Dr. Yagoub Abdalla Mohamed, Dr. Babiker Fadlalla Dr. Alawyia Abdalla and Mr. Mohamed El Amin Abdel Rahman
Abstract: The region of North Kordofan was taken as a case study representing the vulnerable Sahelian zone in the Sudan. The objectives were centered on the assessment of biomass, soil potential and its capability to build up a rich carbon sink following the good rainy season of 1994. The findings concluded that land degradation and ecological imbalance associated with the combined adverse effects of years of below average rainfall and mismanagement may be favourably reversed if rational management practices are applied in accordance with rainfall patterns. The study revealed major trends of recovery both in biomass productivity and soil organic content.
Symptoms of land degradation in the Sudan dates back to 1942 when the government established a Soil Conservation Board to report on the soil condition in the Sudan particularly with regard to erosion and desiccation (Soil Conservation Report, 1944). The Board recommended several soil conservation measures that may help in "The recovery of nature's healthy conditions for the most advantageous production of crops, pasture and forests for the well-being of the inhabitants". Nonetheless, serious and intensive human manipulations of land misuse accelerated after World War II, posing considerable degradation of productive arable land. The result of over-exploitation had been devastating signals of land degradation.
Stepping (1953) did not attribute the causes of degradation to a single factor as he stated that "it is not the unchecked practice of shifting agriculture, plus the largely increased numbers of grazing animals and the annual firing of natural vegetation which is responsible for the present position". It is not equally true that the causes of degradation and degeneration of forests into scrub type can be traced to geological, geographical or marked climatic changes. Lamprey (1975) tried to measure the Southward shift of ecological zones. He concluded that the desert's boundary shifted southward by an average of 90-100 km in the last 17 years as observed in Northern Kordofan. Moreover, Lamprey (1975) reported southward drift of sand dunes on a large scale particularly in areas around Bara and Kheiran. These findings were also noted by DECARP (1976). In a report on the anthropogenic causes of desert advance, Ibrahim (1978) drew the conclusions that cultivation north of the agronomic dry boundary will result in loss of arable land i.e., former productive land will be transformed into desert like conditions. Other authors (Bakhiet, 1983; Mohamed, 1983; Suliman and Darag, 1983) suggested that the spread of desert-like conditions were the results of both physical (drought) conditions and human misuse of resources. Misuse of natural resources and expansion of rain-fed farming into marginal lands were among the causes cited as leading to poor land productivity, while frequent droughts accelerate the process of degradation. In fact, most of these studies link the process of desertification and drought as amplifying each other.
Understanding the nature and causes of desertification attracted other researchers. A group of researchers from Lund University started integrated investigations on desertification using advanced equipment for digital image analysis combined with conventional geographical methods, and arrived at more convincing factors leading to the spread of aridity in Northern Kordofan (Hellden, 1978 & 1988; Olsson, 1985; and hlcrona, 1988). Hellden (1978) questioned the conclusions of Lamprey. Through a satellite-based study in Northern Kordofan Hellden detected no significant shift of the location of the desert margin between 1972, 1976 and 1979. He found that the desert boundary coincided with the 100mm isohyet at latitude 16ºN which again coincided with the desert boundary mapped by Lamprey (1975). Hellden (1978) found no evidence or signals of an active sand creep between Sahara and Kheiran areas of Northern Kordofan. On the contrary, he observed that the dune complex was of a stabilized nature. Olsson (1985) studied three areas in Northern Kordofan taking Bara area as an example of a region with ample subsurface water that permits rather intensive land use in spite of the generally insufficient rainfall in bad years. He also took Um Ruwaba area as an example of extremely intensive land use, and the third area he investigated covered the region between Um Ruwaba and Bara as an example of low land use intensity. Olsson (1985) suggested that reports on desertification have not been based on scientific approach. Most of the studies were based on comparisons of observations on two occasions: one prior to the Sahelian drought and the other during or shortly after it. He further elaborated these statements by stating that "signs of desertifican can mainly be seen during extremely dry periods when rainfall returns to normal, both natural vegetation and agriculture crops recover". According to these hypotheses it could be concluded that the conditions vary with climatic conditions.
