3038

11: Auxiliary plants

Rhizophora apiculata Blume

Enum. pl. Javae 1: 91 (1827).

RHIZOPHORACEAE

2n = 36

Rhizophora candelaria DC. (1828), Rhizophora conjugata Arnott (1838), non L. (1753).

Brunei: bakau minyak, bakau. Indonesia: bakau minyak (general), bako (Javanese), babakoan laut (Sundanese). Malaysia: bakau minyak, bakau tandok, bakau akik. Papua New Guinea: abia (Gulf Province), bahkweh (Northern Province), pana (Central Province). Philippines: bakauan (lalaki), uakatan (Tagalog), bakhau (Samar). Singapore: bakau minyak, red-tree. Burma (Myanmar): pyoo. Cambodia: kaông ka:ng nhi:. Thailand: kongkang-bailek, kongkang. Vietnam: c[aa]y d[uw][ows]c.

Rhizophora apiculata is commonly found in most mangrove swamps in tropical Asia, from the delta of the Indus in Pakistan to Vietnam and Hainan. It occurs throughout the Malesian region and reaches southwards to the Tropic of Capricorn in Queensland, and eastwards as far as New Caledonia and Ponape (Micronesia).

The wood of Rhizophora apiculata can be split easily and has a high energy value, making it in great demand as firewood and for making charcoal. In recent years it has been extensively harvested for production of wood chips in East Malaysia and Indonesia. Rhizophora apiculata is the preferred species in replanting programmes in most mangrove regions in South-East Asia. Poles are used for piling and construction purposes, and as fishing stakes. The timber is suitable for making furniture. The branched stilt roots weighted with stones serve as anchors. The bark is rich in tannin, used for tanning leather and to toughen and dye fishing lines, ropes and nets. The bark provides a medicine against dysentery.
Rhizophora hypocotyls can be eaten after extraction of the tannin, but this is probably only of importance in times of famine.

In all parts of tropical Asia, Rhizophora apiculata is cultivated on a commercial scale for production of firewood and particularly charcoal, poles and tannin. Few production statistics from natural or planted stands are available. In Vietnam, annual wood production is about 60 000 t.

The energy value of the stems, branches and prop roots is 15 000—19 000 kJ/kg, of charcoal 32 200 kJ/kg. The ash content is about 1 g per 100 g wood of the stem and prop roots, and 2 g in branch wood. Leaf samples in the Matang Mangrove Forest which consists of almost pure stands of Rhizophora apiculata contain per 100 g dry matter: N 0.4—1 g, P 0.1 g, K 0.9—1.2 g, Ca 1.1—2.0 g, Mg 0.4—0.8 g, Na 1.6—1.9 g. The quantity of tannin in the bark is very variable, 8—40% in air-dried bark. The tannin of Rhizophora is associated with a substance which darkens gradually; it is used as a deep brown or black dye. The bark, according to some analyses, contains large quantities of pentosans and furfurol. After extraction of the tannins, the ash mainly consists of calcium carbonate (70%) and lime (18%) and can be used as fertilizer.
The wood of Rhizophora apiculata is hard, strong and heavy with an air-dry density of 960—1170 kg/m³. The sapwood is light yellow, 3—5 cm thick, and very distinct from the heartwood which is reddish-brown and darkens with age. Growth rings and parenchyma are indistinct. Pores are small, circular, fairly numerous, straight, solitary and in short radial groups, mostly in pairs, and frequently with dark gummy deposits. Rays are numerous, straight, forming conspicuous silvery grains, narrower than the pores, and visible to the naked eye.

Evergreen tree, up to over 30 m tall and with trunk up to 50 cm in diameter, generally much smaller in exploited forests; bole 10—12 m; stem supported by numerous, lateral, much branched stilt roots; aerial roots sometimes develop from the lower branches; taproot usually abortive; branching primarily sympodial. Bark grey, almost smooth or with vertical fissures. Branchlets swollen at the nodes, solid and pithy. Leaves decussate, rosette-like at the end of twigs; stipules lanceolate, 4—8 cm long, conspicuous, caducous; petiole 1.5—3 cm long, reddish; blade entire, elliptical-oblong to sublanceolate, 7—18 cm x 3—8 cm, leathery, green and shiny, apex acute to apiculate, base cuneate, veins distinct above, obscure beneath, glabrous with minute, scattered black corky warts on the lower surface, visible on older or dried leaves. Inflorescence axillary (in leaf scar below the leaf rosette), 2-flowered; peduncle thick, 0.5—1.5 cm long; bracteoles at the base of flower, cup-shaped, fleshy, crenulate; flowers bisexual, sessile, yellow; calyx deeply 4-lobed, coriaceous, accrescent and reflexed in fruit, lobes ovate, 10—14 mm x 6—8 mm, concave, acute, brown-yellow to reddish, persistent; receptacle with a disk; petals 4, free, lanceolate, 8—11 mm x 1.5—2 mm, membranous, glabrous, early caducous; stamens mostly 12, sessile, anthers 6—7.5 mm long, acute, multi-loculate, opening with a large ventral valve; ovary semi-inferior, 2-celled, superior part enclosed by the disk, bluntly conical, 1.5—3.5 mm long; style 0.5—1 mm long, 2-lobed. Fruit an ovoid or inversely pear-shaped berry, 2—3.5 cm long, rather rough, brown. Hypocotyl cylindrical to club-shaped, up to 40 cm x 1.2 cm before falling, often slightly curved, more or less blunt, smooth and shining, green tinged with red.

