Baikiaea plurijuga (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Forage / feed|
Baikiaea plurijuga Harms
- Protologue: Warb., Kunene-Sambesi Exped.: 248 (1903).
- Family: Caesalpiniaceae (Leguminosae - Caesalpinioideae)
- Chromosome number: 2n = 24
- Zambezi teak, Zambezi redwood, Rhodesian teak (En).
- Teck de Rhodésie (Fr).
Origin and geographic distribution
Baikiaea plurijuga occurs in Zambia, southern Angola, northern Namibia, northern Botswana and Zimbabwe. It has been introduced in Burundi and Madagascar, but with little success.
The wood, also known in trade as ‘umgusi’, is widely used for poles in house building, doors, mine props, vehicle bodies, railway sleepers, furniture, drums, toys, implements, tool handles and dug-out canoes. Its high resistance to abrasion, low shrinkage rates and attractive appearance make it particularly suitable for heavy flooring. Furthermore, it is suitable for interior trim, vats, food containers, carvings, turnery, veneer and plywood.
In Zambia Baikiaea plurijuga is a preferred tree for firewood, and also in Namibia it is valued for firewood, giving a long-lasting fire producing little ash. It is also used for charcoal production. The bark and wood extracts have been used for tanning to produce reddish brown leather. Bark decoctions and infusions are taken as tonic and to treat eye diseases, syphilis and toothache. Sap is used to treat stomach-ache, gum to treat rabies. The seeds are used as beads in strings.
Production and international trade
Selective harvesting of Baikiaea plurijuga has been practised for more than a century. It was not commonly exploited by the indigenous people because the hard wood makes it not easy to fell. In the first half of the 20th century, Baikiaea plurijuga was the most important timber used in southern Africa for railway sleepers and it was the only timber that was exported in any quantity from Zambia. Until the 1980s, the main products in Zambia were railway sleepers for local consumption, mining timber for local consumption and export, and parquet flooring for export. The production of the largest sawmill in Zambia peaked at 4000 m³/year of Zambezi teak in the 1930s and again in 1964. Sales values in Zambia in the 1980s were around US$ 1 million per year, with about 80% traded in the domestic market and 20% for export. The main export market was South Africa. Baikiaea plurijuga still produces the most important and most highly valued timber of Zambia. The utilization of the woodlands of northern Botswana has focused on selective harvesting of Baikiaea plurijuga and Pterocarpus spp., both of which have been marketed at too low prices.
The heartwood is brown with irregular dark markings when freshly cut, turning dark red to dark red-brown upon exposure, and taking a yellow tinge with time. It is distinctly demarcated from the pale pinkish brown, 2.5–5 cm wide sapwood. The grain is straight to interlocked, texture fine and even.
The wood is heavy, with a density of 815–955(–1000) kg/m³ at 12% moisture content, hard and strong. It air dries with little degrade, but should be dried slowly to avoid surface checking. The rates of shrinkage from green to 12% moisture content are about 1.5% radial and 2.6% tangential. Once dry, it is stable in service. At 12% moisture content, the modulus of rupture is 84 N/mm², modulus of elasticity 8500 N/mm², compression parallel to grain 66 N/mm², shear 16 N/mm², Janka side hardness 13,500 N and Janka end hardness 16,000 N.
The wood is difficult to saw and work by both hand and machine tools, blunting saw teeth and cutting edges severely; stellite-tipped saw teeth and tungsten-carbide tipped cutting edges are recommended. Gum may clog up saw teeth, especially when green wood is sawn; water spraying may be practised during sawing to remove the gum. The wood planes well, but a cutting angle of 20° is recommended. It polishes well. Pre-boring is needed for nailing to avoid splitting. Painting and gluing give no problems. The wood has some tendency to char upon boring. The turning properties are good, but bending properties are moderate. In contact with iron, the wood is prone to stain. The heartwood is durable, being resistant to fungi and termites, but the sapwood is susceptible to powder-post beetle and longhorn beetle attack. The heartwood is very resistant to impregnation with preservatives, the sapwood moderately resistant.
The bark contains tannin, reportedly up to 40%. The presence of phlobatannins (phlobaphene condensed tannins) has been demonstrated in the heartwood.
