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Evolution of vegetation on the Niono Ranch since 1969

M. TOGOLA (*), M.I. CISSE (**), and H. BREMAN (***)

(*) M. Togola, Chef de la Section Agrostologique, Station d'Elevage et de Recherche Zootechnique du Sahel, Mali.

(**) M.I. Cisse, Centre Pédagogique Supérieur, Bamako, Mali.

(***) H. Breman, Associate Expert, UNESCO.


Introduction
Conclusion
Bibliography


SUMMARY

A comparison was made of the vegetation conditions on the Niono ranch for the years 1969 and 1974. Indications were found that the intervening drought years had been responsible for a displacement of Sahelian conditions of 75 - 100 km, even in areas free from human influence. The mortality of Bombax costatum and of Andropogon gayanus and the primary productivity of the herb-layer were used as indicators of Sahelization. Overexploitation of the pasture land has a synergic effect, making difficult the return of the Sahelian border to the north during years of normal rainfall.

Introduction

The Niono ranch covers an area of 11,000 ha. Though it was demarcated in 1960, not until 1969 did Boudet (IEMVT 1970) assess and establish its vegetation and productivity. Since then, it has been protected against the invasion of external animals. Until today, however, it has not been possible to make a rational use of its grazing lands, because of the limited number of watering points. In fact, there is only one permanent source of water and a few temporary ponds.

Thus one finds an overexploited area around a permanent source of water beside a large unutilized area. This presents the chance, perhaps unique in the Sahel, for an assessment of the influence of the drought on grazed and unexploited vegetation, by comparing the situation in 1974 with that of 1969.

Of late there has been much talk of the desertification of the Sahel and the Sahelization of the savannah, though there are no data to illustrate these phenomena. It should be possible to determine at the ranch if there has been a displacement of the Sahel to the South, by identifying the influence of human and climatic factors, since Niono is located at the extreme South of the Sahel, in the transition zone, towards the Soudanian savannah.

In 1974, a study was made on the state of the most dominant plant types, and this was compared with the situation in 1969 (1).

(1) The discussions held with Mr. M. Coulibaly and Mr. A. Sow of the Centre National de Recherches Zootechniques were very useful in the preparation of this paper.

1. Study of the environment

The Niono region is included in the dead delta of the Niger river. The ranch is located there at 14°20' N. and 5°50' W. at an altitude of nearly 300 m.

1.1. Climate

The region has a dry tropical climate of the sahelo-Soudanian type, characterized by only one rainy season, from June to September.

The mean annual rainfall (since 1939) is 572 mm, with a variation characterized by a standard deviation of 119 mm. Fig. 1 shows two main periods of dry years, especially between 1940 and 1950 and from 1966. The short fall has been particularly marked since 1970, and the average for the last 4 years is 392 mm, recorded over a period of 34 days against a normal average of 50 days of rainfall. The average monthly temperature is lowest in January (25.8°C). It rises to its maximum in May (34.7°C), falls during the rainy season to 27.8°C in August, rises again to 29.2°C in October, and then decreases gradually. The average daily temperature is highest in May, 40.1°C; and the lowest daily temperature is in January - 12.2°C. The potential evapotranspiration is around 1,700 mm per year.

Fig. 1. - Variation of rainfall at Niono.

Table 1. - Abundance of graminaceae in the various vegetation types of the ranch (IEMVT 1970)

1.2. Geology and pedology

" The. dune plains of the dead delta are made of alluvial layers deposited by the flow of the river without, however, washing out the dune morphology which emerged during the four-year drought period. The winds have sifted the alluvia and have modelled them into dunes. " The soils there are isohumic and tropical ferruginous soils (IEMVT 1970).

1.3. Vegetation

According to several authors, the Niono ranch is situated in the transition zone between the Sahelian and Soudanian zones (see IEMVT 1970). For instance, Bombax costatum is represented there, but besides this there is also Commiphora africana. Andropogon gayanus and other species like Andropogon pseudapricus, Elionurus elegans and Loudetia togoensis could have dominated the herbaceous cover of the savannah but for such dominant Sahelian species as Cenchrus biflorus and Schoenefeldia gracilis.

