Senegalia Nigrescens

Name derivation:

Vachellia  – named after George Harvey Vachell (1789 – 1839), chaplain and plant collector in China. nigressence – becoming black may refer to the pods.

Conservation Status: L C. (Least Concern).  2009 (Raimondo et al.).

Description:

Tree is upright and up to 30m – usually to 18m, with a somewhat spreading crown. Trunk has a cylindrical shape and a diameter up to 76cm. Crown is spreading to round. The rough Bark is dark grey/brown and longitudinally fissured with reddish under-bark visible. Young branches usually yellowish and flaky. A distinguishing characteristic is the hooked downward pointing thorns that occur on the conical, knob-studded young trunks. Similar structures occur in A. Burke but these are less frequent. Large, older trees have a deeply fissured bark and fewer, if any, thorns. Branches are high-up and tend to be upward pointing. Thorns small, strongly hooked, blackish and occur in pairs just below the nodes. 

Leaves are up to 12cm (usually about half this) long with 1-4 pairs of pinnae (each bearing only 1 or 2 pairs of leaflets). The tree is deciduous and is leafless for several months during winter and early spring. Leaves are bipinnate (twice compound). The petiole and rachis may be pinkish on the sunny side. The very large, distinctive Leaflets are pale to grey-green and lighter below. Leaflets are often nearly circular: up to 3 x 3cm and are the largest acacia leaflets. Midrib and veins may be distinctive and glands may be present between pairs of pinna, and sometimes on the petiole. Small recurved spines may be present.  Petiole (leaf stalk) is up to 5,5cm and may be hairy. The base of the petiole and the petiolule (stalk of leaflet) may be swollen. The Stipules (basal appendage of the petiole) are not spinescent.

Flowers have Sepals of the Calyx that is pink or red/brown. They are situated at nodes and are noticeable in large numbers and give the tree a reddish tinge before the leaves appear and before flowers open. Open flowers give rise to an impressive creamy white display. They occur in elongated Spikes (simple indeterminate inflorescence with sessile flowers on a single unbranched stalk) that are sweet scented, up to 10cm long and which are spectacular during early spring. However flowering is erratic. The spikes are clustered at nodes. Flowers are bisexual, actinomorphic (regular, symmetrical). Stamens are mainly responsible for the colour of flowers. There is a single Pistil (the female element of the flower composed of the ovary, style and stigma) and its Ovary is superior (said of an ovary that is free from the calyx or perianth) and contains one locule (compartment). The Style extends beyond the anthers once the pollen has been shed. At the end of the style is a small concave stigma. (Aug-Nov). Pollinating agents are insects. (Aug-Oct).

Fruit is a straight, flattish olive to dark brown to black, usually indehiscent, hairless Pod which is narrowed at both ends and hang down. The pod hangs down and is up to 17 x 2,5cm, rather thinly textured, hairless, dehiscent / indehiscent with the remains of the calyx is visible at the base. The pod is narrowed at both ends. Veins are prominent. (Nov-Jul).

Distribution and Ecology:

Naturally occurring in low altitude bushveld and savanna but not sandy soil. It is common in the drier parts of the lowveld and may be the only tree in some areas. Often found near watercourses and may be the dominant tree in floodplain woodland. Found in Gauteng, Limpopo e.g. in Mapungubwe, Mpumalanga e.g. in the KNP in Punda Maria and noticeable between the Olifants River and Satara, also between Skukuza and Pretorius Kop where it occurs in black turfy soils derived from dolerite (a dark, medium-grained igneous rock which typically occurs in dykes and sills), KwaZulu-Natal, North West, Both sides of the Lebombo Mountains which extend from Hluhluwe in KwaZulu-Natal through Swaziland, parts of Mozambique to Punda Maria in Mpumalanga KNP. Also occurs in Botswana, Northern Namibia, Zimbabwe, Zambia, Malawi, Tanzania and the Caprivi Strip. These trees are generally indicators of good grazing. The trees are fire and drought-resistant but susceptible to frost. Pods are eaten by animal including giraffe, monkeys, baboons and elephant. Flowers are eaten by monkeys, giraffe and baboons. Leaves are eaten by kudu, duiker, impala, steenbok, giraffe and elephant. Seeds produce chemicals which retard germination – ensures survival when tree dies. Trees are often nesting sites for birds e.g. hornbill, glossy starling, hoopoes, parrots, rollers barbets and owlets.  Pearl-spotted Owlet and Scops Owl are often seen in trees. The tree serves as a host plant for larvae of the Demon Emperor butterfly Charaxes phaeus.

Ethnobotany:

Wood is dense, very hard and difficult to saw. It is termite resistant. Sapwood is light cream coloured and the heartwood is dark brown. Trees grow about 0,5m or less per year but are frost sensitive. Wood used for fence poles, furniture and mine props. Wood is also used for fuel and bonsai. It produces good quality firewood which burns at a high temperature with relatively little smoke. The bark contains a lot of tannin and is sometimes used for tanning. It has a high impact toughness and therefore good for parquet flooring. This is a good shade tree and can be planted from seeds – which need to be soaked in hot water overnight beforehand. It should be planted in full sun.

References:

Boon, R. 2010. Pooley’s Trees of eastern South Africa. Flora and Fauna Publications Trust, Durban. 182

Bryant, C. Lombo, B. 2004. Trees of CC Africa, Double Story Books, Cape Town.

Coates Palgrave, M. 2002. Keith Coates Palgrave Trees of Southern Africa, edn 3. Struik, Cape Town.

Lawrence, G. H. M, 1951. Taxonomy of Vascular Plants, The Macmillan Company, New York. Tenth Printing 1965.

Palmer, E. & Pitman, N. 1972. Trees of southern Africa, Balkema, Amsterdam, Cape Town.

Schmidt, S. Lotter, M. & McCleland, W. 2002. Trees and Shrubs of Mpumalanga and the Kruger National Park.

van Wyk, B. & van Wyk, P. 1997 Field guide to Trees of Southern Africa, Struik, Cape Town.

https://en.wikipedia.org/wiki/Senegalia

http://redlist.sanbi.org/species.php?species=15469-11

http://www.plantbook.co.za/senegalia-nigrescens/

http://www.krugerpark.co.za/africa_knob_thorn.html

http://pza.sanbi.org/senegalia-nigrescens

http://iscantree.co.za/senegalia-nigrescens-knoppiesdoring-knob-thorn/