General Info

This Tree up to 20m high.  Bipinnate, drooping Leaves have spinescent stipules.  Golden yellow, spherical Inflorescences at branch ends.  Flowers small, actinomorphic. Fruit is a dehiscent pod.

Description

Acacia karroo, Vachellia karroo, Acacia capensis, Acacia dekindtiana, Acacia Chev, Acacia cacia, Acacia hirtella, Acacia horrida, Acacia inconflagrabilis, Acacia reticulata, Acacia robusta, Acacia seyal, Mimosa capensis, Mimosa eburnean, Mimosa leucacantha, Mimosa nilotica, Mimosa reticulate.

RSA Tree No. 172.

Common names: Cape gum, Cape thorn, Karoo thorn, Sweet thorn, Cape Mimosa, Doringboom, Gum-arabic tree.

Family: Fabaceae or Leguminosae.  (Pea, bean or legume family).  After the Orchidaceae and the Asteraceae, the Fabaceae is the third largest Angiosperm (flowering plants) family with 700+ genera and close to 20 000 species.  Local Tree genera include Acacia (Vauchellia, Senegalia), Albizia, Bauhinia, Bolusanthus, Burkea, Calpurnia, Colophospermum, Cyclopia, Dichrostachys, Erythrina, Erythrophleum, Faidherbia, Indigofera, Mundulea, Peltophorum, Philenoptera, Schotia and Xanthocercis The Fabaceae are recognisable by their fruit and by their pinnately compound Leaves.  Leaves may also be simple and usually have stipules – some of which may be spinescent.  Leaflets are usually entire.  Flowers are bisexual and bracteate.  Regular flowers usually have 4-5 sepals and the same number of petals.  Irregular flowers have 4-5 sepals and 5 or less petals.  Stamens have anthers that have 2 pollen sacs and there are usually at least twice the number of stamens as petals – often 10.  The superior Ovary has one locule that may contain 1 or more ovules.  The Stigma and Style are simple.  The single carpel develops into the Fruit, which is usually a pod.  This pod dehisces on both sides and may break into segments.  Seeds vary.

Name derivation: Acacia – thorny.  Vachellia – named after George H. Vachell (1789 – 1839), chaplain and plant collector in China.  These plants have a capitate inflorescence and stipules that are spinescent.  karroo – most conspicuous tree of the semi desert.

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

Tree

This Tree may reach 20m in height or it may be a several-stemmed shrub.  This is largely habitat dependent.  The Crown is spreading and rounded.  Branching may occur close to the ground.  The trunk may reach 0,6m in diameter but is usually about half this.  On the trunk, the Bark is reddish brown, dark brown or black. It may be smooth or become rough and slightly fissured.  This exposes the reddish non-powdery underbark.  Young stems are reddish (photo 28).

Leaves

This often-deciduous tree has Leaves that are bipinnate (compound: twice pinnate. The central axis or rachis has lateral “branches” not leaflets and the leaflets are on these “side branches”).  Leaves have up to 7 pairs of Pinnae – each containing up to 15 spread out, paired Leaflets.  At higher altitudes more pinna develop.  Each leaflet is oblong and is usually up to 8 x 3mm.  The Apex is rounded to slightly pointed.  The leaf rachis (main axis bearing flowers or leaflets) may be grooved above and Glands may appear between pairs of leaflets.  The darkish green leaves tend to droop on the trees.  The Petiole (leaf stalk) is up to 11mm long.  A Petiolar gland is usually present (photo 69).  Stipules (basal appendage of the petiole) are spinescent.  These straight or slightly curved stout Spines occur in pairs at nodes.  They are usually 0,4-7cm long but may reach 20cm in length.  Spines are white and may be swollen.  More spines occur on the lower branches.  This may be an adaptation to restrict grazing. As the tree ages, the new spines produced may be smaller.

