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Commiphora africana, browse tree of the Sahel**

Jean-Charles Clanet

Maître-assistant, Département de Géographie, Faculté de n' Djamena, Chad

Hubert Gillet

Maître de conférences, Sous-Directeur, Museum National d' Histoire Naturel, Paris


Introduction

Range and habitat

Brief botanical description

Phenology

Commiphora africana as a stabilizing element in pastoral migration patterns

Pastoral utilization of Commiphora africana stands

Advantages of Commiphora africana

Feed value

Reproduction and mortality

Uses


Introduction

Commiphora africana is a small Sahelian tree, not exceeding five metres in height. With its dreary appearance— it looks half-dead during the dry season— it hardly creates a favourable impression. Until now it has hardly attracted any attention from range scientists, being classed amongst the rejects. However, in some areas herdsmen are familiar with this plant and value it on several scores, but particularly as a browse tree. The aim of this short paper is to consider some of the uses of this member of the Burseraceae family, to which so many aromatic species belong.

Range and habitat

Commiphora africana occupies a range which is typical of the southern Sahel in northern tropical Africa, but which also extends at a right angle, southwards into eastern Africa and down to South Africa. North of the equator the range coincides with the southern edge of the Sahel zone. Starting in Mauritania and Senegal (Ferlo), it crosses Mali south of the loop of the Niger, passes through Upper Volta, then through Niger where it widens out on both sides of the Nigerian frontier, reaches Lake Chad and crosses the Chad level with the 14 and 15th parallels; it then enters the Sudan (Darfur, Kordofan), followed by Ethiopia and Eritrea, and finally Somalia, before curving southwards through Kenya and Uganda. Commiphora africana is rare in Tanzania, but becomes abundant again in Mozambique, Rhodesia and south-west Africa.

In its favourite habitat, i.e. on sandy soils, it forms pure stands three or four metres in height, meriting consideration as a plant association in the proper sense. Stands of this kind have been indicated by Aubréville, particularly in Niger, between Gangara and Aberbissinat, or between In Gall and Tahoua, and also by one of the present authors (Jean-Charles Clanet), in a strip bounded by the two meridians passing through Mainé Soroa in Niger and Moussoro in Chad, and by latitudes 13 ° and 16° north. The main features of this strip are the Kadzel district, the valley of the Dillia, the erg of Belaberim in Niger, and the three adjacent ergs of Manga, Liloa and Chichati in Chad. Another stand has been indicated by Hibert Gillet further to the east, in Chad, near the Fama Wadi, north of Biltine (15 ° 30).

In addition to this preferred habitat, the tree grows freely on rocky or stony escarpments (e.g. Aïr. Ouaddaï and Ennedi). It may occasionally be met with in pockets outside its Sahel range, where it may have been introduced by animals, since the kernels of the small drupes are not broken down by the digestive juices. Pockets such as these are found on the Mossi plateau in Upper Volta and in Togo, near Lomé. Although practically pure in its preferred environment, it mixes with other species on either side of this narrow strip, for example with Acacia raddiana to the north and with Combretum glutinosum to the south.

Brief botanical description

Commiphora africana may be recognized unmistakably from a distance by its characteristic outline — it has a spherical top and a short trunk with low branches — and by its bent, misshapen main branches giving rise to short, straight secondaries which are lignified and shaped like elongated thorns. The bark is lustrous while the foliage is being renewed, but dull during periods of non-growth, and as in many species of the Burseraceae family it sheds as a membranous peel. It has a creeping root system which spreads several metres round the tree. Unlike the primary branches, the main roots are straight but not uniformly lignified. If they are laid bare by wind erosion of the surrounding soil, they become sculptured by the wind, the softer part being worn away while the harder part stands out in relief.

