1417

3: Dye and tannin-producing plants

Rhizophora mucronata Poiret

Lamk, Encycl. 6(1): 189 (1804).

RHIZOPHORACEAE

2n = 36

Rhizophora macrorrhiza Griffith (1836), Rhizophora latifolia Miq. (1861), Rhizophora mucronata var. typica A. Schimper (1891).

Brunei: bakau kurap, lenggayong. Indonesia: bakau, bakau hitam (general). Malaysia: bakau hitam, bakau jangkar, bakau kurap. Papua New Guinea: mangoro (Pidgin). Philippines: bakáu (many languages), bakáuan (Tagalog, Bisaya), tong (Bisaya). Singapore: bakau laut, belukap. Burma: pyoo. Cambodia: doeum prasak. Thailand: kon gkang bai yai, phangka. Vietnam: dung, duoc bop, duoc rung cam.

Trees of Rhizophora mucronata grow on the shores of the Old World tropics, from East Africa through Madagascar, islands of the Indian Ocean, the south-eastern mainland of Asia, Indonesia and the Philippines, to north-eastern Australia and the South Pacific islands as far as the Tonga group. In 1922 this species was introduced into Hawaii and is naturalized there.

In the Malesian Archipelago, the bark of mangrove trees (chiefly Rhizophora mucronata and/or Rhizophora apiculata Blume) is an important source of tannin. It is used for tanning leather and to toughen and dye lines, nets, and ropes used by fishermen. According to laboratory investigations, mangrove tannin extracted from the bark could be used to produce adhesive for the manufacture of plywood and particle board. It is used also occasionally as medicine in cases of haematuria. The trees are important for producing good quality charcoal and for firewood. A great advantage of Rhizophora L. in the eyes of firewood dealers is that it can easily be split. The trees are also used for fish-traps. The use of the wood is limited because of its light weight, poor durability and small size of the trunk.

Rhizophora mucronata is hardly cultivated for commercial purposes. It is grown on a very small scale for firewood, chiefly for local consumption, e.g. in Luzon (the Philippines).

The quantity of tannin in the bark may vary greatly. In air-dried bark the tannin content varies from 8—40%. The tannin is sometimes extracted and concentrated into cutch. The bark, according to some chemical analyses, appears to contain high percentages of pentosans and furfurol. The ash, after the cutch has been extracted, consists mainly of lime (18%) and calcium carbonate (70%), and can be used as fertilizer. The tannin of Rhizophora is associated with a substance which darkens gradually; it is used as a deep brown or black dye. The wood shows a beautiful silver grain on radial section and the heartwood is dark orange-red.

Tree up to 27(—30) m tall and with trunk 50—70 cm in diameter; taproot usually abortive; lateral roots numerous, developed from base of the trunk, much branched, usually called stilt roots, hoop or pile-like, supporting the tree; hanging air-roots are sometimes also produced from the lower branches; stem in closed forest cylindrical, or developing a straggling or semi-prostrate habit especially in unfavourable conditions; bark almost black or reddish, rough or sometimes scaly, with prominent, horizontal cracks almost encircling the stem. Leaves leathery, broadly elliptic to oblong-elliptic, (8.5—)11—18(—23) cm x 5—10.5(—15) cm, with very distinct black dots on the undersurface, tapered at both ends and tipped with a fine spine, glossy green above and paler beneath; petiole 2.5—5.5 cm long; stipules large, 5.5—8.5 cm long, pinkish or reddish, sticky. Inflorescences axillary, 2 or 3 times forked, rather loosely (1—)3—5(—12)-flowered; peduncles 2.5—5 cm long; flowers with 4—8 mm long pedicels and united, cup-shaped bracteoles at the base; calyx deeply lobed, 13—19 mm long, pale yellow or almost white; petals lanceolate, 9 mm long, light yellowish, densely hairy along the margins, sparsely hairy on the back; stamens 8, sessile, equal, anthers 6—8 mm long; ovary semi-inferior, free part high conical, 2.5—3 mm high, style very short, 0.5—1.5 mm long, obscurely 2-lobed. Mature fruit an elongately ovoid berry, 5—7 cm x 2.5—3.5 cm, with hardly contracted apex and often rugose base, dull brown-green. Seedlings with cotyledons 2—4 cm protruding from the fruit; hypocotyls hanging, cylindrical, 36—64(—over 100) cm x 1.8—2.5 cm, tuberculate, usually straight, gradually narrowed upwards into a hard, sharp point.