It is possible to distinguish three main vegetation types on the ranch, namely:

- the Pterocarpus lucens shrubby erme, localized in the large muddy colluvial depths,

- the Andropogon gayanus variegated savannah covering the ridges and depressions of the sandy mantle,

- the Schoenefeldia gracilis Come, occurring on the sandy mantle and the bottom of the slopes and sandy peneplaines (IEMVT, 1970).

These types appear in several forms, hence the 8 sub-types observed on the ranch. The most widespread of these have been shown on Table 1. According to Rattray (1960), the graminaceae there are composed of species of both the Savannah and the Sahel.

The type which was studied in detail is one of the sub-species of the Pterocarpus lucens shrubby erme, especially on Savannah variegated by Pterocarpus lucens and Andropogon gayanus. This type covers the lower peneplained soil around the depressions and also the ridges of the low undulating sandy mantle. Bombax costatum, Guiera senegalensis and Acacia seyal are the most abundant woody species besides Pterocarpus lucens. The relatively dense herbaceous cover has three strata:

- a high stratum composed of Andropogon gayanus,

- a medium stratum made up of Andropogon amplectens, Loudetia togoensis, Elionurus elegans, and Indigofera prienreana,

- a low stratum with Polycarpaea corymbosa, and Borreria stachydea (IEMVT, 1970).

Fig. 2. - Rainfall in Mali since 1970 compared with mean annual rainfall.

2. Evolution of the vegetation since 1969

The recent drought has led to a high mortality of some woody species and the perennial graminacea Andropogon gayanus.

2.1. Woody type

Two woody species in particular have suffered during the last few years. These are Bombax costatum, Kapokier, and the leguminous shrub Pterocarpus lucens. Guiera senegalensis, Commiphora africana and Sclerocarya birrea are found dead in a small percentage of the over-exploited dune areas.

Dead Pterocarpus could be seen especially around sources of water where grazing had been intensive. Around them the soil is generally bare, even during the rainy season. It is only at places where the wind has gathered some sand that some grasses, often dominated by Zornia glochidiata could be found. To the east, receding from the permanent watering point (see Fig. 4), dead stalks are no longer seen. The mortality seems " man-made " in that case. Besides, this process is well known and has been described as the growth of the tiger bush (see Boudet, 1972).

Kapokier, for its part, is something else. It is extinct from the entire surface of the ranch, with the highest mortality occurring in the dune areas. As a plant characteristic of the Soudanian savannah, it is found here almost at the extreme north. It is only in the ridges of the inter-dune gorges and at the fossil hydrographic system of the Continental terminal, more to the north, that some stalks have managed to survive under a minimum average annual rainfall of 450 mm (IEMVT, 1970). Fig. 2 shows that the annual precipitation in 1972 and 1973 remained below this limit for this region, which normally receives 650 mm/year; and this after two relatively dry years. It is thus not surprising to observe a high mortality of Kapokier in 1974 north of the 650 mm isohyet, as illustrated in Fig. 3.

Fig. 3. - Mortality of Kapokier in 1974 in relation to annual rainfall.

This figure shows the results of samples taken on the Bamako-Niono and Bamako-Nara roads. Each point on the figure rep-resents the estimated mortality of 100 to 850 trees sampled. The difference between the observations is high because of the soil factor: mortality is high on the sand dunes, but relatively moderate in the depressions. Where the annual precipitation is normally 650 mm, 362 trees out of the 572 trees sampled (or 63 percent) on the dunes were found dead, as compared with 50 out of 160 (or 31 percent) in the depressions. Perhaps, what was at stake was the structure and water retention capacity of the soil, considering the very superficial root system of the Bombax costatum. It was noted that the roots extended up to 25-35 mm from the trunk and penetrated 10-25 cm deep.

These results depict Bombax as an indicator of a Sahelization of the savannah under the impact of climatic fluctuations, considering also its easy identification even by laymen. The large number of young stalks at the ranch stemming from the roots of old stalks could mean a quite rapid recolonization of the region during years of good rainfall. This has been asserted in a book: "Les travaux du Niger" (Bélime, 1940), which shows the 500 mm isohyet as the southern limit of the Sahel and the northern limit of Kapokier; which means that the situation in 1940, after the 1911-1914 drought, was the same as that of 1969 after the drought of the 1940's. Care must be taken to determine the age of the Kapokiers in the north.

