Prota 7(1): Timbers/Bois d’œuvre 1
Sp. pl. 4(2): 1054 (1806).
Mimosaceae (Leguminosae - Mimosoideae)
2n = 52
Racosperma heterophyllum (Lam.) Pedley (2003).
Mountain tamarind (En). Tamarin des hauts, tamarin de la Réunion, chêne de Bourbon (Fr).
Origin and geographic distribution
Acacia heterophylla is endemic to Réunion. It has been reported from Mauritius, but this is probably erroneous. It has been planted in central Madagascar, where it may be naturalized in a few localities.
The wood of Acacia heterophylla is considered valuable and is used for construction, local furniture, joinery, flooring, arts and handicrafts, and for the production of shingles. Formerly it was used for the construction of small boats. The tree is planted as an ornamental along roads and is a source of nectar for honey bees.
Production and international trade
The commercial production of Acacia heterophylla timber is small, with in 1990 about 1200 m³ of logs giving 400 m³ of sawn wood (at an average price of US$ 1200/m³), and 480,000 shingles.
The heartwood is pinkish yellow to orange-brown and distinctly demarcated from the paler sapwood. The density is about 600 kg/m³ at 12% moisture content. At 12% moisture content, the modulus of rupture is 154 N/mm², modulus of elasticity 11,180 N/mm², compression parallel to grain 47 N/mm², cleavage 26 N/mm and Chalais-Meudon side hardness 2.2. The wood is easy to work. The nailing and gluing properties are good, and the wood takes varnish well. It takes an excellent polish.
Shrub or small to medium-sized tree up to 20(–25) m tall; bole often short and crooked, up to 150(–200) cm in diameter; crown spreading. Leaves in young plants alternate and bipinnately compound with 1–5 pairs of pinnae, each bearing 7–14 pairs of ovate-oblong leaflets 6–10(–20) mm long, but soon replaced by elliptical and usually slightly curved phyllodes 6–16 cm × 0.5–1.5(–2) cm, leathery, with numerous longitudinal veins. Inflorescence a head, in small clusters on short axillary shoots, 30–40-flowered; peduncle 5–10 mm long. Flowers bisexual, regular, 5-merous, small, pale yellow, almost sessile; calyx cup-shaped, 1–1.5 mm long, with minute lobes; corolla 2–2.5 mm long, lobed to halfway; stamens numerous, free, 3.5–4 mm long; ovary superior, ellipsoid, sessile, silky hairy, style c. 10 mm long, stigma tubular. Fruit a narrowly oblong, flattened pod 6–11 cm × 0.5–1 cm, glabrous, brown, 5–10-seeded. Seeds oblong-ellipsoid, compressed, 6–7 mm long.
Acacia is a large pantropical genus, comprising more than 1300 species; most of them distributed in Australia (more than 900), more than 200 in America, and about 130 in Africa. Acacia heterophylla belongs to subgenus Heterophyllum, which mainly comprises Australian species with phyllodes and without prickles. Acacia melanoxylon R.Br. (blackwood) from Australia is planted in Réunion and Mauritius. It resembles Acacia heterophylla, but differs in its phyllodes with reticulate veins, its heads arranged in lax racemes and narrower, twisted pods. Cytogenetic investigations suggest a recent autotetraploid origin of Acacia heterophylla from Acacia melanoxylon. Another close relative of Acacia heterophylla seems to be Acacia koa A.Gray from Hawaii, of which a few specimens have been planted in Réunion.
Growth rates of Acacia heterophylla seedlings are high, in good conditions about 1 m per year during 5 years after germination. It is estimated that trees need 100 years to reach a bole diameter of 70 cm. In trials in Madagascar Acacia heterophylla succeeded only at 900 m altitude with an annual rainfall of 1700 mm; 10- year-old trees had a diameter at breast height of 18 cm and were 18 m tall, but they had a bad shape. Rooting is superficial, which explains the often severe damage to the trees by cyclones. Flowering is often abundant, and the flowers are visited by bees.
Acacia heterophylla occurs in humid forest at (800–)1000–1800(–2500) m altitude, and is locally abundant. The annual rainfall should be at least 1500 mm and divided evenly over the year.
Natural germination occurs after disturbance of the forest, e.g. after fire. Under such circumstances regeneration may be abundant. Seeds may remain viable for over 30 years. Clear felling of the natural forest has been practised to establish pure, even-aged stands of Acacia heterophylla as a result of natural regeneration. Regular weeding is necessary, 2 times per year for the first 3 years, to avoid smothering of the seedlings by weedy plants such as Solanum mauritianum Scop. Thinning is practised when the trees are 6 and 10 years old, to 1250 stems/ha and 800 stems/ha, respectively. The eventual density to be reached after 50 years is 200 stems/ha. In 1990 the area managed in this way in Réunion was 1600 ha. Nowadays plantations also exist. Pods are picked from the tree when they are becoming brown and are dried for two weeks. Before sowing, the seeds have to be pre-treated by immersion in boiling water for 15 minutes. Germination takes place 1–3 weeks after sowing. Seedlings stay in the nursery for 6–8 months before being planted out. Direct sowing in the field is also possible. Recently, in-vitro culture techniques have been developed.
The yield of timber per tree in natural forest is often comparatively small due to the poor shape and short length of the bole; wood defects such as knots are common. Proper silvicultural practices may improve bole shape and length, and thus timber yield.
Genetic resources and breeding
The timber of Acacia heterophylla has been exploited for many decades, but important stands still remain in Réunion, which cover approximately 7500 ha, mainly in natural forest. However, monitoring of the populations of this narrow endemic remains necessary.
The demand for the timber, especially for use in the local furniture industry, is much greater than the supply. It has been suggested that a total area of 3000 ha of well-managed Acacia heterophylla stands could be developed in Réunion, with an estimated yield of 4000 m³/year of sawn timber. The high initial investments needed seem worthwhile in view of the high-quality, valuable timber. The use of this indigenous timber tree in Réunion for the establishment of production forest is preferable to the planting of exotic species with regard to biodiversity conservation.
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Correct citation of this article:
Lemmens, R.H.M.J., 2006. Acacia heterophylla (Lam.) Willd. In: Louppe, D., Oteng-Amoako, A.A. & Brink, M. (Editors). Prota 7(1): Timbers/Bois d’œuvre 1. [CD-Rom]. PROTA, Wageningen, Netherlands.