Acacia seyal - multipurpose tree of the Sahara desert
One of few strongly gregarious Sahelian tree species, Acacia
seyal combines tolerance of periodically inundated heavy clays with major roles
in fuel and fodder production in countries at the southern edge of the Sahara
desert, especially Mali, Chad and Sudan. A gum (gum talha) is collected from the
tree and a proportion enters international trade. The epithet seyal derives from
an Arabic word for "torrent" used for the species in Egypt and denotes
association with water courses.
Botany
Acacia seyal Delile (family Leguminosae, subfamily Mimosoideae)
is one of over 60 African acacias referred to the Uniseriae group of subgenus
Acacia. The species usually reaches 9-10 m in height at maturity and in
well-formed individuals a flat-topped crown develops. There arc two varieties,
differing primarily in whether or not pseudogalls ("ant galls") develop and in
bark color. In var. seyal there are no pseudogalls and a reddish bark color
prevails, although periodic bark exfoliation exposes a pale powdery surface
which darkens slowly. In var. fistula pseudo-galls arc present and the powdery
bark typically remains whitish or greenish-yellow. Both varieties have paired,
straight, strong, pale-colored, stipular spines up to 8 cm long which in var.
fistula are often fused at the base into the inflated pseudo-galL The leaves arc
bipinnate - usually with 48 pairs of pinnae, each of which bears 10-20 pairs of
close-set, obscurely veined leaflets. Individual leaflets are 1-15. mm wide and
about 5-8 mm long. Small bundles of up to 5 pedunculate capitate inflorescences
arise in axillary positions on the young parts of shoots. Each inflorescence is
vivid yellow in color, about 15 mm in diameter, and is borne on a peduncle 34 mm
long. The dehiscent pods arc flat and somewhat curved, brown and up to about 20
cm long and 5-10 mm wide when ripe, with slight constrictions between the seeds.
In a well-developed pod 6-10 seeds arc present, each 69 mm long, 4-5 mm wide and
about 2 mm thick - in 1 kg there arc 20,00025,000 seeds. The chromosome number
of 2n = 52 suggests tetraploidy.
Distribution
The range of A. seyal extends from Senegal eastwards to western
Somalia and the coastal lowlands east of the Red Sea, and from the Nile valley
of southern Egypt to southern Zambia. The two varieties differ markedly in their
ranges-var. seyal extends westwards from central Sudan and north of latitude
18°N and var. fistula extends south of latitude 10°S. The ranges
overlap mainly in the upper Nile catchment, the Lake Victoria basin and the
Ethiopian and East African rift valleys. Occurrence beyond the natural range is
limited to arboreta (e.g. Iraq, Portugal) and experimental studies (e.g. India).
Ecology
Given suitable climatic and edaphic conditions closed, and
essentially pure, stands of A. seyal develop but admit sufficient light for
grass to grow in the understory.
Through the greater part of the range of A. seyal mean annual
rainfall is 500-1200 mm and there is a well-defined 6-8 month dry season with
mean annual rainfall less than 50 mm. Occurrences in more arid climates arc
associated with the presence of water in addition to direct rainfall The
phenological cycle relates closely to the rainfall regime. Where there is a well
defined unimodal rainfall pattern, leaf fall takes place by the middle of the
dry period and trees remain leafless for 4-7 months, depending on when the
subsequent wet season begins. Leafless periods are briefer in bimodal equatorial
rainfall regimes. Flowering is concentrated in the middle of the dry season and
ripe fruits arc present about 4 months later.
Temperature regimes vary through the range, particularly for
var. seyal which is subject to mean annual temperatures of 18-25°C. Var.
fistula occurs mostly where mean annual temperatures are 20-25°C, but also
in cooler climates in Ethiopia, at the upper elevation limit (1700-2000 m).
Relationships with extreme temperatures follow a similar pattern - in parts of
West Africa where var. seyal is present, absolute temperature maxima are
50-55°C. Absolute minima through the range of the species are generally
5-10°C but below Sac at the northern limit and at altitudes >1800 m. The
distribution pattern overall is indicative of a frost-sensitive species.
Acacia seyal var. seyal from F.E.M.
Booth and G.E. Wickens. 1988. Non-timber uses of selected arid zone trees and
shrubs in Africa. FAO.
Relationships with soil are well-defined. There is an unusual
degree of adaptation for deep, heavy soils (pH 6-8) accumulated a, low points in
a landscape or formed directly from fine-grouned rocks, such as shales, and
readily weathered volcanic materials. In communities containing both varieties,
var. fistula displays greater tolerance of waterlogging and occupies lower
positions in depressions and along drainage lines. Saline soils are not
suitable.
Uses
Fuel.
Var. seyal, especially, is an important source of rural energy
as both fuelwood and charcoal. Stands managed on a 10-15 year rotation yield
10-35 m³ ha-1 of fuelwood.
Fodder.
