Fig. 3. Phyllode of A. mangium with four longitudinal veins.
Growth phenology
A. mangium 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
Flowering in Acacias is precocious. A. mangium starts to flower and produce seeds 18-20 months after planting (Mergen et al. 1983). Mature fruits occur 3-4 months after flowering period. The time from the onset of flower buds to pod maturity is about 199 days (Zakaria 1993). Flowering phenology differs throughout its natural and planted range. In natural habitat, its 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 Kamis 1991), January in Sabah (Sedgley et al. 1992), October-November in Taiwan (Kiang et al. 1989) and September in Thailand (Kijkar 1992).
Inflorescences, flowers and fruits
The inflorescence consisting of many tiny flowers, occur as rather loose spikes up to 10 cm long, singly or in pairs in the upper axils (Fig. 4). 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 3-5×2-3 mm with a yellow or bright orange (rarely red) funicle folded to form an oily, fleshy aril beneath the seed (Fig. 4).
Fig. 4. A. mangium inflorescence in blooming stage.
Reproductive Biology and Breeding System
Floral biology
A. mangium flower is regular in symmetry, consisting of five sepals, five petals, numerous stamens and one gynoe-cium. It has a mild and sweet fragrance, which is particularly distinct in the early morning when individual flowers are in boom. (Zakaria 1993). Stigma is non-papillate, measure 63 microns in diameter and forms a cup shaped depression at the tip of style. Stigma and anthers lie in same plane. The anther is bilobed and measures 183 microns. Each lobe has four separate loculi with each loculus enclosing a polyad (Composite pollen grains). The polyad is spherical in shape with diameter of 30-40 microns and each polyad consists of 16 pollens. On an average, there are about 113 stamens per flower. The ovary is sessile, normally with minute hairs, with 12-14 ovules per ovary. Flowers are generally hermaphroditic. However, in some inflorescence staminate flowers are also present (Zakaria and Kamis 1991).
Breeding system
A. mangium is generally an outcrossing species with the tendency toward selfing (Zakaria 1993). In A. mangium andromonoecy-spatial separation of sexes is not prominent. In terms of temporal separation of sexes, protogynous dichogamy is not prevalent. Anthesis occurs very early in the day, with flowers opening in the preceding night at about 21:00 hr. The synchronous emergence of styles and stamens, the immediate anther dehiscence, and stigma receptivity after anthesis signify that the flowers of A. mangium are homog-amous (Zakaria 1993). Zakaria (1993) also found that the species index of self-incompatibility (ISI) rating was 0.38, which could lead it to be classified as an out-crossing species with some degree of selfing despite being partially self-incompatible. Its partial self-incompatibility is probably due to the presence post zygotic lethal genes as in case some other Acacia species.
A. mangium requires biotic agents to transfer pollen from anthers to the stigmas. Pollinators are mainly entomophilic, with Trigona and Apis spp., as the consistent pollen vectors. The most active time of day for these pollinators is between 07:30 and 11:00, after which their activity decreases; and very few pollinators are observed in the day. In spite of dense and conspicuous inflorescence, A. mangium fails to attract a more varied spectrum of pollinators, probably because it lacks floral nectaries (Zakaria 1993).
Hybridization
A. mangium has a chromosome number of 2n = 26 as same as in A. auriculiformis, so it often readily hybridizes with A. auriculiformis. Hybrids of A. mangium × A. auriculiformis have the potential to become an important source of planting material for plantation forestry. The hybrid seems to be more resistant to heart rot than A. mangium. Moreover, the hybrid has the straight bole and stem of A. mangium and the self-pruning ability of A. auriculiformis (Zakaria 1993). F1 hybrid trees between A. mangium and A. auriculiformis in Vietnam produced 300-500% greater wood volume than the parental species at 2.5-3 years and at 4.5 years old hybrids, on average, twice the wood volume of A. mangium (Le 1996). Sedgley et al. (1992) found that the cross A. auriculiformis × 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 programmers of these species (Harbard and Sedgley 1994).
Genetics and Improvement
Initial plantings of A. mangium outside its natural distribution range had generally relied on unimproved materials usually from a narrow genetic base. Consequently, the growth obtained was variable and productivity tended to decline over several generations due to genetic erosion (Awang and Bhumibhamon 1993). Elaborate tree improvement activities are now being taken up in many countries where it has been introduced for production of better planting materials with consistent, desirable characteristics.
Provenance variation
A. mangium has a fragmented natural distribution stretching from the Moluccas islands in Indonesia to Western Province of Papua New Guinea and northeastern Queensland in Australia. Many provenances highly adapted to their natural habitats have been identified and studies have shown variation among them in all respects (Awang and Bhumibhamon 1993). For example, 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). One of the international provenance trials are shown in Table 1 (Awang and Taylor 1993).