Session 1 Botany and Distribution

Invited Paper

The Botany and Distribution of Faidherbia albida

P.J.Wood¹

Abstract

Faidherbia albida (syn. Acacia albida) is a unique member of the Acacieae tribe of the Mimosoideae. It is mainly a species of the Sudanian and Sahelian zones of Africa, reaching into the Sahara and beyond, along watercourses. It is particularly well adapted to use as an agroforestry tree, and its ecological optimum is on sites with deep sandy soils and an annual rainfall of 500-1000 mm. This applies equally to western, eastern, and central Africa. It is a plant that loses much water by evaporation and therefore can only develop on moist soils or on soils that allow the development of an extensive root system down to the water table where it can obtain adequate moisture.

Introduction

Faidherbia albida Del. A. Chev. (syn. Acacia albida) is widely distributed throughout the dry zones of tropical Africa. Although of the Acacieae tribe, it has many unique botanical and phytochemical features, as well as botanical naming, that justify its classification as a separate genus.

In western Africa, though not always in eastern, central, or southern Africa, the species has the unique characteristic of shedding its foliage at the start of the rainy season, and of coming into leaf in the dry season. This unexpected "inverted" phenology means that its presence in farmers' fields does not interfere with agriculture, and, indeed, makes it an ideal agroforestry tree for use in combination with crops. It also provides animal fodder in the dry season. These qualities are traditionally well known in rural West Africa, where generations of small farmers have deliberately encouraged it in a kind of cultivated parkland (Pélissier 1980). In southern Africa, mono-specific stands are also thought to result from past cultivation (Fagg and Barnes 1990).

Botanical Characteristics

Morphology and General Features

Under favorable conditions, F. albida can attain a very large size; heights of over 30 m and a diameter of 1.5 m have been noted along the Cunene River on the Namibia-Angola border (NAS 1975). More usually it reaches 15 to 20 m in height and a breast height diameter of up to 1 m. The crown tends to be shaped like an inverted pyramid in young trees, becoming hemispherical with age. The lifespan is generally about 70 to 90 years, although some individuals of over 150 years are reported from Zambia.

The bark of the tree is characteristically dull brown to whitish grey, smooth when young, more fissured and flaky and more cork-like in older specimens. The slash is fibrous, pink to light brown.

Botanical Description

Leaves. The leaves are typical of the Mimosoideae, compound and bipinnate with leaflets borne along the pinnae. They are highly variable: the petiole varies from 0.5 to 3.7 cm long. The rachis is 3-7.5 cm long and carries 2-12 pairs of pinnae, each of which is 2.5-5.5 cm long and carries 6-23 pairs of leaflets. Leaflets are glabrous to pubescent, bluish green, ob-longate and occasionally submucronate, overlapping slightly, 2.5 to 12 mm in length and 0.7 to 5 mm in width (Fig. 1) (CTFT 1989).

Thorns. Thorns occur in pairs at the base of the leaves and are modified, spiny stipules. They are straight and robust, thickened at the base and often (particularly when juvenile) orange or brown at the tip and are 0.2-3.2 cm long. They may be distinguished from those of Acacia species with long thorns, such as A. tortilis subsp raddiana, A. nilotica, or A. seyal, by their basal thickening.

Flowers. The flowers are borne in dense axillary panicles 3.5-16 cm long, with a peduncle 2-4 cm long. They appear about 2 months after the tree comes into leaf and are sessile or with a pedicel of up to 2 mm. In color they are successively white, cream, and then yellow, and are strongly perfumed. The calyx is 1-1.7 mm long, glabrous to pubescent, consisting of five sepals. The corolla, 3-3.5 mm long, carries five free petals. The stamens, numbering 40 to 50, are 4-6 mm long and their filaments are fused for about 1 mm. There is also partial fusing between the stamens and the bases of the petals, (epipetaly). The anthers are 0.2-0.4 mm in diameter, lacking glands even in the bud. The ovary is lightly stipitate and bears small hairs. Flowering commences towards the seventh year (Nongonierma 1976; McGahuey 1985).