- Semi-deciduous, small to medium-sized tree up to 17(–25) m tall; bole branchless for up to 5 m, up to 100(–120) cm in diameter, sometimes swollen at base; bark surface vertically fissured and cracked, pale grey to brown; crown large and dense, with spreading branches; twigs rusty-brown short-hairy, soon becoming glabrous.
- Leaves arranged spirally, paripinnately compound with (3–)4–5(–6) pairs of leaflets; stipules linear-triangular, 0.5–1 cm long, caducous; petiole and rachis together up to 11(–13) cm long, short-hairy; petiolules 1–3 mm long; leaflets opposite, ovate to elliptical or oblong-lanceolate, 3–9 cm × 1–4 cm, usually rounded at apex, with slightly recurved margins, nearly glabrous, pinnately veined with up to 12 pairs of lateral veins.
- Inflorescence a terminal or axillary raceme up to 35 cm long, densely brownish hairy; bracts up to 5 mm long, caducous.
- Flowers bisexual, slightly zygomorphic, scented; pedicel 1–1.5 cm long; sepals 4, ovate to elliptical, 1.5–2 cm long, unequal, brownish short-hairy; petals 5, free, obovate to spatulate, 2–3.5 cm long, crinkled near margins, hairy towards midrib, pale mauve to bluish purple; stamens 10, 9 fused at base and 1 free, up to 3.5 cm long; ovary superior, 1–1.5 cm long, brown hairy, with short stipe, 1-celled, style slightly longer than stamens.
- Fruit a flattened, woody, oblanceolate pod 8–14 cm × 3.5–5 cm, stiped, brown hairy, dehiscing with 2 spirally twisting valves, few-seeded.
- Seeds ellipsoid-rounded, 2–2.5 cm × 1.5–2 cm, flattened, dark reddish brown.
Other botanical information
Baikiaea plurijuga has a very deep root system. Taproots of seedlings rapidly grow down to reach soil layers which are moist throughout the dry season. The rooting depth of trees has been estimated to reach 6–9 m. Saplings and trees have a pronounced development of lateral roots. The roots develop endomycorrhizal associations. Under natural conditions, seedlings require some shade for optimal growth and survival. The stems of seedlings grow very slowly for the first 3 years, when the taproot rapidly penetrates the soil. They often reach no more than 15 cm for several years, but the taproot may reach 1.5 m long in one year. Later, annual growth rates of 50–70 cm can be reached for young trees.
In regularly weeded plantation trials in Zambia at a spacing of 3 m × 3 m, survival of seedlings was 33% after 8 years, and at a spacing of 3 m × 1 m 50–65%. After 18 years, the average height of trees originally planted at a spacing of 3 m × 3 m was 6.7 m and 4.6 m at an original spacing of 3 m × 1 m; the mean diameter of the 200 largest trees was 11 cm. However, in Zimbabwe a mean annual bole diameter increment of 1.5 mm only has been recorded.
Some trees flower already when 7 years old and fruiting may start when 11 years old. Baikiaea plurijuga has protracted asynchronous flowering periods. It can be found flowering from November to May, and ripe seeds develop 5–9 months later. The flowers are pollinated by insects such as bees. The fruits open explosively on the tree and the seeds are dispersed over short distances. The seeds take 1–3.5 weeks to germinate, but often they stay dormant for several months until sufficient rainfall starts germination. Ripening fruits are commonly eaten by monkeys and baboons.
Baikiaea comprises about 4 species and is restricted to mainland Africa. It is related to Copaifera, Detarium, Sindora and Tessmannia.
Baikiaea plurijuga is characteristic for dry deciduous forest on Kalahari and Karroo sands, up to 1200 m altitude. It is well adapted to dry sites on free-draining sandy soils with a pH of 5–5.5. The mean annual rainfall in the area of distribution is (350–)600–1100 mm, with a dry season of 6–8 months, and the temperature variation can be extreme, from a mean minimum temperature of 10–13°C in the cold season to a mean maximum temperature of 32–35°C in the warm season. Frost may occur in the cold season. Baikiaea plurijuga is often found in association with Pterocarpus lucens Lepr., Guibourtia coleosperma (Benth.) J.Léonard, Schinziophyton rautanenii (Schinz) Radcl.-Sm., and Acacia, Combretum and Terminalia spp., but occasionally it occurs in pure or almost pure stands. The area covered by Zambezi teak forest, in which Baikiaea plurijuga dominates, has been estimated in the mid-1980s at 700,000 ha.