Ahlcrona (1988) drew attention to the decrease in the biological productivity that could be caused by aridity or man-made factors, while land degradation is mainly caused by man. According to the investigations carried in the areas west of the White Nile and in East Kordofan, Ahlcrona (1988) concluded that "The major impact on lands biological productivity has been caused by climatic factors and not by man". Among the various variables tested by the researcher as indicative of man-made land degradation, only the qualitative deterioration of natural vegetation cover was regarded as significant.
In a recent general study in 1995 on the magnitude of desertification in the Sudan, carried out by the National Drought & Desertification Control Programme, Co-ordination and Monitoring Unit (NDDU), the desert margin was defined and quantified using NDVI data. In this way it was possible to modify the known ecological zones leading to the production of risk maps. However, this general study contributes little to the debate on the nature and causes of desertification or to the ability of Sudan's Sahel soils to recover as the result of good rainy seasons. Hence this study had been launched to verify recovery using biomass and organic matter as yardsticks. Aerial photos for 1994 good rainy season offered an excellent opportunity to note recovery and help for future evaluation.
For the purpose of the study, an interdisciplinary team composed of soil scientists, botanists and social scientists carried out fieldwork in carefully selected areas in Northern Kordofan (Fig. 1). The fieldwork enabled the team to collect soil samples, make vegetation transects and apply various methods as reported in appropriate places in the subsequent sections. The field work was situated in the areas between latitudes 13ºN and 14ºN from where samples were taken and observations recorded. This finding was supported by satellite images, climatic data and laboratory analyses as will be indicated later.
In arid and semi-arid lands there is a strong correlation between total annual rainfall on the one hand and biomass/organic matter content on the other. Meteorological data obtained from the Sudan Meteorological Department were examined in order to identify the general trends in the annual rainfall pattern in Northern Kordofan during the last two decades for the purpose of defining the relationship between the observed patterns among other factors and the biomass. The main features are summarized in the following:
a) The annual rainfall totals within the area varies from about 170mm in the north to about 350mm in the south.
b) The inter-annual coefficient of variability in the rainfall ranges between 25% to 50% on analysis of readings of various stations.
c) There are periodical changes in the annual rainfall pattern at the stations of the whole area.
d) The average precipitation anomaly index shows that the years 1988, 1993 and 1994 were the wettest, while the years 1984 and 1990 were the driest (Table 1).
Table 1: Total Rainfall in Two Stations in the Study Area
Date |
El Obeid (mm) |
Umm Ruwaba (mm) |
1981 |
364.9 |
235.0 |
1981 |
310.3 |
327.0 |
1982 |
201.9 |
278.0 |
1983 |
351.8 |
- |
1984 |
161.7 |
97.0 |
1985 |
218.6 |
236.0 |
1986 |
375.6 |
N.A. |
1987 |
226.3 |
233.0 |
1988 |
346.0 |
229.0 |
1989 |
267.8 |
300.0 |
1990 |
170.6 |
163.0 |
1991 |
204.4 |
287.0 |
1992 |
513.9 |
475.0 |
1993 |
378.7 |
319.0 |
1994 |
544.7 |
465.0 |
Fig 2 Vegetation Map Based on NDVI 1994
Fig 3 Vegetation Map Based on NDVI 1993 & 1994
Fig 4 Vegetation Map on NDVI 1988 & 1994
Data from NOAA Satellite were used to assess the environment of the areas and detect the changes in vegetation cover in 1994, 1993 & 1994; 1988 & 1994 (Figs. 2, 3 & 4). It is clear that the vegetation cover in 1994 is superior to that of 1993. The comparison between cover of 1994 and that of 1988 shows that the northern part of the area was better in the green cover in 1988 than in 1994, while in the area that lies between latitude 14ºN and latitude 13ºN, some zones were found to be good in their vegetation cover in 1994 than in 1988. South of latitude 13ºN the situation was better in 1988.