Fruits. The fruit is an indehiscent pod varying from bright orange to reddish brown and 7-9 mm thick, 6-35 cm long, and 1.4-6 cm wide. It falls from the tree about 3 months after flowering. The pod surface is convex on one side and becomes concave on the other, and as the fleshy mesocarp lignifies, it tends to roll into a spiral resembling dried apple peel, hence the common name "Apple Ring Acacia." There is much variation in pod shape both within and between trees.

Seeds. Each pod contains 10-29 dark brown shiny seeds, separated by thin septa. They are ovoid, 10 x 6 mm in size and are characterized by an elliptic areolus or hilum measuring about 8x5 mm. The seed coat is tough, waterproof, and leathery, and maintains seed viability for many years.

Taxonomic Classification

Identification and Classification

The first botanical determination of the species under the name A. albida Del. was made by Delille in 1813 and was based on a specimen obtained in Egypt. Bentham (1875) recognized the species as belonging to the Mimosoideae subfamily of the Leguminosae in the Gummiferae series (Table 1), which is characterized by possessing spiny stipules.

Acacia comprises some 1200 species and is the sole genus in the tribe Acacieae, which are characterized by possessing stamens either free or united only at the base.

Table 1. Taxonomic classification after Bentham (1875).

Botanical Nomenclature

Because A. albida (F. albida) is quite distinct botanically, having no other species closely related to it, its classification has given rise to a number of taxonomic studies. Baillon (1863) concluded that the staminal filaments were indeed joined at the base, but that this feature was not sufficiently important to justify separating the species from the genus Acacia. On the other hand, Chevalier (1934), using the same distinctive characteristic as well as other features (which are discussed further below), preferred to exclude the species from the genus Acacia and place it in the new monospecific genus Faidherbia. This move established the link between the tribes Acacieae and Ingeae, the latter being another tribe of the subfamily Mimosoideae characterized by the stamens being more or less fused into a single ring. This taxonomic classification has since been followed by a large number of authorities quoted in CTFT (1989).

Figure 1. Botanical illustrations of Faidherbia albida. (Source: CTFT1989.)

While accepting the distinct nature of the species, some authorities still prefer to keep the name A. albida after nonetheless pointing out the differences between this species and other acacias:

"Although different from the other African acacias, A. albida has nevertheless many features in common with them. It could ultimately turn out to be preferable to transfer the species to the genus Faidherbia" (Ross 1966).

Hutchinson and Dalziel (1958) retain the name A. albida. However, Brain (1987) points out that the chemical constituents of the species are quite different from those of other acacias, and Robbertse (1974) has shown that the pollen also differs. Some authorities still have reservations about keeping Faidherbia separate from the genus Acacia but on balance it seems logical to put the species into the genus proposed by Chevalier (1934), which links the tribes Acacieae and Ingeae. This paper therefore uses the specific epithet F. albida (Del.) A. Chev.

Distinguishing Characteristics. The characteristics distinguishing this species from other Gummiferae may be summarized as follows:

• Phenology: deciduous in the wet season, foliated in the dry.

• Cotyledons: cotyledons are sessile, whereas all the other Gummiferae have petiolate cotyledons (Robbertse 1974; quoted in Ross 1979).

• Foliage: the primordial leaves are bipinnate, whereas all the other known members of the Gummifera have at least one simple pinnate primordial leaf (Vassal 1979).

• Pollen grains; without ridges, unlike those of other Gummiferae, are of four pores and with an exine like those of the Ingeae.

• Wood anatomy: layered structure, and straight rays (Robbertse et al. 1980; Fahn et al. 1986) differ from those of other African accessions.

Natural Distribution

Africa

The main area of natural distribution of F. albida is Africa, as shown in Figure 2. The species occurs right across the African continent from Senegal and Gambia to the Red Sea (Egypt, Sudan, Ethiopia, Somalia, and Kenya). Further south, it is distributed through eastern and central Africa as far as Natal in South Africa, and westwards to Lesotho, Angola, and Namibia, though it is absent from a corridor in the eastern parts of the last two countries. Apart from this area, its distribution corresponds precisely with the Guinean forest zone of Africa.

