Acacia nigrescens

Family: Fabaceae (Mimosoideae)

Acacia nigrescens, locally known as msenjele (in Swahili) and mng'enjele (in Mwera), is one of 10 hardwood timber species – including Mpingo – that are responsibly managed and traded by MCDI-supported communities in south-eastern Tanzania. The English name for the tree is ‘knobthorn’ on account of the conspicuous woody spines on its trunk.

The Tree

A. nigrescens is a deciduous tree that can grow to a medium size (up to 20-30 metres in height) but such individuals are now rare; smaller trees are still a common component of miombo woodland and thicket. It almost always has a single trunk, the bark of which is dark grey. Slashing the bark reveals the red sapwood underneath. Young branches are grey and hairy with short, dark pairs of hooked thorns. The leaves are compound with 6 pairs of smooth rounded leaflets held symmetrically, each leaflet having a prominent central vein. The fragrant, creamy white flowers of A. nigrescens are held in clustered spikes; they appear with new leaves before the short rains and are followed by long, flat, brown-coloured pods.

This species might get confused with Zanthoxylon holtzianum as both trees have corky knobs on the trunk. The knobs found on A. nigrescens, however, do not grow as large as those of Z. holtzianum, neither do they form ridges. The leaves of A. nigrescens are also much smaller and unscented.

Ecology & Disribution

A. nigrescens is relatively widespread and occurs from Tanzania down to north-eastern Namibia, Botswana and north-eastern South Africa. The trees are a common component of miombo woodland and thicket, where they often grow on alluvial soils in valleys and near rivers, as well as on shallow soils of rocky hillsides. The trees are naturally resistant to fire.

Timber

The timber of A. nigrescens starts off with a reddish colour that turns dark brown on exposure to air. The heartwood is very dense, hard and extremely durable; it has high resistance to preservative treatment, as well as to fungal, borer and termite attack. A good finish can be obtained by applying waxes and oil.

Physical & Woodworking Properties
Density (kg/m3)Very high (1,000-1,200)MachiningDifficult
Hardness (Kgf)Very high (1,945)Response to hand toolsPoor
DurabilityVery highMovement in serviceModerate
GrainIrregularResistance to impregnationHigh
TextureMedium-coarseResistance to insectsVery high

 

Other similar timbers

Although considerably harder, A. nigrescens has similar woodworking properties to greenheart (Chlorocardium rodiei) and cumaru (Dipteryx odorata). It also has similar durability and machining characteristics to ekki (Lophira alata) and ipe (Handroanthus spp.). The timber offers a good substitute for lignum vitae (Guaiacum officinale), a now endangered species with almost identical physical and woodworking properties to A. nigrescens.

Uses

Timber

Owing to its termite resistance, the timber of A. nigrensis is the preferred material for local house construction. The trunk is split lengthwise into rough sections and the pale sapwood is often left attached when the poles are used as uprights in house building. The heartwood has historically been used to support mine shafts and as railway sleepers.

Other uses

Young trees are often used to produce bark rope. However, if all of the bark is stripped - i.e. the tree is ‘ring-barked’ - the tree will die. The leaves are eaten by game, and sometimes by people in times of food shortage.