ACACIA ALBIDA DEL. - A MULTIPURPOSE TREE FOR ARID AND SEMI-ARID ZONES

by

Edouard G. Bonkoungou
Institut de Recherche en Biologie
et Ecologie Tropicale
C.P. 7047, Ouagadougou

Bourkina Faso

INTRODUCTION

At its Fourth Session in 1977, the FAO Panel of Experts on Forest Gene Resources drew up a list of 41 tree species of arid and semi-arid environments that are particularly important for fuelwood. Fourteen out of these 41 species were considered by the Panel as priority species for the improvement of agricultural environment and rural living (FAO 1977, Palmberg 1981).

Acacia albida is one of these 14 priority species. Contrary to usual tree behaviour, Acacia albida sheds its leaves during the rainy season. This has several benefits (NAS 1979):

• the lack of leaves during the rainy season enables sunlight to reach the crops planted beneath and around the tree;

• during the hot months of the dry season, the dense foliage of the tree provides cool shade and forage for livestock and protects the soil from wind erosion and excessive dessication;

• the drop of leaves at the beginning of the rainy season and dung from continuous presence of livestock under the shade of the trees, enrich the soil. In addition, A. albida contributes to soil fertility through nitrogen fixation.

A. albida has substantially increased the well-being of present-day small farmers in Sahelian regions by increasing soil fertility and crop yields and by providing fodder. However, the importance of A. albida as a multipurpose tree has also earlier been recognized and fully exploited in traditional farming systems of West Africa. The Haoussa people in Niger, for example, held A. albida in high respect. The tree was protected by law and unauthorized cutting of mature trees of A. albida could lead to death penalty (Giffard 1974; Pelissier 1966). The Serere people in Senegal and Bambara people in Mali are also known to have protected Acacia albida and developed successful, intensive management regimes of A. albida parklands (Pelissier 1966).

TAXONOMY AND INTRASPECIFIC VARIATION

Knowledge of taxonomy and intraspecific variation of a species provides the basis for its conservation and improved utilization.

Taxonomic studies on A. albida are reported by e.g. Ross (1966), Wickens (1969), Nongonierma (1978) and Brenan (1959, 1983).

Brenan (1959) mentions the presence of two well-defined geographical races within A. albida, based on leaflet size and the presence or absence of pubescence. In his study of West African Acacias, Nongonierma (1978) also identifies several intraspecific categories within the species.

An important conclusion which may be drawn from the above is that A. albida contains important intraspecific variation and, thus, that there is considerable potential for genetic improvement.

DISTRIBUTION

The natural distribution range of A. albida extends through northern Africa, from Senegal to Ethiopia, and southwards through East Africa to the Transvaal and Lesotho. The largest concentration of the species occurs on the sandy alluvial soils of the Jebel Mara drainage system in the Sudan, where pure stands are found in belts often a mile wide (Wickens 1969). Outside Africa, A. albida occurs in Yemen, Israel, Lebanon and Jordan. It has been introduced to Cyprus and Pakistan (Brenan 1983) and is presently being tested in India and Peru under the FAO project on Genetic Resources of Arid and Semi-Arid Zone Arboreal Species for the Improvement of Rural Living (Palmberg, FAO pers. comm.).

Acacia albida is one of the largest trees in the genus Acacia, reaching 30 m in height and 1.5 m in diameter. It occurs naturally in a wide diversity of habitats, ranging from the fringing vegetation on alluvial soils of perennial or seasonal water-courses, to open savanna woodland and cultivated lands. Altitudinal distribution ranges from sea level in Israel to 2500 m on Jebel Mara, Sudan.

REGENERATION

Although generally there seems to be adequate seed production (Lemaître 1954; Wickens 1969; Palmer and Pitman 1961), natural regeneration by seed may be limited because of heavy seed predation and high seedling mortality (Wickens 1969).

Regeneration by vegetative means is often more successful in nature and suckers may develop in a radius as wide as 20–25 m from the mother tree. Vegetative reproduction is the main means of natural regeneration where the natural stands produce little viable seed (Halevy 1971).

Artificial plantations of more than 100 ha are reported to exist in Senegal, Chad (Felker 1978) and Niger.

Although growth of A. albida may be relatively slow during the first few years after planting, while the plant develops a deep tap root, saplings and trees are relatively fast growing (Palgrave 1981).