The natural distribution does not appear always to follow the climatic zone that characterizes the Guinean zone, however, because the species also occurs along watercourses and in other areas where ground-water is present, or where there is residual water in the alluvium of a seasonal water course. In such places, the distribution is discontinuous, and includes sites in the mountain massifs of the Sahara (Air, Tibesti, Ennedi, and Hoggar, for instance). It also occurs along the Nile in Egypt.

Outside Africa

Outside Africa, spontaneous occurrences of the species are found in Yemen, Israel, Jordan, and Lebanon, where the northernmost is 30 km north of Beirut. The species does not occur in Madagascar.

Introductions have been made to Ascension Island, the Cape Verde Islands (Wickens 1969), Cy-prus, and Pakistan (Brenan 1983). More recent introductions have been made to other countries, notably Peru and India; in India, it is of interest as occupying an ecological niche similar to that of the indigenous Prosopis cineraria.

With a species that has been cultivated so widely and for such a long period, it is now difficult to determine its actual origin. Aubréville (1937) considered that it was originally a riverine tree of eastern and southern Africa, and was introduced into the northern part of its range in West Africa, where it is only found on cultivated or previously cultivated lands. Chevalier (1934) was of the view that it originated in the Sahara before the current desertification, and that it would have become domesticated in the Sudanian zone. Trochain (1969) claimed to have identified a "semi-climax of F.albida in the south-western part of Senegal," where the species would have been brought in by nomadic peoples (through the effects of their animals) in particularly hard times. Human influences thus developed "stable tree populations which, on the sandy clay soils, replaced the different natural open forest climax formations of the Sudanian zone, these being Combretiun glutinosum on the lighter, sandier soils and Acacia seyal on the heavier sandy clays" (Felker 1978).  

Figure 2. Distribution map of Faidherbia albida. (Source: CTFT 1989.)

Many foresters also believe that the species is not indigenous to the natural forest areas of West Africa, having been introduced there through pastoral ism and agriculture. Many of the places where it is common today can be identified as old animal compounds, water holes, or common grazing grounds. It is also found in religious forests, where it may have been planted in the past, though there are no records of this.

On the other hand, the two separate races of the species are regarded by Wickens (1969) as an indication of a northern origin for the species.

Intraspecific Variation

Fagg and Barnes (1990) report great variation between provenances of the species both in morphology and phenology. The existence of two well-defined races of F. albida in eastern Africa has been described by Brenan (1959):

• Race A: young branches glabrous or nearly so, similarly the inflorescence, peduncle, calyx, and corolla. Leaflets ciliale along the margins, otherwise glabrous or nearly so, and generally small, up to 6 mm long by 1.5 mm wide.

• Race B: young branches pubescent, similarly the inflorescence, peduncle, calyx, and often the corolla. Leaflets pubescent on the surface, generally larger than in Race A, up to 12 mm long and 4 mm wide.

The two races are not distinguished taxonomically because a large number of intermediate forms are encountered in certain parts of the range. Ross (1966) states that the characteristics of the two races described by Brenan are not strongly correlated and, as Brenan points out, they appear more as heterogeneous features to the extent that intermediate forms show peculiarities of both races. However, there is confusion only in some regions. Ross indicates that most specimens from the southern part of the range fall within Race B, which is the only one represented in Lesotho, Namibia, Zambia, Zimbabwe, Mozambique, Malawi, and most of Tanzania.

Race A is found in northern and eastern Tanzania, Kenya, Uganda, Somalia, and Ethiopia. In Sudan and in West Africa, Race B is common, but many specimens combine characteristics of both races; i.e., large leaves and glabrousness or small leaves and pubescence. In Egypt and in the northern populations, most specimens appear to be intermediate between the two races, being generally more or less pubescent but with smaller leaves than normal in Race B. The situation is probably the same in the Yemen, but botanical material is currently inadequate. In Cyprus, glabrous leaves predominate, but they are larger than usual in Race A.

