Common name: Gao tree

Native to dry areas of Africa, extending from southern Egypt to South Africa, this timber, fuelwood, forage and nitrogen-fixing tree typically grows 15 to 20 m (50 to 65 ft) tall. However, it may attain heights of 30 m (98 ft) or more on favourable sites and is fast-growing.

It usually develops a slim, straight trunk supporting a columnar crown when young, with age becoming rounded to wide-spreading and with a weeping canopy of finely textured foliage. Pairs of straight, sharp thorns arm the branches. The bark is pale brown, smooth on young trees, becoming fissured and rough with age.

Leaves are 5 to 10 cm (2 to 4 in) long and twice-feathered, consisting of small, dull green oblong leaflets arranged in pairs along each leaf branch and in a dense arrangement that casts a deep shade. They fall from the tree to conserve water, though curiously at the start of the rainy season, the opposite of most tropical deciduous trees. New leaves emerge at the start of the dry season, causing the tree to stand out from other trees in what is mostly then a bare-branched landscape.

Flowers are small, pale yellow and tightly packed in flower spikes up to 16 cm (6 in) long, resembling bottle brushes. They bloom in the dry season, about two to three months after the new leaves emerge.

Fertilised flowers develop into curved and ribbed seedpods up to 14 cm (5.5 in) long. Green when young, they become bright orange, then reddish-brown and enclose up to twenty hard, dark brown oval seed. When fully mature, they detach and fall to the ground, usually towards the end of the dry season.

Use

The wood is medium-weight, averaging 600 to 700 kg per cubic meter (37 to 44 lbs per cubic ft.) and has low to moderate natural resistance to rot and decay. This puts it in the non-durable hardwood class, making it suitable for indoor use only and generally unsuitable for outdoor or in-ground construction.

Sufficiently sized, well-formed logs are sawn into planks used mainly for interior flooring, furniture, cabinets and boxes. Small diameter logs are cut into lengths for poles, posts, woodcraft or pulp to process into paper products. Alternatively, and more widely, the wood is used for firewood and charcoal making, though more out of necessity than as a highly regarded fuelwood.

The tree's habit of shedding its leaves and leaflets at the start of the rainy season is taken advantage of by dryland farmers, who must wait for the rains to irrigate their crops. The bare branches allow rain and sunlight to pass through to the ground below, and the fallen leaflets break down quickly, enriching the soil. This encourages the farmers to plough the ground directly beneath the trees to sow their crops. There is also little to no competition for water because the trees have a deep root system and, being leafless then, are dormant. 

The leaves and mature seedpods are an important feed in areas dry season, not only for ruminant livestock such as cattle, goats and camels but also for wild herbivores. When mature, the seedpods fall to the ground, making them accessible to livestock, but farmers must lop the branches for the leaves. The leaves have a dry weight crude protein content of about 17% and the seedpods 12%.

It is one of a few select tree species deemed suitable for inclusion in the Great Green Wall project, an African-led initiative with ambitions to grow a 7,000 km (4350 mile) natural wall against the encroaching Sahara Desert. Other species identified as suitable for the project include Gum Arabic (Senegalia senegal), Jujube (Ziziphus jujuba) and Desert date (Balanites aegyptiacus).

Honeybees can be observed working the flowers, but the tree's value to honey production is unknown.

Climate

Grows naturally in dry to moderately humid subtropical and tropical lowland to mid-elevation climates, generally areas with annual lows of 12 to 25°C, annual highs of 25 to 38°C, annual rainfall of 400 to 1400 mm and a dry season of 6 to 8 months, extending to drier areas with irrigation or groundwater. However, its best development is in areas with an annual rainfall of 700 mm or more.

Growing

New plants are usually grown from seed which have a hard coat and benefit from being scarified (nicked). Germination occurs within about two weeks when sown in a free-draining potting mix. Seedlings are then planted out when around 20 cm (8 in) tall and at the start of the rainy season.

Performs best on free-draining loam and sand soils of a moderately acid to neutral nature, generally with a pH of 5.5 to 7.5, and on sites with full-sun exposure. The tree's ability to fix nitrogen from its roots is widely reported. 

Problem features

It produces large quantities of seed and is considered an invasive species in some areas where it is introduced.

Books

• Allen, O. N. & Allen, E. K. 1981, The Leguminosae : a source book of characteristics, uses, and nodulation, University of Wisconsin Press, Madison, Wisconsin

• Booth, F. E. M. & Wickens, G. E. 1988, Non-timber uses of selected arid zone trees and shrubs in Africa, Food and Agriculture Organization of the United Nations (FAO), Rome

• C.A.B. International 2013, The CABI encyclopedia of forest trees, CABI Publishing, Wallingford, Oxfordshire

• Food and Agriculture Organization of the United Nations (FAO) 1986, Databook on endangered tree and shrub species and provenances, FAO Forestry Paper 77, Forest Resources Division, Rome

• Gohl, B. 1981, Tropical Feeds : feed information summaries and nutritive values (Revised edition), Food and Agriculture Organization of the United Nations (FAO), Rome

• Hines, D. A., & Eckman, K. 1993, Indigenous multipurpose trees of Tanzania: uses and economic benefits for people, Cultural Survival Canada

• Hocking, D. 1993, Trees for drylands, International Science Publisher, New York

• Howes, F. N. 1949, Vegetable gums and resins, Chronica Botanica Company, Waltham, Massachusetts

• National Research Council (Board on Science and Technology for International Development) 1975, Underexploited tropical plants with promising economic value, National Academic Press, Washington D. C.

• National Research Council (Board on Science and Technology for International Development) 1979, Tropical legumes : resources for the future, The National Academies Press, Washington D. C.

• Nussinovitch, A. 2010, Plant gum exudates of the world : sources, distribution, properties, and applications, CRC Press / Taylor & Francis, Boca Raton, Florida

• Randall, R. P. 2002, A global compendium of weeds, R.G. and F.J. Richardson Press, Melbourne

• Webb, D. B. 1984, A Guide to species selection for tropical and sub-tropical plantations, 2nd ed., Unit of Tropical Silviculture, Commonwealth Forestry Institute, University of Oxford, Oxfordshire

• Wickens, G. E & Day, Peter R., 1928- & Haq, N & International Symposium on New Crops for Food and Industry 1989, New crops for food and industry, Chapman and Hall, London ; New York

Articles, Journals, Reports and Working Papers

• Nair, P. K. R., Fernandes, E. C. M., & Wambugu, P. N. (1985). Multipurpose leguminous trees and shrubs for agroforestry. Agroforestry systems, 2(3), 145-163.