This umbrella Tree with thick branches may reach 18m high. Bark is corky or papery. Leaves are bipinnate with spinescent stipules. The many tiny whitish regular, bisexual Flowers are in condensed spikes. Fruit is a pod up to 25cm.
Acacia sieberiana var. woodii, Vachellia sieberiana var. woodii, Acacia woodii, Acacia vermoesenii.
Common names: Paperbark Thorn, Paperbark Acacia, Natal Camel Thorn, Flat-topped Thorn.
RSA Tree No. 187.
Family: Fabaceae, 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 – typically 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. sieberiana named after W. B. Sieber – plant collector in the 18th century. woodii named after a Natal botanist: J. Wood – curator of Durban Botanical Gardens from 1882 – 1913 and founder of the National Herbarium in 1882. The species of the genus Vachellia were considered members of the genus Acacia until 2005.
Conservation Status: L C. (Least Concern). 2009 (Raimondo et al.).
The single stemmed Tree is usually umbrella shaped or flat-toped and up to 18+m high. It has a spread, which is often greater than its height. The tree grows up to 1,5m high per year. Trunk has a diameter of up to 0,8m. Bark is corky or papery, light brown to greyish yellow, sometimes flaking in strips – mainly on the branches and this exposes a yellowish underbark. Branches are thick. Young branches are yellowish or greyish brown and hairy. The Bark is flammable and the passage of fire may blacken it.
On this deciduous tree, the dark green Leaves may be hairy. They are are bipinnate (compound: twice pinnate. The central axis or rachis has lateral “branches” not leaflets and the leaflets are on these pinnae “side branches”). The leaves may have as many as 30 pairs of pinnae, each bearing up to 45 pairs of leaflets. Leaves may be up to 12cm long. Leaflets are narrowly oblong up to 6,5 x 1,5mm and hairy when young. Near the base of each, leaf just below the bottom pair of pinnae on the upper surface, is a small knob-like gland (photo 31). Leaflets are often hairy above and on the margins. They are usually hairless below. There is almost no space between leaflets. The Rachis (main axis bearing flowers or leaflets) is hairy (photo 31 & 707). The Petiole (leaf stalk) is very short and hairy, up to 1cm long and may be swollen. The Stipules (basal appendage of the petiole) are spinescent. These paired Thorns are greyish-white, straight or almost so, and are short or up to 9cm long. They have no swellings and are less common or absent in older trees.
The scented Flowers occur in condensed Spikes (simple indeterminate inflorescence with sessile flowers on a single unbranched stalk). Each inflorescence appears as a white or pale to yellow, creamy sphere – up to 2cm wide. This colour is largely due the mass of exserted stamens that emerge in each flower. Flowers are usually bisexual and actinomorphic (regular and symmetrical). The Perianth (the 2 floral envelopes considered together; a collective term for the calyx and corolla) has light green Calyx lobes up to 3mm long. The Corolla is up to 4,5mm long. It also has light green lobes. The Anthers are yellow and the free filaments are white – together generating the overall colour. There is a single Pistil (a unit of the Gynoecium, the female element of the flower, composed of the Ovary, Style and Stigma), which has a superior Ovary with a single locule. A single Stigma and Style are present. Flowers are clustered in leaf axils, or they may appear singly on slender stalks. (Sep-Mar).
The stout and usually creamy-brown, thickened Fruit is a belatedly dehiscent Pod. It is woody, straight or slightly curved and up to 21 x 3,5cm. The pods may be slightly hairy. Seeds are often parasitized (photo 602). (Mar-Aug).
Distribution & Ecology
The Trees are located in open bushveld. In wooded grassland, they may be the only tree. They occur alongside rivers in deep soil and at relatively high altitudes. Rainwater may collect in the bark and this may make the tree more susceptible to lightning strikes. There are excellent specimens near Estcourt, Ladysmith and Colenso. The Bark is flammable. Trees are naturally located from KwaZulu-Natal, Gauteng – Walter Sisulu NBG and Pretoria NBG, Limpopo, Mpumalanga, Botswana, Northern Namibia and northwards as far as Sudan and Ethiopia. A different variety of this species (var. sieberiana) occurs in northern Mozambique. The local tree has a symbiotic relationship with a Rhizobium bacterium through which the tree, and surrounding plants, gain form the fixing of atmospheric Nitrogen. Buck eat fallen Pods. Flowers attract many insects, including bees, thrips (minute, slender insects with fringed wings – they puncture and suck out the cell contents), butterflies and, partly as a result, birds. The tree is a nesting place for barbets.
The inner Bark is a source of fibre and is useable for bead stringing. Grey louries (Corythaixoides concolor) eat the Pods. Young pods and young leaves may contain prussic acid (hydrocyanic acid HCN). Prussic acid can be fatal to livestock when eaten in large quantities over a short time span. Cattle consume fallen pods but they may give milk an unpleasant taste. The tree is slightly frost-tolerant and quick growing. The Gum (a water-soluble sugary polysaccharide that is exuded to seal wounds and prevent infection by bacteria and fungi and prevents trees from freezing) produced is edible. The gum is used as an adhesive and for making marking ink. The Wood is not much used. It is prone to termite and beetle (Order Coleoptera) attacks but with seasoning, the wood has a stronger finish. The wood has a density close to 650kg per cubic metre and the wood will float in fresh water.
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