Hubert Gillet
Maitre de Conférence, Sous Directeur, Laboratoire d` Ethonobotanique et d` Ethnozoologie,
Muséum National d'Histoire Naturelle, Paris
1. The importance of ligneous forages
3.2 Trunks cut some distance above the ground
The Sahel is one of the most inhospitable areas of the world. Men, animals and plants are all subject to severe tests. They have to survive a very long dry season, which grows increasingly severe as the months pass. Each dry season is a different length to its predecessor and to the one following it, and the same is naturally true of the wet season also. There are few countries in the world where the amount of rainfall from one year to the next is as subject to freak fluctuation.
Nevertheless, animals are able to survive there permanently and apparently tolerate the climatic stresses well. Amongst them are the desert gazelles, notably the dorcas gazelle. In April or May, the hardest part of the dry season, it is striking to note how these animals are well fleshed with lustrous coats and how they leap and frolic freely, while the cattle are thin and visibily in a poor state of physical resistance. The former are able to abstain from water, searching out their food and finding in the leaves and young branches, as well as in their fruits and seeds, a complete diet, while the latter have to make do with eating whatever they find in the places to which they are led, usually dried grasses and occasionally leaves of Acacia, Balanites or Cadaba.
This behavioural difference may appear secondary. In fact it is of capital importance. Many bromatological analyses have shown that ligneous forage, especially that of Sahel trees, is far richer in protein than dry grass (22% for Balanites leaves, 20% for those of Maerua crassifolia, 15% for those of Acacia raddiana, to quote only three of the most significant examples, as against 4–5% for straw). Ligneous plants also provide fruit.
Grass seeds, some of which have a high starch content (e.g. Panicum turgidum, Panicum laetum, Brachiaria deflexa, etc.) fall to the ground where they mingle with grains of sand, escaping out of reach of heavy livestock.
It is nonetheless true that many Sahel trees provide a secondary but nevertheless essential source of feed for livestock at a time when the only other resources available are strawy residues with a high cellulose content and very low in DCP.
Although therophytes are equipped with a survival method which is particularly effective through drought and especially so during a run of dry years, since the seed remains in a dormant state and can stay in cold storage for several years, ligneous phanerophytes are also able to adapt with great efficiency. The following adaptations, for example, enable the plant to continue to assimilate, building up carbohydrates by a process of photosynthesis: the thick leaves of Boscia senegalensis and B. angustifolia, leaves protected by a thick cuticle such as Balanites aegyptiaca, and especially the chlorophyll content of the branches of Capparia decidua, Leptodenia pyrotechnica and even of Balanites aegyptiaca. The consumption of these young shoots is a resource which is often underestimated.
Tree cover in the Sahel is becoming thinner year by year. Overstocking has taken its toll. In many areas of Niger, the regeneration rate is zero. No new stocks are appearing to replace the old trees which die off one after the other. All young plants are relentlessly devoured by livestock after the first few months of their life. As a result the tree cover, owing to the lack of young replacement stock, is growing older and older.
Although browsing is one of the main causes of destruction for ligneous plants, it is unfortunately not the only one. In areas where man turns to his own use anything that comes to hand free of charge, the tree cover pays dear in terms of human depredation.
Wood remains the main source of fuel, and consumption of wood for cooking may be estimated at 100 kg per inhabitant per month. The large capital cities such as Bamako, Ouagadougou and N'Djamena burn a hundred thousand tonnes of wood annually. This tonnage has to be found and inevitably, despite the new eucalyptus plantations, the large cities become the centre of a desert area devoid of ligneous plants, the radius of which grows year by year.
Beyond a certain distance, the cost of transporting wood becomes prohibitive, so that in terms of weight it becomes advantageous to turn the wood into charcoal, a lighter material despite the loss in calorific value per unit of wood. This loss varies as a function of the species of tree and the manufacturing method used.
However, it is the larger trees which bear the brunt of these operations, since charcoal can only be made from large branches. Charcoal burnes, as they are called, are making serious inroads into the ligneous stratum. When the trunk is too large to be felled by axe the vandals light a fire at its base, which slowly consumes the wood and finally breaks down the resistance of even the strongest trees. This method leaves the tree no chance whatsoever of regeneration. It is very much in evidence in the area around N'Djamena, according to observations made by the present author in April 1978, concerning Acacia nilotica.
Zeribas are enclosures for livestock erected by the pastoralists, who cut large branches, usually with plenty of subsidiary branches, from thorny species and build them into hedges. The purpose of these enclosures is to contain the livestock overnight. Acacia species, particularly those with large thorns such as A. raddiana, A seyal and a. nilotica, are the species most often used. Balanites is also cut and the branches combined with those of Acacia. For reason of economy and the problem of transport, Acacia trees are mercilessly slashed and very often all the main branches are taken so that the tree is completely lopped. It does not die, but emerges far weaker after this mutilation.
Zeribas use up a large number of branches and have the serious disadvantage of not lasting, since they are destroyed by termites and have to be rebuilt on average every two years. Moreover, the accumulation of dead wood is highly combustible, and wherever bush fires break out they flare up spectacularly.