The colour of the foliage is unlike any other in the Sahel. The upper surface of the leaves is a waxy gray-green, while the lower surface is lighter in colour. The leaves are made up of three folioles, cuneate at the base and unevenly serrated with a slightly longer median (up to 4 cm long × 2.5 cm wide). The leaflets are lightly pubescent on the upper side, especially when young, but more densely coated on the underside. This covering of soft, short hair makes them soft to the touch. When bruised the leaves are aromatic.

The flowers are discrete (6 mm), occurring in groups of about 10 growing in small auxiliary clusters. That the four petals will be red may be seen when the flower is still in bud, since it is poorly concealed by the four sepals. When open the flower reveals eight stamens arranged in two rows of four, uneven in height. Many flowers abort, and the fruit consists of reddish-coloured drupes 17 to 18 mm across, containing a white kernel and a resinous pulp.

Phenology

Commiphora africana is a true humidity detector. It is sensitive to the very first breath of the humid wind from the sea, or in other words as soon as the prevailing northeasterly changes so that the wind blows from the opposite direction, its leaf buds expand and in a few days send out young leaves. The Commiphora is the first tree to come into leaf and the leaves are nice and tender, so that the full significance of this early return to greenery, after a long dry season in which livestock have been deprived of fresh foliage, can be appreciated. Every year the Commiphora's return to the green stage is an event awaited with great impatience by the herdsmen, who immediately drive their hungry herds towards this hoped-for source of food.

The leaves then spread out and the tree remains astonishingly green throughout the rainy season. Of all the trees of the Sahel, the Commiphora and the Salvadora persica are the ones with the most tender foliage, and for this reason as well as others it is greatly appreciated by camels, who can easily reach the leaves, as also can goats.

However, the leaves are only able to survive in moist air: they are very sensitive to drought and dry out as soon as the season of continuous rainfall is over. At this time the Commiphora turns a beautiful golden colour, just like the yellow shades of autumn leaves in temperate countries (e.g. the oak or the chestnut). The leaves then fall. The leafy phase is thus synchronous with the rainy season. In his thesis, Poupon notes that in 1972 at Fété Olé (with only 38 mm of rainfall, as against an average of over 300 mm), the duration of the leafy phase was extremely short and the tree came into leaf a second time at the beginning of October, when rain fell again.

Somewhat later in the dry season (from October to January, according to the year and habitat), the small flowers appear. Flowering, and to an even greater extent fruiting, does not occur regularly every year. The growth cycle of Commiphora is that of a Sudanian tree adapted to a Sahelian climate. Its foliage shows none of the features associated with adaptation to xeric conditions. Nonetheless, the tree has to withstand the long dry season, which may last nine or even eleven months as it did in 1973. It protects itself by disfiguring its primary branches and by lignifying its secondaries at an early stage so that these become perennial, hard and morphologically thorn-like. The bark thickens and the root system spreads itself out as far as possible.

Herdsmen, acute observers as they are, have for a long time observed this curious tree, in which the foliation biology has remained that of a humid tree although its habitat has become dry. Over the centuries a pastoral way of life making generous use of the Commiphora has evolved. This way of life deserves closer examination.

Commiphora africana as a stabilizing element in pastoral migration patterns

The inhabitants of the sub-Sahara, who derive a living from the various natural environments of the Sahel by using livestock, primarily camels, cattle, and goats, lead agropastoral lives of a wide variety of different shades and combinations. These range from a completely sedentary life, which is rare outside the towns, to a life of permanent migration. The latter is a necessary response to the constraints inherent to desert and subdesert environments. However, when they can be used, stands of Commiphora africana help to reduce the movements of nomadic herdsmen by supplying them with green browse at the most critical period of the dry season. Unfortunately, water shortages force the herdsmen to migrate even when plant formations in which Commiphora africana dominate are present.