Natural regeneration is often poor and hampered by erratic seed production, long periods of drought, damage to the seedlings by wild animals, competition with dense undergrowth, and fires. Seeds retain their viability for at least one year when stored at room temperature; in cold storage they are still viable after 3 years. Pre-treatment of seeds by soaking in cold water for 24 hours is recommended, or soaking in hot water for 2 minutes and then staying overnight in the cooled-down water. In general, seeds germinate easily. Germination is best (85–90%) they are sown at a depth of 4 cm; there is no germination at all when the seeds are sown deeper than 15 cm. In a nursery in Namibia, a germination rate of 96% has been recorded. Seeds can be sown in a 2:1 mixture of river sand and compost. When seedlings have developed two leaves they can be transplanted in nursery bags.
In Zambia it was concluded in 1976 that the lower felling limit for Baikiaea plurijuga boles should be raised from 35 cm to 50 cm diameter. The rotation cycle has been estimated to be 80–100 years for trees naturally regenerated.
Fire has a detrimental effect on the structure of Baikiaea plurijuga woodland, with a reduction in stocking, especially in small-diameter classes. However, protecting sites from fire is expensive and results in the accumulation of litter, which contributes to the destructiveness of fires. Controlled burning in the early part of the dry season has been recommended to reduce the danger of damage caused by fires in the late dry season. Another method to reduce fire damage is by minimizing grass fuel loads through grazing of the woodland in the mid or late-growing season.
Baikiaea plurijuga seems to be almost free of diseases. In Zambia severe problems have been reported for natural regeneration of Baikiaea plurijuga caused by various rodents and duikers (small antelopes). The foliage of trees is often browsed by elephants, which may seriously damage the trees, but which on the other hand may promote natural regeneration. Sawn timber is easily infested with wood-boring beetles in the course of seasoning; the most harmful are Lyctidae and Bostrichidae beetles, especially Heterobostrychus brunneus.
Natural forest can yield 5 m³/ha of Baikiaea plurijuga timber, although 0.5–1 m³ is much more common. A log of 4.9 m long and 35 cm in diameter felled in Zambia yielded 0.35 m³ of wood, and one of 4.9 m long and 50 cm in diameter 0.7 m³.
In Zambia two reserves have been established already in the early 1980s for the conservation of Zambezi teak. In the same country, a conservation plan for the genetic resources of Baikiaea plurijuga has been presented in 2002; a number of populations were suggested as gene conservation stands, representing all ecological zones in order to maintain the genetic variation to the best possible extent.
In Zimbabwe in the 1980s, Baikiaea plurijuga has also been reported to be in danger of genetic impoverishment or even extinction. In Namibia Baikiaea plurijuga is protected by the Forestry Ordinance, because it is threatened by over-exploitation for the timber and by forest clearing for agricultural purposes. Populations of older individuals have now disappeared in many regions throughout the distribution area of Baikiaea plurijuga. However, populations in fallow fields and national parks are regenerating locally well, and thickets with Baikiaea plurijuga are fairly widespread.
Baikiaea plurijuga is included in the IUCN Red List of threatened species, although classified as lower risk / near threatened and with the note that updating is needed.
Baikiaea plurijuga is regarded one of the most valuable timber trees in drier parts of tropical Africa. It has been tested in plantation trials in Zambia and Zimbabwe in the 1960s, but the high costs of protection and maintenance made widespread plantation establishment prohibitive. Therefore, timber production is still dependent on natural forest, for which very long cutting cycles are needed because of the low growth rates. Sustainable economic production seems unrealizable, and probably the wood will stay a luxury product for special applications. Baikiaea plurijuga has prospects as ornamental tree for larger gardens and parks.
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- R.H.M.J. Lemmens, PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
Correct citation of this article
Lemmens, R.H.M.J., 2012. Baikiaea plurijuga Harms. [Internet] Record from PROTA4U. Lemmens, R.H.M.J., Louppe, D. & Oteng-Amoako, A.A. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. <http://www.prota4u.org/search.asp>.
Accessed 10 February 2020.
- See the Prota4U database.