowns and fast diameter growth. The proposed management regime for A. mangium in Peninsular Malaysia is outlined by Buford Briscoe (1995) and involves singling at 1.5 m average height, two prunings at 6 and 9 m and four thinnings at 9, 13, 22 and 27 m leading up to clear felling of the 100 stems/ha remaining at 15 years when an average height of 28 m is anticipated.

Under optimal conditions, A. mangium grows very fast and reaches 20-25 m tall and 20-30 cm diameter at 10-13 years in Sabah (Jones, 1983). At Sabah Softwoods Sdn. Bhd. unthinned stands planted at 1075-1680 stems/ha reach their maximum MAI in volume (to a 10 cm top) at 6 years after planting on high quality sites (>21 cubic metres/ha at 8 years) and at age 7 years on poorer sites. Soil depth and topographic position can influence yield in A. mangium. Volume production on deep alluvium may be almost double that on the skeletal soils.Choice of the correct provenance for a particular planting site can have a major influence on growth rate and yield. On an Imperata grassland site in South Kalimantan, Indonesia, Tuomela et al. (1996) reported up to a threefold difference in volume production between the best growing provenances (60-90 cubic metres/ha) and the poorest performers (30-50 cubic metres/ha) at 26 months. In the same study, growth after singling and pruning at 8 months was found to be only about 70% of that of untreated plots. These treatments were deemed by the authors as being undesirable if growth rate is the first priority. In another trial in the same region, Otsamo et al. (1996) reported MAIs for A. mangium at 41 months of up to 39 cubic metres/ha. At Kampar Kiri-Riau, Indonesia, the best provenance in a trial was reported as giving an MAI of 41.4 cubic metres/ha at 2.5 years (Leksono et al., 1996).

There are large provenance differences in growth rate, stem straightness and frequency of multiple leaders. International provenance trials were established during the 1980s (Doran and Skelton, 1982). Results of these trials were reported by Harwood and Williams (1992). There were highly significant differences in performance between experimental sites (19), between provenance regions (5) and among the local provenances within provenance regions. Growth was generally fast at near-equatorial trial sites, with mean annual height increment around 3-4 m, and slower for sites further from the equator. Papua New Guinea provenances were consistently the best performers, closely followed by the Claudie River provenance from north Queensland. The slowest growing provenances were from the Maluku province of Indonesia and southerly parts of the distribution in Queensland. Other studies have given similar results with local seed sources such as the extensively used Subanjeriji seed production area, South Sumatra, Indonesia, amongst the poorer performers (Nguyen Hoang Nghia and Le Dinh Kha, 1996; Otsamo et al., 1996; Tuomela et al., 1996).Since establishment of these trials, further comprehensive seed collections have been undertaken and Australian and several South East Asian organizations are developing improved breeds from wide bases of the best provenances, there being low levels of genotype-by-environment interaction at the provenance level (Harwood, 1996). Expected gain in volume production in progeny from seedling seed orchards of A. mangium in Indonesia is on the order of 50-70% compared to local selections planted in the same orchards (Kurinobu and Nirsatmanto, 1996). Selection criteria of importance in addition to fast growth are propensity for producing single straight stems, light branching and high wood density. The level of genetic diversity in the nuclear genome of A. mangium is moderately high (Butcher et al., 1996). This contrasts with the results of isozyme studies that indicated low levels of genetic variability within and between populations in this species (Moran et al., 1989), although Harwood and Williams (1992) suggested that isozyme studies only examine a small sub-set of the genome, and therefore do not provide an absolute measure of overall genetic variation. Butcher et al. (1996) found that the genetic differentiation between populations as estimated by RFLP analysis was consistent with geographic discontinuities in the species distribution. The level of genetic diversity was highest in New Guinea and declined from north to south in the Australian populations. The lowest diversity was recorded in Ceram (Muluku, Indonesia) and Sidei (Vogelkop Peninsula, Irian Jaya). The ranking of provenances based on growth performance in provenance trials corresponded with rankings based on levels of genetic diversity. The Subanjerijii seed production area had relatively low levels of genetic diversity consistent with populations in the Daintree-Townsville region which are the putative source populations. Acacia mangium is known to form natural hybrids with A. aulacocarpa, A. auriculiformis and A. polystachya. The hybrids with A. auriculiformis are intermediate between the two parents in morphology and wood properties. They inherit the better stem straightness of A. mangium and the self-pruning ability and better stem roundness of A. auriculiformis (Turnbull and Awang, 1997). Their growth is sometimes more vigorous and resistance to heart rot is better. There is much interest in the domestication of this hybrid because of this combination of commercially desirable characteristics. Aspects of seed production and vegetative propagation of the hybrids are covered in papers in Carron and Aken (1992).The use of A. mangium as a parent of hybrids, particularly in combination with A. auriculiformis, is of great potential. Many hybrids show desirable commercial characteristics such as fast growth, fine branching and straight boles. F1 hybrid trees in Vietnam produced 300-500% greater wood volume than the parental species at 2.5-3 years and at 4.5 years old hybrids produced, on average, twice the wood volume of A. mangium (Le Dinh Kha, 1996). Sedgley et al. (1992) found that the cross A. auriculiformis x A. mangium was more successful than the reciprocal, but fertile seed was 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 and Sedgley, 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 and Havmoller, 1994). Hybrid clones selected for outstanding growth and form and propagated by tissue culture of meristems are being tested on suitable sites in Vietnam (Le Dinh Kha, 1996). Vegetative propagation of the hybrid by striking cuttings from coppice shoots is being used in Bangladesh (Banik et al., 1995).The Australian Tree Seed Centre of CSIRO Forestry and Forest Products, Canberra, Australia, maintains seed stocks of representative provenances from throughout the natural range of the species.

A. mangium is unlikely to grow well on sites where there is a severe period of drought, incidence of frost or high soil pH. It may produce multiple stems and heavy persistent branching, particularly when growing fast, requiring selection and breeding within fast-growing provenances and singling to improve form.A. mangium is susceptible to heart rots, which degrade logs for exploitation as sawn timber, and to wind damage, especially in typhoon-prone areas. This species could become a weed under certain conditions.

Considering its known weediness and invasiveness in various parts of the world beyond its native range, including parts of South America, the Caribbean and Asia Pacific, recommended areas of research include the extent to which this species escapes from cultivation, particularly in places where it is widely cultivated for commercial use. Prevention is difficult due to the dispersal of seeds by birds; research on additional methods of mechanical, chemical and biological control is needed.