Thus, for example, in eastern Chad and western Sudan, where Commiphora africana is abundant, available water resources rapidly dry up after four or five months of the dry season. Camel herdsmen are compelled to follow the classic form of nomadism proper to the "endodromic" areas found at this latitude. After a prolonged stay on the dry-season rangelands they become increasingly mobile, migrating in two opposite directions in order to draw in the plant resources of the two neighbouring zones. The first is a southward migration towards the outlying green pastures of the Soudano-Sahelian areas, while the second, which takes place in the middle of the rainy season, has the achebes of the Sahara as its destination.

In some areas which are privileged in terms of water resources, where the drop in the water level is minimum, the nomads are able to use local supplies of Commiphora africana as end of dry season browse, thereby enabling them to miss out a stage along their migratory route which is a sensitive one owing to the ever more densely woven fabric of sedentary fields through which they must weave their way, meeting frequent opposition. As may be seen, the opportunity of using Commiphora africana sets up a special type of "endodromia", the importance and special characteristics of which deserve to be emphasized.

Pastoral utilization of Commiphora africana stands

The Toubous, Dazas and Arabs who inhabit the area defined in paragraph 2 manage to make the best of the grazing available to them. For these peoples, formations of Commiphora africana represent not merely a providential browse resource to be used only at a clearly defined moment of the season, but also determine all camel and goat movements before the onset of the rainy season.

In the ergs north of Lake Chad, and on the sandy slopes ending in the tableland which stretches from Koutous to the Niger, water is never very far from the surface. In the depressions and basins between dunes the traditional wells reach water at between three and eight metres. Thus the bridging period between dry feed supplies and the new green shoots following the first rains is not further complicated by the problem of finding water. However, in the area described the bridging period may sometimes be reduced by more than a month and a half thanks to the early appearance of the leaves of Commiphora africana. For the local herdsmen this event marks the onset of the period of dry tornadoes which the Toubous call "Archat" and the Arabs "Richach". It corresponds to the change in the direction of the wind which precedes the rains. This short intermediate season ends with the first showers.

During this period, which may last from mid-May to July, the camel and goat herds are led down to the basins, the steep sides of which are covered with Commiphora Africana, which the Toubous call "digi" and the Arabs "gafal". To some extent it is an example of genuine transhumance, representing a point in the annual migration pattern which the pastoralists await with a great deal of anticipation, similar to the time at which European farmers send their animals up to the high alpine pastures. The local terminology is significant in this respect: "goni digou dertété", it is the time for taking the camels to browse on the "digi". The appearance of new leaves on Commiphora africana thus marks a precise moment for the society, and a happy one as well since it is the prelude to the joys of the rainy season. Similarly, the Arabs decide to "Nensara elbil fil gafal", i.e. to go and herd their camels "over" the "gafal". The particular importance of this feed supply at the end of the dry season will only be fully understood if it is borne in mind that the other herdsmen of the Sahel in these latitudes, who are less favoured by local conditions, have to make do with dried straw and stunted shrubs which are severely overgrazed and eroded by trampling as well as partly destroyed by bush fires. All this comes at a time when the animals, weakened by seven months of the dry season, are suffering from heat and exhaustion owing to the increasing distances they must travel in the search for food.

Advantages of Commiphora africana

The Toubous and the Arabs claim that with Commiphora africana the animals make up the flesh which they have lost during the dry months, and that they are subsequently in a better position to profit from the Sahara pastures to which they return in the rainy season. The Daza camel men also say that when the leaves appear very early they are relieved of the additional burden of taking the animals fed on them to the natron wells on a salt cure (in June 1974 the only female camel in our caravan gave more milk after browsing for three days on Commiphora africana at Téfidinga, near the frontier between Niger and Chad). To these nutritive and diuretic qualities may be added other advantages in using this type of browse, and the herdsmen are inexhaustible on the subject.