The economy of Kordofan state is predominantly agro-pastoral. In the sporadic human settlements the two types of economies are normally integrated forms of production. Crop production is mainly traditional. Cereal and cash crops cultivated in fertile soils/stabilized sands during good rainy years include millet as the dominant crop, sorghum and sesame. Millet occupies the largest cropped area accounting for more than 50% of the area under different crops. In areas around Um Ruwaba, Er Rahad and Tendalti sesame occupies substantial area. In recent years, natural fallow periods became less, giving only limited chance to woody herbaceous vegetation to recover.
Acacia senegal plantation is another form of land use. Because of its economic benefits for gum arabic collection, it is the only tree that is planted and protected by the farmers.
Livestock production is primarily based on systems of range location and utilization. There are two types of livestock management: sedentary livestock and nomadic pastoralism. Settled villagers keep goats, sheep and few cattle, while nomads are mainly camel owners who traditionally move with their livestock in north-south axis.
Forestry activities are locally practised mainly for the supply of fuelwood and construction material for local and commercial purposes.
Crop production declined during the years of drought. The low yields were compensated for by increased areas under crops to produce the amounts needed for family consumption. Such practices might cause land degradation. However, the good rainy season of 1994 resulted in a complete change in the ecological conditions. Field observations and NDVI image interpretation have revealed contrast with those described during years of the Sahelian drought. The considerably rapid recovery and improved ecological conditions may be attributed to the following factors, which will be discussed in more detail in the subsequent sections:
a) Throughout the prolonged Sahelian drought large-scale out-migration of the inhabitants and their livestock gave a resting period for degraded areas to recover.
b) The flora and fauna of Northern Kordofan are ecologically resilient and when good conditions prevail, mainly during good rainy seasons, the recovery is rapid and almost guaranteed.
c) Successive years of drought posed human/animal suffering; the inhabitants developed a reasonable degree of awareness of irrational practices and hence became more concerned with careful management of their land and water resources.
In the study area, the years of drought (1973-1984) manifested disastrous physical and biological consequences. The biological impact had many indicators of stress such as considerable shrinkage in vegetation cover, diminishing specific diversity, a sharp decline in abundance of plants and scarcity of above-ground biomass upon which the local agro-pastoral life depended for subsistence and income generation. Those drought years also resulted in sharp decline in number of herds and in very poor agricultural productivity.
The study on biomass productivity was carried out during April 1995 to assess the signs of recovery in the region after the exceptionally good rainy season of 1994. The vegetation maps based on NDVI for the years, 1988, 1993 and 1994 indicated sharp favourable changes in vegetation cover. This feature coincided with a well-above average total rainfall.
For the purpose of data collection, six carefully selected sites (Sectors I-VI) (Fig. 5) were used. The criteria for sector selection had been: (a) that the sectors collectively represent major habitats along N...S trend of increasing regional rainfall (b) that they express a similar trend on E...W axis and (c) that within each sector there is one or more community types that characterize Northern Kordofan. The sectors included all the major ecologically prominent vegetation sub-units (Table 2).