Flowers

The spectacular golden yellow Flowers are in spherical inflorescences.  They are up to 18mm in diameter and arise from short spur branches and not among the leaves as in Vachellia erioloba but near the end of stems.  The Peduncle (stalk of flower cluster or of a single flower when that flower is the remaining member of an inflorescence) is green and hairless.  Individual flowers are sweetly scented, small, bisexual (with a few male flowers) and actinomorphic (regular, symmetrical.  Perianth, the calyx and corolla, is divisible into 3 or more identical sectors).  The Calyx is warty and its 5 Sepals are connate (united or joined) and tiny – up to 3mm long.  The Corolla has 5 united Petals that are up to 3mm long and valvate (meeting by the edge without overlapping).  The many Stamens are exserted (sticking out; projecting beyond) and have golden filaments.  This, together with the golden sepals and petals, gives the inflorescence its colour.  The superior Ovary has many ovules.  The Style is filiform (thread or filament like) and terminates in a small stigma.  This tree is one of the latest “acacias” to flower – (Dec-Jan+) and is often rain dependent.  The tree is typically insect pollinated.

Fruit

The Fruit is a slightly woody Pod, which is initially green but become rusty to dark brown when mature.  The pod is usually up to 10 x 0,7cm and dehisces on the tree.  The pods vary from almost straight to sickle shaped.  They are constricted between seeds.  Seeds may remain on the trees in open pods for some time (photo 853).  Seeds are flat and elliptic or lenticular and may be olive green to brown.  Dispersal is mainly through the gut of ruminates.

Distribution & Ecology

These Trees are Located in the Northern Cape, Western Cape, Eastern Cape, Free State, KwaZulu-Natal, Gauteng, Limpopo, Mpumalanga, North West, Namibia, Angola, Botswana, Zimbabwe and Zambia.  This is the most common tree in southern Africa.  It grows in a variety of habitats and different soil types – including sand dunes.  It is common to the west of the Drakensberg.  The tree has a deep tap root system (reportedly in excess of 50m) and, as a result, is often found slightly further away from a water source than other trees in dry areas.  In more moist environments, trees are often associated with Themeda triandra (a blue-green tufted grass).  Trees are not common in winter rainfall areas or at high altitudes because of sensitivity to frost.  The trees occur from coastal dunes to an altitude of 1 800+m. Many butterfly larvae feed on the leaflets.  Many other insects visit the tree and this, in turn, attracts birds.  The spines offer protection and encourage bird nesting.  Antelope consume the flowers, pods and leaves.  Leaves are poison free.  The flowers are good bee food.  Plants have the ability to regenerate after a fire.  Other plants are encouraged to grow under the tree because of the symbiotic relationship of the roots with a fungus. During this process, atmospheric nitrogen is fixed. These additional plants encouraged because of the low density (higher light) of the leaves.  A substitute for coffee can be made from the Seeds.  This tree is often subjected to severe attacks by the wattle bagworm Kotochalia junodii.  This is a moth species – the female is worm like.  The hemi-parasite (a plant which obtains or may obtain part of its food by parasitism, and by photosynthesizes) Agelanthus natalitius may occur on the branches (photo 669).

Ethnobotany

The Wood is white to slightly yellow, hard and is relatively dense – up to 890kg per cubic metre.  At this density, it will just float in water.  It is used for building.  The wood saws easily and planes to a smooth finish.  It glues and varnishes well. The wood is used as fuel and makes good charcoal. It burns with little smoke and is odourless.  The Bark has a high level of Tannins – up to 19%, which tan to a red colour.  In its lifetime, a tree can produce over a cubic metre of wood.  Because of its deep root system, the Trees give a good indication of where to find water.  The inner bark is used to make rope, which, unlike leather, does not stretch when wet.  The red-brown Gum is edible – hence the common name sweet thorn.  People and animals including the bush baby consume the gum.  The gum is also used for glazing pottery.  This hardy tree is fast growing.  In dry areas, the tree provides good shade. Seeds should be soaked in hot water for a while then immersed in cold water, left overnight, planted and lightly covered with soil.  Seeds germinate in less than 2 weeks.  In over utilized areas the plant may become invasive.  This is an excellent fodder tree.  The tree may live for up to about 40 years.  Best growth is where the rainfall is in the 80-90cm range.  Roots are invasive – keep clear of buildings.  The plants can form an effective brushwood fence.  The fallen tree in the photo below was about 40 years old.

References

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

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.

Ross, J. H. A conspectus of the African Acacia Species. 1979. Botanical Research Institute.

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.

 

http://redlist.sanbi.org/species.php?species=15470-13

http://plantzafrica.com/plantab/acaciakar.htm

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

http://www.worldagroforestry.org/treedb/AFTPDFS/Acacia_karroo.PDF

http://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:77087189-1

http://posa.sanbi.org/flora/browse.php?src=SP