Bruguiera (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Bruguiera Lamk

Protologue: Tabl. Encycl. Meth., Bot.: t. 397 (1793).

Family: Rhizophoraceae

Chromosome number: x= 18;B. gymnorhiza: 2n= 36 (+ 1B)

Vernacular names

• Bakau (trade name). Black mangrove (En)
• Indonesia: tanjang (Java)
• Malaysia: berus, lenggadai, tumu (Peninsular)
• Papua New Guinea: black mangrove (En), mangro (pidgin)
• Philippines: pototan (general), bakauan (Tagalog). Burma (Myanmar): byu
• Cambodia: prasak
• Thailand: prasak
• Vietnam: vẹt.

Bakau is the standard Malaysian name for the timber of the Rhizophoraceae genera Bruguiera , Ceriops and Rhizophora , whereas in Indonesia it applies only to Bruguiera and Rhizophora .

Origin and geographic distribution

Bruguiera consists of 6 species and is distributed from tropical East Africa and Madagascar, through South and South-East Asia to north-eastern Australia, Micronesia and Polynesia. B. gymnorhiza has the broadest range of distribution, covering the entire area of the genus, but all species are comparatively widespread. The South and South-East Asian region probably represents the centre of origin.

Uses

The wood of Bruguiera is used for construction (columns, beams and rafters), piles, telegraph poles, pit props, railway sleepers, fish stakes, tool handles, scaffolding, occasionally for furniture, cabinet work and parquet flooring, but the use for firewood and charcoal is generally more important. Moreover, it is suitable for the production of pulp by the sulphate process and the trees are increasingly felled for this purpose, especially in Borneo.

The germinating seeds and sometimes the leaves are boiled and eaten as a vegetable or preserved. A condiment and an adhesive can be obtained from the bark. The bark of B. gymnorhiza is extensively used for tanning leather and fishing nets. The fruit may be applied as an astringent in betel quid or may be chewed as a substitute for betel nut ( Areca catechu L.). The medicinal use of the fruit includes its application in the treatment of shingles and eye diseases. The bark is used as an astringent medicine against diarrhoea and sometimes malaria. The roots and leaves are used to cure burns.

Production and international trade

Although Bruguiera wood is usually lighter in weight and in colour than Rhizophora wood, it is generally traded together. The production of tannin from B. gymnorhiza is important, and at present Borneo accounts for an important portion of world supplies. In Malaysia the production of firewood and charcoal are more important.

Properties

Bruguiera yields a medium-weight to heavy hardwood with a density of 630-1010 kg/m³at 15% moisture content. The heartwood is pale pinkish-brown to red-brown or dark brown (grey in B. parviflora ), not clearly demarcated, but sometimes distinct from the pale brown sapwood; grain usually interlocked or sometimes straight; texture fine and even; wood with conspicuous silver grain. Growth rings absent, occasionally visible due to zones of fewer vessels; vessels small to very small and indistinct to the naked eye, mostly in radial multiples of 2-3, occasional tyloses present; parenchyma scanty paratracheal, indistinct even with a lens; rays generally visible except in B. parviflora ; ripple marks absent.

Logs shrink and check excessively during seasoning. The wood is strong to very strong and hard to very hard. The wood works and finishes well. It is non-durable to moderately durable when exposed to the weather or in contact with the ground. Poles of B. gymnorhiza , however, may have a service life of 10 years. B. parviflora is susceptible to attack by dry-wood termites, the sapwood is generally regarded as non-susceptible to Lyctus . The wood is easy to treat with preservatives: B. gymnorhiza wood absorbs 200 kg/m³.

The energy value of the wood of is 18 950-20 200 kJ/kg. The tannin content of the bark (based on dry weight) is 6-26% for B. cylindrica , 21-36% for B. gymnorhiza , 9-17% for B. parviflora and 27-32% for B. sexangula .

See also the tables on microscopic wood anatomy and wood properties.

Botany

Small to fairly large trees up to 40 m tall, bole up to 70(-90) cm in diameter, buttresses present, with kneed pneumatophores, sometimes with stilt roots; bark surface fissured to smooth, distinctly or obscurely lenticellate, greyish to blackish or brown, inner bark usually pinkish, often fibrous. Leaves decussate, simple, leathery, entire, usually pointed at apex, black dotted below, petiolate; stipules present. Flowers either solitary and large, or in up to 6-flowered cymes and small, bisexual, 8-16-merous; calyx lobed; petals 2-lobed, with appendages; stamens twice as many as the petals, enclosed in pairs by the pouched petals; ovary inferior, 2-4-locular with 2 ovules in each cell, style obscurely (2-)3(-4)-lobed. Fruit a berry, included in and adnate to the calyx tube, with persistent calyx lobes, 1(-2)-seeded, viviparous. Seedling with cotyledons united at base; hypocotyl cigar-shaped, terete or obscurely ribbed, blunt, falling with the fruit.