Table 2: Ecological Data and Biomass Productivity (A)
Sector |
Community |
% Cover |
Density m2 |
M.H cm |
Litter Depth |
Dry Wt. gms. |
|
I |
Acacia tortilis Acacia senegal Indigofera oblongifolia |
15-20 20-30 40-50 |
20-50 30-50 35-50 |
30 36 40 |
2.5 2.5 3.0 |
7.2 7.8 6.0 | |
II |
Leptadima Acacia senegal |
35-40 50-60 |
30/50 40/50 |
38 46 |
3.25 3.7 |
8.7 9.0 | |
III |
Acacia senegal Maerua Crassifolia Maeua/boscia |
40 50 50 |
40/50 38/50 46/50 |
37 40 52 |
3.6 3.9 3.6 |
9.1 10.4 11.7 | |
IV |
Acacia senegal Acacia maerua Acacia/Boscia |
60 50 60 |
35/50 40/50 40/50 |
45 50 56 |
4.0 4.5 4.2 |
14.0 14.0 15.9 | |
V |
Acacia tortils Acacia senegal "Maerua/Boscia |
60 70 40 |
35/50 40/50 40/50 |
66 66 70 |
5.5 5.5 6.0 |
16.6 18.7 20.0 | |
VI |
Acacia tortilis Acacia senegal |
70 80 |
40/50 56/50 |
70 77 |
7.0 7.0 |
20.6 24.8 | |
During the field work, an appropriate land survey approach was applied. The eco-taxonomical data have been obtained from sample units located at random in the centre of each community type in the sector. the eco-taxonomical data focused on recording the prominent characteristics of the general landscape, a broad description of the vegetation and the species present. The sampling units which were located in the centre of each community gave evident expression of the optimal level of biomass components, the highest specific diversity, peak in visible ground cover and maximum indices of abundance of the main species, herbaceous as well as woody perennials. As expected, the richness of high indices noted in the community centre followed a gradual decline on either side of the centre of each, while northerly locations in the community contained plant species of dry affinities, the southerly locations contained species of relatively less dry affinities. Accumulations of litter in each community type was recorded as indicative of relative quantities of litter on the soil surface. The data served the purpose of comparison amongst the different vegetation units (Figs 6 & 7).
There is no reliable comprehensive data base for vegetation cover and above ground biomass in the study area. Under such circumstances and because of the short field work duration, the task of determining biomass productivity was limited to using ecological parameters suited to vegetation monitoring over large areas. Each of the two strata of community was treated separately: (a) the biomass of ground cover and (b) small to medium size woody trees. In each, biomass estimates were made by percentage ground cover. While quadrates (400m2) were used for the herbaceous ground vegetation, crown diameters were measured to obtain the canopy area of mature common woody trees that characterized the landscape in each study site.
Data for percentage cover and dry weight of herbaceous vegetation represented important attributes of the plant community in each type. The data revealed trends that match (a) availability of optimal soil type (b) species affinities and responses to different levels of impacts and (c) bulk of seed bank that would contribute to the relative magnitude of recovery in good rainy years.
Herbaceous biomass productivity and cover variables were determined in terms of the ground strata community, not according to individual species. Hence the biomass of the association, as represented by litter, rather than individual species, was used to evaluate the productivity. Alongside with specific composition list, a scale of abundance was made to reflect biomass productivity of the vegetation assemblage. Patterns of abundance of prominent species reflected on biomass productivity (Figs 6 & 7).
Diversity of plant species in arid and semi-arid lands was under siege from the combined pressure of recent drought, hence the northerly habitats of Kordofan experienced a serious decline in range desirable species and the prevalence of drought resistant rejected forms. However, species list prepared during the survey, suggested clear trend of improved diversity levels and hence relatively better system stability. The trend coincided with higher soil moisture conditions in accordance with higher means of annual rainfall. The present survey tended to confirm the possibility of recovery when moisture requirements were satisfied in rainy years.
Territorial transitions were detected near "hafirs", seasonal streams, village outskirts and edges of agricultural plots where trees had been removed and in well-defined undulations. Hence, the interpretation of the dynamics of vegetation and the condition of the biomass productivity should be based on soil type/moisture regime on the one hand and grazing/agriculture and human settlement on the other. The latter manifested itself on the progressively fading destructive efforts of villages upon the natural vegetation and biomass productivity. A radial pattern of diminishing ecological imbalance was evident in estimated vegetation cover, litter depth, species diversity and vegetative performance. This was noted in random transects starting from the village centre and extending to the margin of undisturbed natural habitat.
Soils of the study area were mostly stabilized sand dunes (Goz) consisting of yellowish red sandy loam and loamy sand soils (Fig.8). The study area was part of the stabilized Goz described by Lebon (1965) as occupying a very large area of west central Sudan forming a belt from the White Nile to the Western borders with Chad. For the purpose of assessing soil organic matter content, the same six sectors described in section 2 (Fig. 5) were used. Along these sectors 43 soil samples were taken from 15 observation points for laboratory analysis. A version of Walkley and Black method of wet oxidation of organic matter percentage for the representative samples was computed and the pressure of roots was also recorded.