BENEFICIAL EFFECTS ON THE ENVIRONMENT

The beneficial effects of A. albida on soil chemical, physical, and microbiological characteristics have been reported by several workers, see e.g. Dugain (1960); Dancette and Poulain (1969); Radwanski and Wickens (1969); Charreau and Vidal (1965); and Jung (1966). Increased soil microbiological activities have also been reported (Jung 1966; 1967).

Intermingling crops and A. albida has been reported to increase millet yields between 430 kg/ha (Sarlin 1963) and more than 1000 kg/ha (Dancette and Poulain 1968); sorghum yields by 480 kg/ha (Dancette and Poulain 1968); and peanut yields by between 300 kg/ha (Dancette and Poulain 1968) and 330 kg/ha (IRHO 1966). Occasional reports on depressed yields have been explained by excessive soil fertility, which favours vegetative growth at the expense of reproductive growth (seeds and pods).

FODDER VALUE

The leaves, small branches and pods make excellent fodder and are avidly eaten by sheep, goats, camels, cattle (NAS 1979) and horses (Palgrave 1981). Nomadic graziers throughout the northern savanna belt lop the branches in order to provide dry season browse for their stock (Wickens 1969).

Niger's dry farmlands will normally support only 10 cattle per km²; however, the carrying capacity can be increased to up to 20 in areas where A. albida is common. The pods of A. albida can be dried and stored for subsequent use as fodder; this is not possible with most of the other species of the genus, in which pods open and shed their seeds when dry (NAS 1979).

RESEARCH NEEDS

Research required to manage and develop the full potential of the genetic resources of A. albida covers a wide range of topics which cannot be adequately summarized within the space limits of this note. For a start, however, it may be suggested that research efforts concentrate on the areas of biology, silviculture, and genetics.

Biology

• Intraspecific variation, conservation of genetic resources

  As mentioned above, A. albida is a variable species. Additional field collections and studies are needed to specify the geographic and ecological limits of the taxa identified; and to bring together the works of Brenan (1959, 1983) and Nongonierma (1978).

  Paleobotanical evidence indicates that A. albida is a very ancient species (Bonnefille 1975; Butzer 1966). Thus, records of its natural occurrence outside the main distribution range must be given due consideration, as such occurrences may represent relics and contain valuable genetic resources; isolated pockets of A. albida stands should be urgently investigated with genetic conservation objectives in mind.

  Action on genetic conservation of A. albida is underway in Israel within the framework of the FAO Project on Genetic Resources of Arid/Semi-Arid Zone Arboreal Species (Palmberg 1981). Conservation action in the species has also been taken in South Africa (Pitman 1961). However, a more systematic approach to conservation is needed.

• Plant physiology and nitrogen fixation

  Basic information on the phenology of the species and on symbiotic relationships and nitrogen fixation should be gathered to form a basis for applied research.

• Plant protection

  Reports from Brunk (1972 and 1974) and Felker (1978) indicate that insect problems in A. albida can be quite severe. Research needs in plant protection should include the following issues: studies of damaging insects of A. albida; identification of non-phytotoxic chemical control methods; and identification of sources of genetic resistance to insects.

  A recent FAO manual (Southgate 1983) deals with the problem of insects that attack flowers, pods and seeds in Acacia species. Follow-up action in the field to solve these problems is warranted.

• Silviculture

  A recent handbook by FAO outlines the methodology of seed collection, handling and treatment (Doran et al. 1983).

  Additional research is needed in seed handling, nursery and establishment techniques; intercropping; use of A. albida as browse, fodder, shade, shelter etc.

• Tree Improvement and Genetics

  Systematic efforts in exploration, conservation, collection and evaluation of a part of the range of A. albida (Yemen, Sudan, Senegal) are presently underway within the framework of the FAO Project on Genetic Resources of Arid/Semi-Arid Zone Arboreal Species (Palmberg 1981, 1983; FAO 1980).

  Follow-up action should include the development of breeding strategies using techniques of sexual and vegetative propagation in order to improve traits such as nitrogen-fixing capability; browse and pod yield and quality; disease resistance; precocious flowering; and fast growth.

CONCLUSIONS

A. albida has been shown to increase crop yield, soil organic matter and soil nitrogen content, soil microbiological activity and soil water holding capacity. In addition, the leaves, small branches and pods make excellent fodder much prized by domestic stock and game. Since the species sheds its leaves during the rainy season it can be used in agroforestry systems together with annual crops. It will, in addition, provide wood, fuelwood, shade and shelter.