Nongonierma (1976) identified four intraspecific taxa, which do not, however, compare with Brenan's races, and are based only on northern and western African material (Fagg and Barnes 1990). The identified varieties (Acacia albida var senegalensis Benth., A. albida var microfoliata De Wild, and A. albida var variofoliata De Wild.) all belong to Race B (Brenan 1983).

Ecology

The wide natural distribution of F. albida has already been noted. It is clearly a species with considerable ecological adaptability and it is important to examine the relationship between climate, soil, and growth and, if possible, to define an ecological optimum.

Climate

Trochain (1969) gives climatic diagrams for natural sites for F. albida (Figure 3). These are:

• Dry Mediterranean: long summer, dry season with long days, and annual rainfall between 100 and 400 mm (e.g., Gaza).

• Tropical: dry season with short days and rainfall varying from less than 300 mm to over 1000 mm (Northern hemisphere: Dakar and Sahr (southern Chad), with a rainy season from May to October-November. Southern hemisphere: Lusaka, with a rainy season from September-October to April-May.

These climatic diagrams illustrate the wide range of climatic conditions in which the species will grow, the only constant feature being a long and well-defined dry season. Saturation deficit, particularly in the Sudan and Sahelian Zones, is an important parameter, not often included in climatic records. Obviously the species will stand very low air humidities, as in the mountains of the Sahara. In some countries, however, the air humidity in the dry season is often relatively high, as in eastern Africa. In the eastern Mediterranean, the species survives frost.

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Figure 3. Climatic diagrams for four sites of origin for Faidherbia albida. (Source: Trochain 1969; reproduced from CTFT1989.)

Recent seed (17 collections) and botanical collections in southern and eastern Africa, including Namibia (Fagg and Barnes 1990) have recorded the species in less than SO mm of rainfall as well as in a zone receiving over 1500 mm.

Altitude

F. albida will grow at a wide range of altitudes, from 270 m below sea level near the Dead Sea in Palestine to over 2000 m in Ethiopia and the Jebel Marra of the Sudan.

Soil Requirements

In eastern, southern, and central Africa the species is often found growing near sources of water, such as along rivers, on the shores of lakes, and in gullies and ravines. In all these situations, the soils tend to be alluvial or hydromorphic. These seem to be the ideal sites; in Sudan, for instance, the greatest concentrations of the species occur in wide belts along the sandy alluvia of the drainage basin of the Jebel Marra (Wickens 1969). It is also found on stabilized sand dunes. In western Africa, as we have seen already, the distribution of the species is generally a result of human activity. Here, it prefers deep, lighter sandy or silty soils, though occasionally it is found on lateritic soils with a shallow pan.

Water Requirements

The tree does not conserve moisture but is effective in obtaining water for transpiration, as amply demonstrated by its vegetative vigor in the dry season, in the total absence of rain, when temperatures are high, humidity is low, and evapotranspiration is at a maximum. To achieve this, its root system has a taproot that grows very rapidly and will reach the water table if it is within range; depths of about 40 m have been recorded (Lemaitre 1954). When on its preferred sites, notably in association with water, the species prefers deep sandy soils easily exploited by its root system.

Acknowledgment. The bulk of this paper relies heavily on the standard monograph on the species produced by the Centre technique forestier tropical (CTFT 1989), to which grateful acknowledgment is made.

References

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Footnote__________

I. Overseas Development Administration, London, UK.

Wood, P.J. 1992. The botany and distribution of Faidherbia albida. Pages 9-17 in Faidherbia albida in the West African semi-arid tropics: proceedings of a workshop, 22-26 Apr 1991, Niamey, Niger (Vandenbeldt. R.J., ed.). Patancheru, A.P. 502 324, India: International Crops Research Institute for the Semi-Arid Tropics; and Nairobi, Kenya: International Centre for Research in Agroforestry.