Generally the soil texture was loamy sand to friable sand loam showing high sand percentage ranging from 66-88% for the top soil to 62-92% at subsoil level, indicating slight differences in sand content in a downward direction. Silt percentage was very low (0-9%) while clay varied between 12% and 20%.
Results of the laboratory analysis show that the organic matter percentage ranges from 0.09 to 0.31% with and average of 0.17% for the top soil which is really low compared to similar soils under arid moisture regime elsewhere in the tropics (Table 3). However, the 1994 rainy season contributed forage of grass roots to the soil, but it was so fresh when samples were taken that it could not be easily detected as soil organic carbon. This remarkable addition of roots indicates that under good rainy seasons a considerable volume of Co2 is extracted from the atmosphere by plants, sinks down into the soil and kept there for humification and mineralization processes.
Low organic matter percentage in arid areas is very common. Young (1976) cited that organic matter content of the semi-arid zones is generally low compared to other soils in the tropics. The very low figures obtained could be attributed to the following:
a) The results of the analysis show that the present organic matter had accumulated in the years prior to 1994.
b) Organic matter added during 1994 is undecomposed and could not be easily determined under the present available facilities.
c) Considerable increase in millet cultivation depletes the soil organic matter which had been added during former good rainy years.
d) Frequent dry years limit the organic matter annually added to compensate for losses due to cultivation.
e) Soil micro-organisms are very much affected by moisture availability, hence consecutive dry years- which are very frequent in this area - retard the process of humification.
Despite the low organic matter percentages obtained, there is ample evidence that repeated studies, in years following good rainy seasons will reveal values similar to soils under arid regime in the tropics.
The widely prevalent instability of annual rain input in the semi-desert of Northern Kordofan is reflected in low biomass productivity in poor rainy years. Evident indicators in recovery occur in good rainy years. This study assessed the 1994 good rainy year with respect to impacts on biomass productivity and soil organic matter content in these areas. However, due to factors related to the time when field work was carried out the study could only provide trends and qualitative description of the signals of recovery.
Ideally, two visits of land survey must be made: one at approximately 10-12 weeks after the commencement of the rainy season, and the second visit 10-12 weeks after the first visit. The former visit would reflect the peak of the green mantle and vigour, the latter visit signals the beginning of vegetation anti-climax, though it gives information on the magnitude of disturbance and the residual seed bank stored in the soil for future recovery.
Successional changes since the mid 1970s highlighted degradational changes that have been related to progressive decline of rainfall means below the normal average. However, the 1988 rainfall had been exceptional, giving an undoubted recovery (Fig. 4). The subsequent years were of below average means leading to slow recovery as revealed by Fig. 3. The rainfall of 1994 was high and the good recovery was manifested in NOAA Stellite images and confirmed by the present observations based on land surveys (Fig. 2).
Despite the limitations mentioned, the fieldwork revealed major trends in biomass recovery, which are manifested in high species diversity and abundance of plant litter. It was possible to identify and record palatable species thought to have been lost. In fact there was no total loss but the good rainy conditions led to large scale recovery of many species. During years of drought, researchers observed the south-westerly movement of vegetation units as species of (e.g. Adansania "Baobab", Balanites" Higlig", Boscia "Mukheit" Eragrostis "Bano grass", Arilid "Gaw" Acacia tortilis" Seyal" and Acacia nubica "Laot"). The present study found dense cover of these species in areas described by previous researchers as decidedly desertified.
Such results indicate the resilience of the semi-desert areas of North Kordofan and its potential for recovery if good moisture conditions prevail. Hence it is possible to postulate here that the observed trends of recovery are the result of climatic factors. This does not rule out man's impact, through farming and grazing, in creating qualitative deterioration of vegetation in locations near settlements leading to changes in plant cover, usually becoming thinner as observed in areas near Bara. In fact, years of drought are associated with a drastic decline in pastoral potential leading to both population and herd migration to areas of good grazing resources. This feature allows the meagre plants that grow to reach maturity and produce seeds that succeed to germinate when optimum moisture conditions prevail.