Both varieties of A. seyal are viewed favorable as forage. Dry
matter net energy contents are high: 6-8 MJ kg- 1 (foliage) and 4-7 MJ kg1
(fruits). The associated digestible protein levels are also :gh: 100-150 g kg1
in the foliage, and higher in the fruits. For both foliage and fruits, analyses
indicate a well balanced supply of minerals and very favorable qualities in
terms of proximate fractions (e.g. crude fiber 10-20%; ether extract <7%).
The foliage of var. seyal has been shown to contain secondary metabolites but
experience suggests that levels are not a matter of serious concern.
Gum talha.
Gum talha has not been toxicologically evaluated and is not
listed as an approved food additive. It contrasts with gum arabic in several
significant respects, being strongly dextrorotatory, of high molecular weight
and low in nitrogen (0.06-0.24%) and rhamnose (<4% sugar composition). Ash
contents of cobalt, copper, iron, nickel and, especially, aluminum (>6000
ppm) are high and tannin is present (2%), restricting acceptable use to such
applications as a binder for foundry molding and a sizing agent in the textile
industry.
Management of natural stands
Both varieties of A. seyal are noteworthy for occurrence in the
undisturbed state in seral, even-aged stands. Reconstitution of an exploited
var. seyal stand depends not on coppice shoots but on the presence of abundant
seed and its exposure to a mild fire which enhances the germination of var.
seyal but checks the regeneration of competing species. Stands 15 years old when
harvested are likely to have produced a seed reserve sufficient to regenerate
the stand. However, individual trees or uncut patches of the original cover
should be left as seed sources to insure abundant regeneration. Where management
for fodder production is concerned, evaluation of responses to lopping and
cutting of var. seyal indicate limited recovery capacity in mature trees.
Beating branches to detach leaves and fruits without damage to axillary buds is
therefore preferred to exploit these as dry season resources.
Propagation
Unopened, full-sized fruits are gathered off the trees and
allowed to release seed. After cleaning, seed stores well in cool, dry
conditions, remaining viable for up to 8 years. Pretreatment in the nursery is
advantageous, although not essential, to accelerate the germination rate.
Scarification and acid treatments have proved favorable. However, germination
rates have rarely exceeded 30% in 7 days. Seeds can be pregerminated in contact
with moist cotton wool or filter paper to allow rapid identification of viable
non-dormant seed. Transfer to containers filled with a silt-rich medium.
Seedlings require shade until the second leaf expands and watering at intervals
of 1-3 days as necessary to keep the medium moist but not waterlogged.
Formal stands
Stands of var. seyal have been established in Sudan, often by
direct sowing of pretreated seeds to prepared planting spots. Sowing seed in
batches ensures a high proportion of spots become occupied. Competition from
weed growth is overcome by using taungya, with mechanized site preparation and
sowing. Sesamum or Sorghum is intercropped among widely spaced (ca 4 m) rows of
trees. For poles and fuelwood a 20 year rotation is projected. Initial stocking
is 1000 stems ha -1. Thinnings after 10 and 14 years reduce stocking
to 675 and 450 stems ha -1, respectively.
Nodulation
Nodulation occurs in natural populations. In artificial
regeneration it has been achieved by pelleting seed with culture of bacterial
isolates, sowing into an infected medium or germinating in unsterilized soil.
Uninfected seedlings have been inoculated successfully by treatment with a
suspension of a symbiont. Rhizobium strains from A. mellifera and A. senegal and
Bradyrhizobium from the latter have proved to be effective symbionts.
Pests
Over 40 species of insects are reported associated with A.
seyal. These include 10 species of bruchid beetles which may damage high
proportions of stored seeds. Beetles of various other families attack the wood,
the postrychid Sinoxylon senegalense being the most notorious and swiftly
locating and infesting freshly cut wood, especially if lying on the ground.
Attacks are much reduced if the bark is removed and the cut stems stacked
upright Subsequent creosote treatment ensures extended durability.
Selected references
Adams, M.E. 1967. A study of the ecology of Acacia mellifera, A.
seyal and Balanites aegyptiaca in relation to land clearing. Journal of Applied
Ecology, 4: 221-237.
Booth, F.E.M. & Wickens, G.E. 1988. Non-timber uses of
selected arid zone trees and shrubs in Africa FAO Conservation Guide, 19: 8-12.
Hall, J.B. & McAllan, A. 1993. Acacia seyal: a monograph.
School of Agricultural and Forest Sciences, University of Wales, Bangor.
Le Houérou, H. N. 1980. Browse in Africa the current state of
knowledge. International Livestock Centre for Africa Addis Ababa. 491 pp.
Tybirk, K. 1991. Regeneration of woody legumes in the sahel.
Aarhus University Botanical Institute. AAU Report, 27: 1-81.
For a complete set of references contact the author or NFTA.
A Publication of the Nitrogen Fixing Tree Association c/o
Winrock International Rt. 3, Box 376 Morrilton, Arkansas 72110 USA Tel:
501-727-5435; Fax: 501-727-5417
NFTA 91-01 April
1991