The stem of Rhizophora apiculata is upright and cylindrical in closed forest, but plants develop a straggling or semi-prostrate habit in unfavourable sites. Flowers are self-compatible and usually wind-pollinated. Insects have occasionally been observed foraging for pollen. Vivipary is characteristic for Rhizophora species. One-seeded fruits start to germinate when still hanging on the tree. The root protrudes from the fruit, producing a green, spindle-shaped rod (hypocotyl) of up to 40 cm long. Eventually, the seedling falls from the fruit, floats with the high tide and establishes if it reaches a suitable site. Seedlings may retain their viability for several months.
Average annual increase in diameter over a 30-year period in Matang, Malaysia, was 0.32 cm. Litterfall varies with stand vigour and age. Estimates of annual litterfall vary from 6—11.5 t/ha.

In South-East Asia 3 Rhizophora species occur: Rhizophora apiculata, Rhizophora mucronata Poiret and Rhizophora stylosa Griffith. Rhizophora apiculata is slightly more common than Rhizophora mucronata to which it is closely related. They can be distinguished in the field by some easily observed characters: bark grey, almost smooth, with vertical fissures in Rhizophora apiculata; in Rhizophora mucronata the bark is nearly black or reddish, rough or sometimes scaly. Inflorescence in Rhizophora mucronata longer, forked 2 or 3 times, with more numerous flowers; hypocotyl longer (35—65(—90) cm). Rhizophora stylosa has broadly elliptical leaf blades, up to 12 cm x 7 cm, flowers with styles 4—6 mm long and the hypocotyl up to 30 cm long. A few specimens have been collected with characters intermediate between Rhizophora apiculata, Rhizophora mucronata and Rhizophora stylosa in western Malesia and western New Guinea.

Rhizophora apiculata is the most common mangrove species. It grows gregariously in swamps flooded by normal high tide, on deep soft mud of estuaries, often consolidated and sheltered from surf and currents by pioneer species of Avicennia L. and Sonneratia L. Rhizophora apiculata avoids hard soils and develops well in per-humid regions where it can form almost pure stands, sometimes in association with Bruguiera spp. or Rhizophora mucronata. It does not occur in fresh water swamps. It is killed by frost and by extended periods of near-freezing temperatures.

The best way to regenerate a mangrove stand at the least cost is to encourage reproduction in the period before the final harvest by thinning and by minimizing damage to young plants during harvesting. Damaged young trees are capable of recovering by sprouting from dormant buds and bending upwards to form another erect stem. Additional planting and planting in denuded areas mostly succeeds well, provided the ecological conditions are suitable. Natural regeneration is often good, provided sufficient seed trees are left after harvesting.
In nurseries shade is neither advantageous nor harmful. Seedlings tend to be taller in shade and produce fewer roots.
Planting programmes often coincide with the fruiting season. Mature propagules that remain viable for 4—5 months are gathered from the forest floor. The planting procedure is simple, involving inserting the propagules vertically into the muddy soil along predetermined lines and spacings. In sites where attempts at planting had previously failed due to pest problems, planting nursery-raised seedlings and transplanting of wildings proved successful. Wildings are readily available, as natural regeneration is often profuse.
More intensive site preparation is required where flooding is limited due to large numbers of crab mounds and where there is severe infestation of Acrostychum ferns.

In Peninsular Malaysia it takes 35 years for Rhizophora apiculata to reach a stem diameter of 19 cm at breast height. Judging by the greatest volume production of firewood, a 40-year rotation is preferable. Current rotations vary from 15 years (in firewood plantations in Thailand) to 20—30 years, but may be even shorter. Thinning is important for good stand development. Three thinnings are prescribed in Matang, Malaysia (at 15, 20, and 25 years of age), one thinning in Indonesia. In practice, thinning is irregular, incomplete and selective, and poorly accessible stands are often neglected. As thinning is a commercial operation, its timing, as well as the selection of tree species and stem diameter are influenced by market demand.