Increase in biomass recovery, as reported for 1994 rainy season, has influenced soil organic content. Generally the soil organic matter content of the semi-arid zones such as that in Northern Kordofan is considered to be low. However, when good rains prevail, substantial amount of organic mater will be added to the soil in the form of plant roots and litter. The amount of organic matter during such good rainy years form a good reservoir for future use. Taking into consideration the ecological nature of the study area and for reasons mentioned earlier, the soils showed percentages lower than normal as compared to soils of similar areas. This result does not rule out the fact that the year 1994 added substantial amount of organic matter to the soils because of good biomass productivity adding tons of grass roots to the soil, but it was undecomposed that could not be determined as soil organic matter. However, it will remain in the soil for the humification and mineralization processes. This suggests that under good rainy conditions, a considerable volume of carbon dioxide is absorbed from the atmosphere by plants and sinks down into the soil (Table 3).
Table 3: The Status of Soil Organic Matter
Lab. No. |
Profile No. |
Depth (cm) |
Rooting Depth (cm) |
Organic carbon % |
Organic Matter |
1 |
A1 |
0-30 |
20 |
0.07 |
0.12 |
2 |
|
30-60 |
|
0.08 |
0.14 |
3 |
A2 |
0-30 |
21 |
0.08 |
0.14 |
4 |
|
30-60 |
|
0.08 |
0.14 |
5 |
|
60-90 |
|
0.07 |
0.12 |
6 |
A3 |
0-30 |
15 |
0.18 |
0.31 |
7 |
|
30-60 |
|
0.11 |
0.19 |
8 |
|
60-90 |
|
0.08 |
0.14 |
9 |
A4 |
0-30 |
29 |
0.05 |
0.09 |
10 |
|
30-60 |
|
0.06 |
0.10 |
11 |
|
60-90 |
|
0.07 |
0.12 |
12 |
A5 |
0-30 |
34 |
0.08 |
0.14 |
13 |
|
30-60 |
|
0.08 |
0.14 |
14 |
|
60-90 |
|
0.08 |
0.14 |
15 |
A6 |
0-30 |
24 |
0.11 |
0.19 |
16 |
|
30-60 |
|
0.07 |
0.12 |
17 |
|
60-90 |
|
0.05 |
0.09 |
18 |
A17 |
0-30 |
26 |
0.12 |
0.21 |
19 |
|
30-60 |
|
0.07 |
0.12 |
20 |
|
30-60 |
|
0.04 |
0.17 |
21 |
A8 |
0-30 |
37 |
0.09 |
0.15 |
22 |
|
30-60 |
|
0.05 |
0.09 |
23 |
|
60-90 |
|
0.05 |
0.09 |
24 |
A4 |
0-30 |
28 |
0.07 |
0.12 |
25 |
|
30-60 |
|
0.07 |
0.12 |
26 |
|
60-90 |
|
0.08 |
0.14 |
27 |
A10 |
0-30 |
48 |
0.11 |
0.19 |
28 |
|
30-60 |
|
0.03 |
0.05 |
29 |
|
60-90 |
|
0.05 |
0.09 |
30 |
A11 |
0-30 |
29 |
0.09 |
0.15 |
31 |
|
30-60 |
|
0.07 |
0.12 |
32 |
|
60-90 |
|
0.07 |
0.12 |
33 |
A12 |
0-30 |
62 |
0.11 |
0.19 |
34 |
|
30-60 |
|
0.07 |
0.12 |
35 |
|
60-90 |
|
0.07 |
0.12 |
36 |
A13 |
0-30 |
26 |
0.11 |
0.18 |
37 |
|
30-60 |
|
0.08 |
0.14 |
38 |
|
60-90 |
|
0.07 |
0.12 |
39 |
A14 |
0-30 |
- |
0.13 |
0.22 |
40 |
|
30-60 |
|
0.07 |
0.12 |
41 |
|
60-90 |
|
0.05 |
0.09 |
42 |
P1 |
0-28 |
56 |
0.07 |
0.12 |
43 |
|
28-120 |
|
0.02 |
0.03 |
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