Unfortunately, A. albida, like many other arid and semi-arid zone arboreal species, is threatened by the destruction of natural ecosystems as a consequence of changing land use patterns and increasing human pressure.

Urgent action is needed to conserve this valuable species and to realize more fully its potential as a provider of goods and services in rural areas.

REFERENCES

Bonnefille, R. 1975 Présence de pollen fossiles d'Acacia albida Delile et Cycadales dans les échantillons pleistocènes de Melka Kontouré (Ethiopie). Boissiera 24 a: 33–42.

Brenan, J.P.M. 1959 Flora of Tropical East Africa: Leguminosae - Mimosoideae. Crown Agents for Overseas Governments, London.

Brenan, J.P.M. 1983 Manual on taxonomy of Acacia species. Present taxonomy of four species of Acacia (A. albida, A. senegal, A. nilotica, A. tortilis). FAO, Rome (47 pp.).

Brunck, F. 1972 Entomologie et pathologie forestières. Première étude sur le dépérissement des Gaos dans les arrondissements de Magaria et de Matameye. CTFT, Division Entomologie et Pathologie Forestières (10 pp.).

Brunck, F. 1974 Comptes rendus d'un déplacement effectué au Niger du 28 Novembre au 6 Décembre 1973. Etude sur le dépérissement des Gaos (suite). CTFT, Paris (10 pp.).

Butzer, K.W. 1966 Climatic changes in the arid zones of Africa. In: Proc. of the International Symposium on World Climate from 8000 BC to 0 BC. Roy. Met Soc. London: 72–83.

Charreau, C. and Vidal, P. 1965 L'influence de l'Acacia albida Del. sur le sol, la nutrition minérale et les rendements des mils Pennisetum au Sénégal. Agron. Trop 6–7: 660–626.

Dancette, C. 1968 Note on the advantages of a planned utilization of the Acacia albida in Senegal. IRAT, Bambey, Senegal (4 pp.).

Dancette, C. and Poulain, J.F. 1968 Influence de l'Acacia albida sur les facteurs pédoclimatiques et les rendements des cultures. Sols Africains 13 (3): 197–239.

Doran, J.C., Boland, D.J., Turnbull J.W. and Gunn, B.V. 1983 Handbook on Seeds of dry-zone Acacias. A guide for collection, extracting, cleaning and storing the seed and for treatment to promote germination of dry-zone acacias. FAO, Rome (92 pp.).

Dugain, F. 1960 Rapport de mission au Niger. Centre de Pedologie de Hann-Dakar. Mimeo (22 pp.).

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Felker, P. 1978 State of the Art: Acacia albida as a complementary permanent intercrop with annual crops. University of California, USA (133 pp.).

Giffard, P.L. 1974 Les essences de reboisement au Sénégal: le Kad Acacia albida Del (Faidherbia albida Chev). CTFT, Dakar, Mimeo (35 pp.).

Halevy, G. 1971 A study of Acacia albida in Israel. La-Yaaran 21: 86–97.

IRHO 1966 Influence de l'Acacia albida sur la culture de l'arachide. Rapport annuel, IRHO, Sénégal: 19–29.

Jung, G. 1966 Etude de l'influence de l'Acacia albida Del. sur les processus microbiologiques dans le sol et sur leur variation saisonnières. Centre ORSTOM - Dakar, Mimeo (49 pp.).

Jung, G. 1967 Influence de l'Acacia albida Del. sur la biologie des sols dior. Centre ORSTOM. Dakar. Mimeo (66 pp.).

Lemaître, C. 1954 Les problèmes de la conservation des sols au Niger et le “Gao”. C.R. 2è Conférence Interafricaine des sols.

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Palmberg, C. 1983 FAO Project on Genetic Resources of Arid and Semi-Arid Zone Arboreal Species for the Improvement of Rural Living. Report on Progress. Forest Genetic Resources Information no. 12: 32–35. FAO, Rome.

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Radwanski, S.A. and Wickens, G.E. 1967 The ecology of Acacia albida on mantle soils in Zalingei, Jebel Mara, Sudan. J. Appl. Ecol. 4: 569–579.

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Sarlin, P. 1963 Le Faidherbia albida (Acacia albida Del.) à Ouahigouya. CTFT. Mimeo (36 pp.).

Southgate, B.J. 1983 Handbook on seed insects of Acacia species. FAO, Rome (30 pp.).

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Manuscript received in May 1984