Propagules of Rhizophora spp. are sometimes attacked by a scolytid beetle (Poecilips fallax). Occasionally, bagworms and larvae of the moth Strelote lipara cause localized defoliation of trees and seedlings. In some plantations, long-tailed macaques (Macaca fascularis) and grapsid crabs (Sesarma spp.) have been reported to be the major pests of newly planted propagules.
The ferns Acrostichum aureum L. and Acrostichum speciosum Willd. may occur throughout South-East Asia as a low, tufted ground cover under the canopy. With the opening of the canopy, the ferns may form up to 4 m tall, dense, continuous thickets, making it impossible for propagules of Rhizophora to enter the area. Large areas of formerly productive Rhizophora stands have been made unproductive in this way. Uprooting the ferns manually with iron bars or spraying with a herbicide can solve the problem, unless the inundation regime has changed. Derris trifoliata Loureiro can also be a serious strangling weed.

Felling of Rhizophora apiculata for poles is essentially a thinning operation in which straight pole-sized individuals are cut. If cut at least 20 cm above the stilt roots, regeneration is without problems. The poles are carried out of the swamp. In Malaysia, stick thinning is practised. This process involves selecting a well-formed tree and felling all pole-sized trees around it within a radius drawn by a stick 1.2—1.8 m long. The system of final harvesting of trees for fuelwood varies between countries. Thailand and Indonesia adopt strip-felling, the Philippines and East Malaysia practise minimum diameter harvesting, while clear-felling has been traditionally carried out in Peninsular Malaysia. Minimum diameter for charcoal production varies from 20.5 cm in Sabah to 4 cm in Vietnam. Felled trees are bucked into billets of about 1 m long, and sometimes debarked before they are transported out of the swamp forest. In large concessions of 2000—4000 ha for wood chipping in Sabah, a diameter limit of 10.2 cm is used, provided that 100 seed trees per ha are left. Large concessions in Indonesia require felling of trees down to a stem diameter of 7 cm in 50 m wide strips interspaced with 20 m undisturbed strips in a proposed 20-year rotation. Thailand has prescribed the maintainance of a 10 m uncut strip along waterways to reduce the effects of waves and currents on regeneration.

A stand of Rhizophora apiculata in southern Thailand had an annual leaf production of 7 t/ha and 20 t/ha of wood. Total aboveground dry matter has been estimated to be 160—190 t/ha in 15-year-old stands in Thailand and 257 t/ha in a 28-year-old stand in Peninsular Malaysia.

It is unlikely that any substantial germplasm collections of Rhizophora apiculata are being maintained. There are no known breeding programmes.

Rhizophora forests are being heavily exploited for fuelwood and poles. Recently, they have also been extensively harvested for woodchips or converted for agricultural and aquacultural purposes. Long-term, multiple-use management plans have to be developed and implemented to ensure sustainable use of the remaining resource. Rhizophora apiculata, being among the most easily regenerated and widely planted species, will play an important role in those sustainably managed systems.

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Hou, D., 1958. Rhizophoraceae. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana Series I, Vol. 5. Noordhoff-Kolff, Jakarta, Indonesia. pp. 448-457.
Putz, F.E. & Chan, H.T., 1986. Tree growth, dynamics, and productivity in a mature mangrove forest in Malaysia. Forest Ecology and Management 17: 211-230.
Srivastava, P.B.L., Keong, G.B. & Muktar, A., 1987. Role of Acrostichum species in natural regeneration of Rhizophora species in Malaysia. Tropical Ecology 28: 274-288.
Srivastava, P.B.L., Majid, N.M. & Shariff, A.H., 1980. Foliage and soil nutrients in Rhizophora apiculata Bl. stands. Tropical Ecology 21: 113-124.
Tamai, S. & Iampa, P., 1988. Establishment and growth of mangrove seedlings in mangrove forests of southern Thailand. Ecological Research 3: 227-238.
Tomlinson, P.B., 1986. The botany of mangroves. Cambridge University Press, Cambridge, United Kingdom. 413 pp.
van Vliet, G.J.C.M., 1976. Wood anatomy of Rhizophoraceae. Leiden Botanical Series 3: 20-75.

D. Hou & H.T. Chan

Hou, D. & Chan, H.T., 1997. Rhizophora apiculata Blume. In: Faridah Hanum, I & van der Maesen, L.J.G. (Editors): Plant Resources of South-East Asia No 11: Auxiliary plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea