Sahelian rangeland response to changes in rainfall over two decades in the Gourma region, Mali
Introduction
Major droughts occurred through out the Sahel in 1972–1973 and again in 1983–1984. They followed a relatively wet period of 20 years 1948–1968 (Nicholson, 2001) and were included within a 25 years dry period 1968–1993. Since 1994 rainfall vary around the overall average. Both droughts had severe impact on the vegetation, crops, livestock and the population of the Sahel. The disturbances on vegetation and soils by the first drought were studied and monitored over a few sites in the Gourma by Boudet, 1972, Boudet, 1977, Boudet, 1979 and Leprun (1992). First reports (e.g. Boudet, 1972) were alarming and warning on the risks of rapid desertification. The following reports (Boudet, 1977, Boudet, 1979, Boudet, 1984, Coulibaly, 1979) were less pessimistic, herbaceous layer had rapidly recovered, at least on sandy soils, and some of the decimated woody plant populations had started to regenerate soon after the drought. However, the desertification trend was confirmed, and responsibility for a larger fraction of vegetation and land degradation attributed to natural resource management, especially to pastoralist through grazing livestock at rates believed unsustainable (Gallais, 1975). Then occurred the second drought that peaked in 1984 with repeated crop failure for two or three years in a row, spectacular losses in vegetation cover that triggered wind and run-off soil erosion and massive losses in livestock, famine, impoverishment and emigration of the population (Hiernaux, 1996). In the framework of the impact assessment of that drought, 25 rangeland sites were sampled and described (Hiernaux et al., 1984). Some of the sites were selected at the locations where Boudet and his colleagues had made their observations in order to capitalise on the dynamics already described. Other sites were added to sample the North–South bioclimatic gradient, the three main soil types and the range of grazing intensity. The monitoring of these 25 sites was carried out over ten years (1984–1993) and progressively intensified in 2000 onwards under the AMMA project (Redelsperger et al., 2006, Mougin et al., 2009).
Located at the northern edge of the area reached by the West African monsoon, Gourma has recorded extremes in inter-annual variation of rainfall and resulting variations in vegetation growth (de Leeuw et al., 1992). This paper aims at presenting, first, the methods used to monitor range vegetation, climate and environment, second, the resulting data base, and third, the analyses of the herbage yield dynamics over years. Yield trends are discussed and related to rainfall, grazing status and species composition. The interpretation of yield trends also rely on previous surveys on soil seed stocks and germinations (Hiernaux and Diarra, 1993), root masses and grass tillering (Hiernaux et al., 1994), and on the results of controlled burning and grazing experiments conducted at the same sites (Hiernaux and Turner, 1996).
The first hypothesis tested in these analyses, is that Sahel vegetation, dominated by annual herbaceous, would linearly respond to rainfall (Le Houérou et al., 1988, Prince, 1991, Olsson et al., 2005). And because variation in rainfall are larger, at least in relative term, toward the north of the gradient, variation in vegetation production would also be greater toward the drier end of the bioclimatic transect (Le Houérou et al., 1988). Moreover, the impact of rainfall being mediated by the redistribution of rain water at the soil surface, a corollary is that vegetation on shallow soil or poorly permeable soil should vary more than in deep permeable soils. A complementary hypothesis is that the herbaceous layer would suffer more and recover less from drought when subjected to heavy grazing by livestock (Boudet, 1972, Breman and de Wit, 1983, Bille, 1992, Hein and de Ridder, 2006). Because the herbaceous layer is dominated by annuals, it is also hypothesised that the duration of the wet and dry periods that succeeded in the second half of the 20th century are long enough to trigger a shift in species composition toward species more adapted to dryer climate, including some perennial grasses (Breman and Cissé, 1977, Hiernaux and Le Houérou, 2006).
Section snippets
Material and methods
Monitoring sites are sampled along the South–North bioclimatic gradient in four groups (Fig. 1, Table 1). In each or these four sets of sites, three main edaphic situations were sampled: deep sandy soils, deep loamy-clay soils, shallow soils on rock or hard pans. On the sandy soils that extend over about half the landscape (Kammerud, 1996), three levels of grazing pressure where systematically sampled: low, medium and high, in relation to the proximity, size and seasonal duration of
Seasonal herbaceous growth
Being largely dominated by annual plants, herbaceous growth starts by seed germination following the first rains, sometimes between late May (as in year 2005 at Agoufou, Fig. 3) and July (as in 2007, Fig. 3). In a first period, which duration depends on earliness of germination and on soil moisture availability, above ground growth remains modest, below 15–30 kg of Dry Matter (DM) d−1, with seedling establishing their root system, while above-ground plant tillers or branches. Rapid growth (45–60
Is the yield response to rainfall of rangeland herbaceous linear?
Although statistic relationships have been established between herbaceous yield and total annual rainfall in Sahel (e.g. Boudet, 1984, Breman and de Wit, 1983, Le Houérou et al., 1988) and are classically used to get yields estimates, the relations established at one site between herbaceous yield and rainfall are often poor (Tracol et al., 2006, Hiernaux et al., 2009a). Linear regressions between yield relative anomalies at a site and the corresponding rainfall relative anomalies at the closest
Conclusion
Soil texture and topography largely determine the average yield of the herbaceous layer of Sahel rangelands. They also determine its spatial heterogeneity assessed by the coefficient of variation of the mean yield calculated from hundred 1 m2 random virtual plots. The means of these standard deviations calculated over 1984–2006 range from 295.1% in shallow soil, to 86.0% in clay soils, 79.5% in lightly grazed sandy soils and 76.6% in heavily grazed sandy soils, with an overall mean of 124.5%.
Acknowledgments
This research was funded by INSU and CESBIO in the framework of the AMMA program. AMMA is an international research program funded by a large number of agencies, especially from France, UK, USA and Africa. It benefits from a major financial contribution from the European Community’ Sixth Framework Research Program (AMMA-EU). Detailed information on scientific coordination and funding is available on the AMMA International web site: http://www.amma-international.org. Special thanks to INSU for
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