Mangrove trees of Rhizophora have a characteristic development of the seed. One seed is developed per fruit and starts to germinate when the fruit is still attached or hanging on the tree; this phenomenon is known as viviparous germination and is common among mangrove plants. The root (radicle) gradually protrudes from the fruit, at first like a green cigar, then grows into a rod-like structure. In Rhizophora mucronata, such a seedling root (hypocotyl) with a rough and warty surface may attain a considerable length (sometimes over 100 cm), the largest and longest in the genus. Later the seedling falls out of the fruit, drops into the mud and sooner or later begins to grow. The seedlings that have fallen into the water at high tide commonly drift to another place or are washed up on the shore; they retain their vitality for several months, and will survive and grow if the spot is ecologically suitable. The main root of the seedling is usually abortive and lateral roots take over its function.
Rhizophora is usually wind-pollinated. The flowers are bisexual, self-compatible and therefore may be able to self-pollinate. Insects (e.g. bees) have been observed sometimes visiting flowers to look for pollen.

One should keep in mind that, at least in Malesia, botanical information on Rhizophora mucronata can often also be applied to a closely allied, also common, widely distributed species Rhizophora apiculata Blume. The latter species can be distinguished from Rhizophora mucronata in the field by some easily observed characters. The bark is grey, almost smooth, with vertical fissures. Inflorescences are shorter, fork only once, and are always 2-flowered. Seedling hypocotyls are usually less than 30 cm long, smooth.
In western Malaysia and west from New Guinea a few specimens occur with characters intermediate between Rhizophora mucronata, Rhizophora apiculata and a third species, Rhizophora stylosa Griffith. Hybridization might occur in nature between these species.

Plants of Rhizophora mucronata are most profusely developed, generally gregariously, on the banks of tidal creeks, in estuaries and on low coastal areas flooded by normal, daily, high tides. In general they prefer deep soft mud rich in humus with suitable salinity and they are often found well developed in wet climates. The trees of this species form a rather uniform, evergreen fringe to the mangrove forest. In certain favourable regions in Malesia they may occupy considerable large areas, sometimes associated with Rhizophora apiculata, sometimes forming almost pure stands. Communities of the two common species of Rhizophora can sometimes be identified at a glance by their different shades of green.

Natural regeneration always occurs near mature trees. There is a form of vegetative spread of the trees by horizontal growth of the lower branches supported by stilt roots; these branches can continue to grow if the parent trunk dies. Young seedlings can be used for planting.

The tree grows slowly. In Peninsular Malaysia it takes 35—40 years to reach up to 19 cm in diameter. A 40-year rotation is favourable.

The radicles and hypocotyls of germinated Rhizophora mucronata seeds occasionally suffer from a peculiar disease which is characterized by a brown discolouration and dying of the tissues.
Crabs are great enemies to seedlings and will damage plantations. In the Philippines it has been reported that drying the seedlings for several days in the shade before planting can stave off crab attack. Beetles (e.g. from the family Scolythidae) may damage the root tips, resulting in deformed roots.

For tannin production the bark is removed by hand from living trees or from trees just felled for firewood, charcoal or timber.

In Malesia the bulk of the mangrove bark used for tannin appears to originate from Rhizophora mucronata, which has a considerably higher proportion of bark and tannin content than Rhizophora apiculata. It was reported that Rhizophora mucronata is likely to be the most profitable species and its yield of bark is 23—27% of the volume or 18—20% of the weight.

The bark stripped off from the tree should not be allowed to become dry, otherwise it will be regarded as worthless. If it is not required for immediate use, it should be stacked and kept moist by frequent watering.

As frequently stripping most of the bark from the bole kills the tree, it is better to integrate bark production with the production of firewood and charcoal. Felling the trees should be well planned and care must be taken not to destroy the possibilities for natural regeneration. Too much mangrove forest has been destroyed in recent years.

Hou, D., 1958. Rhizophoraceae. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana, Series I, Vol. 5. pp. 429—493.
Rollet, B., 1981. Bibliography on mangrove research 1600—1975. Unesco, Paris. xxviii + 479 pp.
Tomlinson, P.B., 1986. The botany of mangroves. Cambridge University Press, Cambridge, Great Britain. xii + 413 pp.
Watson, J.G., 1928. Mangrove forests of the Malay Peninsula. Malayan Forest Records 6: 1—275.

D. Hou

Hou, D., 1991. Rhizophora mucronata Poiret. In: Lemmens, R.H.M.J. and Wulijarni-Soetjipto, N. (Editors): Plant Resources of South-East Asia No 3: Dye and tannin-producing plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea