Common name: Gum arabic

Originating in the arid Sahel zone of north Africa, this small tree yields Gum Arabic, a commercially important natural gum. It is used in various products, including food, pharmaceuticals and cosmetics.

It is a variable species, with several naturally evolved varieties reported in its native range, from shrubs 2 to 5 m (7 to 16 ft) tall with multiple stems to trees up to 12 m (40 ft) tall. Typically, it is a small, low-branching tree 3 to 5 m (10 to 16 ft) tall with a wide-spreading, flat-topped crown. The bark is smooth yellowish-brown on young trees, on older trees becoming greyish-brown.

The leaves are twice-feathered, with lateral branches that hold small, green, oval leaflets arranged in pairs. In the dry season, they fall off the tree to conserve water and are replaced by the new leaves, which emerge and grow in the rainy season that follows.

The flowers are fragrant, yellowish-white to yellow, in finger-like clusters that bloom during the rainy season, coinciding with new leaf growth. They are followed by a flat, papery seedpod that are light brown when mature and many in number.  

Use

The main interest in the plant is the edible gum that exudes from its stems when wounded. After wounding, the gum is allowed to collect, dry and harden, then is broken off and traded as 'Gum Arabic'.

Gum Arabic has natural emulsifying, gelling, stabilising and thickening properties, which has led to its use as a gum in a range of products, including food and confectionery. For example, it is used to stabilise emulsions in candy coatings and pharmaceuticals and, to some extent, cosmetics, where its gelling and film-forming properties have found application in products such as face-peeling masks.

The foliage, including the leaves and the numerous small flat seedpods, are eagerly browsed by goats and cattle. The fresh leaves and seedpods have a crude protein content of around 20% of dry weight.

The wood is heavy, with a density of around 720 kilograms per cubic meter (45 lbs per cubic ft), but is mostly available as small-diameter roundwood used for firewood, charcoal making and shaping into tool handles. The stems selected for cutting are ringed to allow the valuable gum to drain and be collected before being removed completely. 

It is classed as a honey plant, but there does not appear to be much information published on its value to honey production. The honey is amber coloured, with a very mild aroma, and very quick to granulate.

Its ability to fix nitrogen and its tolerance to extreme drought conditions make it suitable for land rehabilitation and dune stabilisation initiatives, particularly in dry or desert areas. 

General interest

Gum Arabic, together with other drought-tolerant species of the Sahel region, has been planted in significant numbers to create what has become known as the 'Great Green Wall', a mammoth initiative against the encroaching Sahara Desert.

The goal is to create a multi-species belt of vegetation across the African continent, spanning over 7,000 kilometres (4350 miles) east to west and 15 kilometres (9 miles) in width. Other species identified as being suitable for the project include Desert date (Balanites aegyptiacus), Jujube (Ziziphus jujuba) and Gao tree (Faidherbia albida).

Climate

Grows naturally and yields the greatest amount of gum in dry to sub-humid subtropical and tropical lowland climates, generally in areas with annual lows of 16 to 24°C, annual highs of 29 to 35°C, annual rainfall of 200 to 700 mm and a dry season of 7 to 11 months. 

Although it also grows naturally in moderately humid areas, with annual rainfall of up to 1200 mm, the plants are reported to produce much less gum and of poorer quality than those from drier regions.

Growing

New plants are usually grown from seed, which benefit from pretreatment to improve germination by immersing them for a few seconds in boiling water that is then allowed to cool. It performs best on free-draining sand, gravel and other coarse soils of a moderately acid to alkaline nature, generally with a pH of 5.0 to 8.0, and on sites with full sun exposure. It has a low tolerance for poorly drained or waterlogged soils.

Problem features

Seed that fall to the ground will sprout when conditions are favourable. If left unmanaged, the seedlings can grow to form dense, impenetrable thickets. It is recorded as a weed in at least one reference publication. Still, there does not appear to be any record of it anywhere as a serious weed, despite its introduction to Australia, the Caribbean and other areas outside its native range.

The branches are armed with sharp spines or thorns that can cause injury to the unwary.

Books

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

• Arbonnier, M. 2004, Trees, shrubs and lianas of West African dry zones, CIRAD, Margraf Publishers Gmbh, MNHN, Paris

• 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

• Champion, H. G. & Seth, S. K. 1968, A revised survey of the forest types of India, Natraj Publishers, Dehradun, India

• Coppen, J. J. W. 1995, Gums, resins and latexes of plant origin, Food and Agriculture Organization of the United Nations (FAO), Rome

• Crane, E., Walker, P. & Day, R. 1984, Directory of important world honey sources, International Bee Research Association, London

• Duke, J. A. 1983, Handbook of energy crops (unpublished), Center for New Crops & Plants Products, Purdue University, West Lafayette, Indiana

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

• Hill, A. F. 1952, Economic botany : a textbook of useful plants and plant products, 2nd ed, McGraw-Hill, New York

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

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

• Imeson, A. 2010, Food stabilisers, thickeners and gelling agents, Wiley-Blackwell Publishing, Chichester, United Kingdom

• Khan, I. A. & Abourashed, E. A. 2010, Leung's encyclopedia of common natural ingredients : used in food, drugs and cosmetics, 3rd edition, Wiley Publishing, Hoboken, New Jersey

• Krishen, P. 2006, Trees of Delhi : a field guide, Dorling Kindersley Publishers, Delhi

• Leakey, R. R. B et al. 1996, Domestication and commercialization of non-timber forest products in agroforestry systems : proceedings of an international conference held in Nairobi, Kenya, Food and Agriculture Organization of the United Nations (FAO) Press, Rome.

• Letourneux, C. 1957, Tree planting practices in tropical Asia, Food and Agriculture Organization of the United Nations (FAO), Rome

• Liegel, L. H. 1987. A technical guide for forest nursery management in the Caribbean and Latin America, U.S. Dept. of Agriculture, Forest Service, Southern Forest Experiment Station, New Orleans

• Luna, R. K 1996, Plantation trees, International Book Distributors, Dehradun, Uttarakhand

• Mollison, B. 1993, The permaculture book of ferment and human nutrition, Tagari Publications, Tyalgum, New South Wales

• Nair, P. K. R. 1993, An introduction to agroforestry, International Centre for Research in Agroforestry (ICRAF), Kluwer Academic Publishers, Dordrecht

• 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.

• National Research Council (Board on Science and Technology for International Development) 1980, Firewood crops : shrub and tree species for energy production (Volume 1), The National Academies Press, Washington D. C.

• Sheikh M. I. 1993, Trees of Pakistan, USAID Forestry Planning and Development Project, Pictorial Printers, Islamabad

• Stewart, A. 2013, The drunken botanist : the plants that create the world's great drinks, 1st ed., Algonquin Books, Chapel Hill, North Carolina

• 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

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.