2.2. Herbaceae

The study of the influence of the drought on herbaceae is more complicated than that for the woody species. The major perennial graminaceae are, however, an exception.

The most productive plant groups of the ranch have, as common important element, the perennial graminacea Andropogon gayanus. These types occupy 60 percent of the total area, and their productivity is between 3-4 t/ha dry matter; 2-3 tons of which are palatable (IEMVT, 1970). One observation is enough to show clearly that the situation is no longer the same. Andropogon gayanus has become a rare species on almost the entire ranch. At places described in 1969 as " wide tracts of land covered by Andropogon gayanus to the exception of any other graminaceae" (Coulibaly's personal notes), there are now only a few pockets sparsely occupied by the graminaceae. The species is no longer characteristic of the zone, and its productivity must consequently have changed a great deal (see next chapter).

3. Evolution of the savannah variegated by Pterocarpus lucens and Andropogon gayanus

In 1969, the savannah variegated by Pterocarpus lucens and Andropogon gayanus occupied 32.4 percent of the grazing grounds of the ranch (see Fig. 1 under A4). The influence of effective drought and the impact of drought plus intensive exploitation were studied from phytosociological data and samples of the biomass at various distances from the unique permanent source of water.

Three zones were identified, notably, the stretch of grassland at a maximum distance of 4 km from the source of water; the zone between 4 to 8 km from the water and the remote perimeters (see Fig. 4.)

Fig. 4. - The Niono Ranch.

3.1. The spectrum of species

The abundance code of Table 1 was used in estimating the abundance of a number of easily recognizable species. In each zone, 3 or 4 samples were taken. Table 2 sums up the results, showing that two graminaceae of the savannah have undergone serious changes: Andropogon gayanus was replaced by the weed Loudetia togoensis under the influence of effective drought. The combination of drought and intensive exploitation has completely thinned out Andropogon gayanus. The Sahel graminacea Andropogon amplectens too withstands the combination poorly, though it is not influenced by effective drought. Like Zornia glochidiata, Dactyloctenium aegyptium seems to resist exploitation. Finally, Blepharis lineariifolia is more abundant than in 1969.

3.2. Productivity

It is obvious that the disappearance of a perennial graminacea like Andropogon gayanus will occasion a change in primary productivity. The extent to which the disappearance is felt is illustrated in Table 3, which represents the productivity of the grass stratum of the type in question, expressed in grassland at a maximum distance of 4 km from the source of water; the zone between 4 to 8 km from the water, and the remote perimeters (see Fig. 4). The stocking rate there was respectively estimated at below 10 ha/LSU, 10-20 ha/LSU, and over 20 ha/LSU. (This stocking rate was calculated on an annual basis, even in the case of temporary grazing. The period of intensive exploitation is from September to June.) terms of biomass of dry matter at the end of the growth period in 1974.

Productivity was only 0.9 t/ha for the most exploited zone and 1.6 t/ha for the practically unused zone, as compared with an average 4.0 t/ha in 1969. The deficit rainfall of the year under review (see 1.1.) is far from being the only explanation for this low productivity, because scattered areas where Andropogon gayanus was still dominant recorded a productive yield of 5.1 t/ha during that year (see Cissé and Breman, this volume).

The drought therefore had a significant impact on the value of the rangelands. From the productivity assessed, the 3,560 ha of Group A4, which could have sufficed in maintaining almost half the 3,000 LSU envisaged for the ranch, could maintain only 680 LSU. In comparing Tables 1 and 2, and in estimating that only the Andropogon gayanus groups would be seriously affected, it could be expected that of the 11,000 ha of the ranch, 2,000 at the most instead of 3,000 LSU could be maintained. According to Breman (1975), this is the carrying capacity of a region with an annual rainfall of 450-500 mm. This perhaps is an indication that should be used to explain the significant increase in the productivity of the transition zone from the Sahel to the savannah with the appearance of the perennial graminacea Andropogon gayanus.