The density of Commiphora africana stands mean that the animals hardly have to move to obtain their feed ration. When their appetites are satisfied they can lie down to ruminate under the shade of the trees, which, although limited, is valuable in these areas during the sultry months. A further advantage is the fact that the trees are concentrated, thereby preventing the camels, which are always very difficult to supervise at this time of year and are thus easily rustled 1, from wandering too far, since as soon as they scent the first smell of distant rain they head straight towards it, often covering twenty or thirty kilometres before stopping. Fetching them requires extensive journeys. Finally, "digi" grazings avoid the need for southward migration, thereby keeping the animals away from mosquitoes and flies, and thus from the diseases for which these insects act as vectors. Again from the point of view of disease prevention, the fact that each basin contains a single isolated colony of Commiphora africana prevents the joint utilization of this shrub land grazing by herds from other localities, each depression harbouring no more than one or two families of nomads with their livestock.

1 This may sound ludicrous, until it is remembered that 50% of court cases still consist of livestock thefts.

At this point it is appropriate to undertake a bromatological enquiry in an attempt to gain a better understanding of the yearly quest for digi grazing.

Feed value

The leaves and fruit of Commiphora africana have been analysed at the Laboratoire de Nutrition of the Institut d'Elevage et de Médecine Vétérinaire des Pays Tropicaux (IEMVT). The results are as follows:

 

Young leaves

Fruit

Dry matter (DM)

  91.70

85.50

Crude protein (CP)

  14.25

  8.40

Cellulose

  12.90

21.50

Fat content

    2.47

21.50

Mineral content

939

  7.00

Ca

   0.98

  0.88

P

   0.18

  0.16

Mg

   0.48

  0.39

K

   2.47

  1.95

Ca/P

   4.45

  5.57

Ca/Mg

   2.94

  2.25

In terms of the CP content of its leaves, Commiphora africana occupies a halfway position in the list of trees, but it should be taken into account that its 14% CP is available at a time when CP resources generally are extremely low. Commiphora foliage also provides an appreciable mineral content, but like most Sahel trees it suffers from a deficiency in P and Mg, so that the Ca/P ratio is too high owing to the lack of P. The Ca content is well above the necessary (0.2% of DM), while K is also in surplus.

Reproduction and mortality

The tree will only bear fruit when the climatic conditions are right, i.e. only in certain years. How long the germinating power of the scattered seeds lasts is not precisely known. The kernel is hard and the indications are that germinating capacity is certainly improved by passage through the intestines. It is also possible that fruit-eating birds play a part in spreading Commiphora seeds (stocks have been found in Togo).

In the course of eight years of observations at the Fété Olé station, Poupon found that the germination of Commiphora was sporadic, i.e. zero in certain years but considerable in others (e.g. 406 germinations observed over 25 hectares in 1973).

Although in some areas the trees resisted the recent period of drought very well, this was not the case at Kanem in Chad, where Jean-Charles Clanet observed whole burial-grounds of dead trees lying on the ground, especially in the triangle between Tarey, Zigueiï and Faragoul, and also to the southwest of Bir Tchao, in western Soulias as far as Bir Edet, and between Ourel and Karkour, in the Préfecture of Batha.

Owing to the fact that their roots are near the surface, the trees are vulnerable to wind. Each year the violent gusts which precede the tornadoes uproot a certain number of them, even succeeding in turning them upside-down, so that the tree lies with its roots stranded in mid-air. These trees are certain to die, but others, which are simply flattened, recover by a process of natural layering. The growth rate of Commiphora is regular until the base of the trunk reaches 40 cm. They are then about 30 years old.

Uses

The wood is resistant and easy to work. It has the extraordinary advantage of being very little susceptible to attack by termites. It is preferred to all other woods for using in bedsteads (in the Batha area). The trunks of the largest trees can be cut up into planks. Together with the wood of Balanites, to which it is preferred owing to its finer texture and resistance to splintering, it is the favourite wood used in Koranic wooden-engravings.

Elegant fragments of root, which are straight and naturally sculptured by the wind, can become ornaments in the decoration of huts. According to the claims of the Quaddai hunters in Chad, the greater kudus like to rub the base of their horns against the trunks and junctions of the main branches, doubtless in order to rid themselves of excoriations.

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