Bruguiera trees grow slowly. In Sarawak B. cylindrica takes 60 years to attain a diameter of 18.5 cm, whereas B. parviflora and B. gymnorhiza take 40 years to attain 16 cm in diameter. Growth decreases with decreasing salinity as in the process of sedimentation in mangrove forest. Tree form is according to Aubréville's architectural model, characterized by a monopodial trunk having rhythmic growth and plagiotropic branches (e.g. B. gymnorhiza ), or according to Attims's model, with a monopodial trunk having more continuous growth and orthotropic branches (e.g. B. parviflora ). Large-flowered species with pendent flowers are visited by birds such as honey eaters or by insects; small-flowered species with erect flowers are often visited by butterflies. The pouched petals "explode" when triggered, scattering pollen on the visitor. In Indonesia trees bear fruit in July-December. In north-eastern Papua New Guinea they flower in July-August. The fruits are dispersed by water.

Bruguiera is related to the other "mangrove genera" Ceriops , Kandelia and Rhizophora . It can be distinguished particularly by its more numerous calyx lobes, its hypocotyl falling with the fruit, and the explosive mechanism of pollen release.

Ecology

Bruguiera is a characteristic element of the middle and inner mangrove community, extending into the transitional landward communities, and often ascending tidal parts of rivers. Its salinity tolerance is 1-3%. Annual rainfall should be over 1000 mm. B. gymnorhiza trees are often the largest and oldest ones of the mangrove, and their presence marks the final stage of the succession of mangrove towards swamp or dry-land rain forest. Some species, e.g. B. cylindrica and B. parviflora , behave like pioneers and may form pure stands. B. cylindrica is characteristic of stiff clay soils with negligible aeration but does not tolerate prolonged immersion of its pneumatophores.

Silviculture In the Philippines seeds of B. cylindrica germinate completely in 2-18 days after sowing, compared with 2-5 days for B. gymnorhiza and 5-10 days for B. sexangula . In Indonesia seedlings of B. gymnorhiza are collected from the trees or from the ground and can be planted out at 3 m × 1 m after a period of 3-4 months in the nursery. Nurseries outside the mangrove forest should be made by planting fruits in polythene bags filled with brackish clay continuously soaked with seawater or a solution of approximately 30 g NaCl/l of water. Plants develop best where the tidal range is only about 0.35 m and the salinity is 1-2.5%. The seedlings can remain alive in the water for 5-6 months. Little is still known about the silvicultural management of mangrove forests, although some experimental work has been done on natural and artificial regeneration. B. gymnorhiza inhibits natural regeneration under its canopy. In Thailand the mortality during the first year of B. gymnorhiza planted at 1 m × 1 m was 80% in mangrove forest and 9% on cleared areas. In general, clear felling with natural regeneration can be carried out in strips perpendicular to the coast, leaving several mother trees. B. cylindrica is a prolific seed bearer and a healthy forest normally develops even after clear felling. The young and pure stands thus formed may be extremely dense and may contain 55 000-70 000 poles per ha. Due to its extremely slow growth the rotation should be longer than 20 years, even when harvested for charcoal and chipwood. B. cylindrica does not reach exploitable size until about 40 years old. For B. gymnorhiza a cutting cycle of 10-20 years is suggested, and a rotation of 30 years is applied in mangrove forests in Thailand. Bruguiera does not coppice. In mangrove forest in Peninsular Malaysia the fern Acrostichum became a weed and had to be controlled after the forest had been logged. In West Kalimantan the total standing volume of B. gymnorhiza trees over 10 cm diameter is estimated at 17.8 m³/ha, accounting for over 20% of the total standing volume of this tree class.

Genetic resources and breeding

Most Bruguiera species occur commonly or even gregariously in the mangroves of South-East Asia. However, they are still locally threatened in areas where mangrove vegetation is indiscriminately cleared.

Prospects

Since B. cylindrica is one of the few economic species growing in brackish, anaerobic soil conditions and having a very long harvesting cycle, it needs special attention in mangrove management programmes. Cutting Bruguiera to extract tannin and produce charcoal leads to a gradual transformation into a Rhizophora -dominated vegetation. This process is hastened by the introduction of Rhizophora seedlings. The slow growth of Bruguiera makes it more suitable for planting for charcoal or chipwood than for sawn timber.

Literature

37, 61, 151, 186, 260, 267, 300, 304, 333, 341, 343, 348, 402, 439, 696, 708, 773, 829, 845, 934, 1038, 1048, 1101, 1221, 1242.