Table 2. - Abundance of some grasses after 4 years of drought under various intensities of exploitation.

Types

Situation in 1969 (IEMVT, 1970)

Situation in 1974

< 10 ha/LSU

10-20 ha/LSU

> 20 ha/LSU

Graminaceae of the savannah

Andropogon gayanus

3

-

-

1

Andropogon pseudapricus

+

+

+

+

Setaria pallide-fusca

3!

2

?

?

Elionorus elegans

2

2

1

2

Loudetia togoensis

4!

1

3

3

Graminaceae of the Sahel

Eragrostis tremula

-

+

+

+

Aristida mutabilis

-

+

+

-

Pennisetum pedicellatum

2!

1!

+

!

Dactyloctenium aegyptium

-

2

+

-

Andropogon amplectens

3

1

2

3

Schoenefeldia gracilis

3!

+

2

1

Misc. herbaceae

Blepharis lineariifolia

-

+

2

1

Zornia glochidiata

-

3

1

-

Polycarpea corymbosa

2

-

1

2

Borreria stachydea

2

2

2

2

Table 3. - The productivity of plant type A4 after 4 years of drought under different intensities of exploitation.

Intensity of exploitation

Productivity in t/ha/year

Interval t/ha/year

Surface cut in m²

< 10 ha/LSU *

0.9

-

12

10-20 ha/LSU

1.3

0.39

60

> 20 ha/LSU

1.6

0.33

36

(*) The studies made in this zone were difficult because of intensive exploitation.

Here again, the question arises as to the length of time required under at least normal rainfall for a restoration of the 1969 situation. An automatic reconstitution would be very difficult for areas where the human factor of overexploitation has been added to the drought phenomenon because of the complete disappearance of Andropogon gayanus there.

Conclusion

It could be said that 4 years of drought, with a mean annual precipitation of almost 400 mm instead of the normal 575 mm, have created a situation resembling that of a region with an average annual rainfall of 450 to 500 mm. This has direct bearings on the exploitation of the ranch, because the carrying capacity is at the most only two-thirds of the carrying capacity in 1969.

It will be necessary, in the interests of livestock planning in general, to pursue this study; first by analysing the effects of the drought on the other vegetation types - exploited or unexploited - and subsequently, by estimating the speed at which decimated species would recur. It is thus recommended to set aside specific areas on the ranch and to protect them fully against any human activity.

Three factors should be taken into account in following up the alterations in the transition zone between the Sahel and the savannah as a result of climatic changes: notably, the frequency and mortality of Bombax costatum and of Andropogon gayanus, two species known to everyone; and the immediate changing of the rainfall/primary productivity ratio between the Sahel and the Soudanian savannah.

Bibliography

1. BELIME, E.L. - Les travaux du Niger. Paris, Martin-Mamy/Crouan et Roques, 1940.

2. BOUDET, G. - Désertification de l'Afrique tropicale sèche, Adamsonia (sér. 2: 505-524) 12: 1972.

3. CLAUDIN, J. - Etude Phytoécologique du Hodna. Algérie. M.A.R.A./F.A.O., 1973.

4. I.E.M.V.T. - Etude Agrostologique pour la création d'une Station d'embouche dans la région de Niono, no. 29, le Mali, 1970.

5. I.E.M.V.T. - Esquisse Pastorale et Esquisse de Transhumance de la région du Gourma, le Mali, 1971.

6. I.E.M.V.T. - Aménagement du berceau de la race N'Dama dans le cercle de Yanfolila, no. 36, le Mali, 1972.

7. I.E.M.V.T. - Projet de Développement de l'Elevage dans la Région de Mopti, no. 37, le Mali, 1972.

8. Mali. - Annuaire Statistique 1970, Service de la Statistique générale de la Comptabilité Nationale et de la Mécanographie, 1970.

9. Mali. - Rapport sur l'Etude Sectorielle concernant l'Approvisionnement en eau du pays. D. Balfour et Sons, 1973.

10. RATTRAY, J.M. - The Grass Cover of Africa, F.A.O., 1960.


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