Bush clearance for the cultivation of pearly millet (Pennisetum typhoideum) only takes place in the south of the Sahel, where cultivation is possible. As the population increases and the soil becomes progressively exhausted, the areas cultivated grow and grow, at the cost of areas traditionally reserved for grazing, thereby depriving the trees of all chances of regeneration.
Pastoralists habitually cut the high branches of trees, throwing them to the ground so that they are accessible to cattle and small stock. When it is skillfully carried out this type of trimming does not have too great a destructive effect and has the advantage of providing livestock with a feed source which is rich in proteins, while still allowing the tree to survive.
More serious, however, is the method by which large branches are bent down to ground level. The branch remains attached to the tree by a section which is half split through, constituting an open door for bacteria and fungi. Moreover, the bare branch in contact with the ground is an easy prey to flames when fire occurs, which then spreads to the whole tree, whereas normally only the trunk would have been lightly damaged.
The vitality of some species of Sahel tree is remarkable and many types of tree would doubtless have disappeared long ago if they did not possess this extraordinary ability to bounce back. A number of examples may be examined.
Trees are cut clean to the ground whenever bush is cleared, on cultivated perimeters or around huts, etc. Some trees disappear for ever but others re-emerge some time later.
The most resistant species without doubt is Ziziphus mauritiana. In this species the cut surface of the stump is covered with vigorous green shoots capable of assimilation and protected by supple thorns. These new shoots are tasty and are a much appreciated source of green feed for small stock. They grow during the dry season, as was the case for the shoots observed by the author on 12.4.78 at Walia, 30 km north of N'Djamena, and on 24.12.78 at Komsilga 20 km south of Ouagadougou. Many examples could be given.
The advantage with new shoots is that they are more tender than ordinary branches and grow more rapidly. Quite possibly, if browsed at regular intervals, these new shoots would not be able to maintain themselves. A trial plot would enable us to obtain precise indications, especially with regard to the resistance of the stump to this kind of utilization and to the optimal period to allow between successive grazings. This ability to put out new shoots is all the more worth investigating since it occurs during the dry season. Bauhinia rufescens possess the same property, and the edges of stumps which are cut to the ground become covered with minute bilobular leaves. However, this species is not very palatable except to goats, and does not deserve so much attention.
This is very often the case for thorny acacias cut for constructing zeribas. The most resistant of these trees is Acacia seyal. Whether the trunk is cut short at 1 m, 1.5 m or 2 m seems hardly to matter, since during the following wet season or even before then the cross-section of the trunk will be crowned with a cluster of young shoots showing strong upward growth and covered with dense foliage and soft thorns.
The geotropic development of these new branches is extremely negative. In one specific case the stocks of acacia which had been cut in this way were covered with tuft-like clumps of new shoots, as observed by the author on 6/8/79,10 km north of N'Djamena.
Clearly, a faculty for regeneration of this kind should be studied experimentally. It would be extremely constructive to measure this genuine productivity, in real terms to gain an understanding of its variation in accordance with the annual cycle as well as to carry out bromatological analyses at regular intervals in order to monitor the development of CP and DCP.
In some species, e.g. Combretum glutinosum, Acacia seyal, trunks lying on the ground put out suckers which grow skywards from the uppermost surface of the trunk. In Combretum spp. these young stems are consumed although the adult leaves are left.
This kind of ramification is frequently met in young trees which are regularly overbrowsed. Each year, during the rainy season, the young tree (Balanites, for example) puts out shoots which grow outwards from their "phyllosphere". During this period livestock are usually absent, having moved northwards in search of annual grazing, or if they are present animals are more attracted towards the young grasses such as Brachiaria or Dactyloctenium which are tender and plentiful during this period.
When, the dry season arrives these young shoots will be mercilessly consumed and only those which are the least accessible will escape, i.e. those enjoying a greater or lesser degree of natural protection from a ligneous branch positioned in front of them. As the years go by, the tree takes on a bushy habit. Eaten away at the edges each year, it grows only very slowly, but every year produces a mass of new leafy branches which are much utilized by livestock. Despite appearances, trees of this kind play an active part in the feed supply of camels, cattle and goats.
It would be very instructive to know for how many years a tree can tolerate this kind of overutilization. What proportion of energy does the tree succeed in retaining and how much is taken by the animals? Answers to questions such as these could only be given if trial plots were set up.
Finally, instead of allowing livestock to overgraze the ligneous stratum to a point at which regeneration is seriously threatened, a stage which has unfortunately been reached already in vast areas of the Sahel, would it not be more appropriate to curb this chaotic and savagely destructive process, replacing it by rational utilization of woodland grazings? Of course, the practical ways and means have yet to be worked out. Nonetheless, there are already certain rules which could be obeyed, such as those pertaining to systematic pruning, for example, in which the central bud is always left intact, enabling the tree to regenerate more rapidly. Just at the tufts of short-stemmed Andropogon grasses do, browse trees recover, and the pastoralist must learn to utilize browse as he has learnt to do so with grassland. His efforts will be greatly rewarded.
