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Resume Cette etude compare les pâturages sous et hors couvert ligneux d'une savane a Combretaceae de la zone soudano-sahelienne de Thysse Kayemor (Senegal). L'analyse des parametres floristiques et de production de ces herbages permet de distinguer nettement les biotopes couverts de ceux decouverts. Il existe donc un effet couvert, caracterise par une plus importante richesse specifique de la vegetation pastorale, liee aux especes exclusives. La production de matieres seches, bien que significativement plus elevee sous couvert, ne varie pas selon l'espece ligneuse assurant l'ombrage. L'abondance relative des especes apparait toutefois plus faible sous les arbres que dans les milieux decouverts.

Carbohydrates represent 5–25% of the organic matter in soils. They constrain microbial activities and mineral nutrient production in soil and also reflect the whole microorganism community dynamic. The objective of this study was to determine the contents and composition of hydrolysable carbohydrates in soils collected in a forest–savanna mosaic landscape in the region of Lamto (Côte d’Ivoire). Capillary gas chromatography was used to identify and determine carbohydrates in soil profile under 4 tropical ecosystems: gallery and plateau forests, and grass and shrub tree savannas. Forest soils were higher in organic matter than savanna soils (0.50–2.96% C v. 0.53–1.22% C). The carbohydrate-C content of soils, expressed as percentage of total soil organic C, was low, a likely consequence of the tropical climate that promotes a rapid decomposition of surface plant debris. The carbohydrate-C content was higher under savanna soils (5–7%) than under forest soils (3–4%). Glucose, ribose, mannose, xylose, and galactose were the 5 most abundant extractable monosaccharides in all soils. Between them, only xylose and ribose are controlled by the vegetation type. The [(galactose + mannose) : (arabinose + xylose)] and [mannose : xylose] ratios suggested that most soil sugars derive from microbial biomass. The large abundance of microbial carbohydrates indicates intense microbial activities in the soil, and then rapid decomposition of soil organic matter favoured by the long wet season, with high temperatures and soil water availability at the site study. Results suggest clearly that the climate likely controls the amount and composition of carbohydrates in Lamto soils.

Around Niamey, in Niger, the shrub Guiera senegalensisdominates all of the fallows in almost totally monospecific stands. The species has a strong resprouting ability enabling it to survive cutting when clearing land for crops, whereas all other species succumb to cutting. However, in this dry environment (560 mm of annual rainfall from June through September), this remnant-increaser species has to cope with variations of available water in space and time. The daily course of its water status is an indicator of the variation of the intensity of water flux between the soil, vegetation and atmosphere, and it is often estimated with Soil Vegetation Atmosphere Transfer (SVAT) models. We assessed the water status of G. senegalensis through its daily cycle leaf water potential ( Ψ ), using six populations representative of the local diversity by measuring three shrubs per population on two excised leaves per shrub for each measurement. We made our measurements over two vegetative cycles (from July 1994 to December 1995), fortnightly during the rainy season, and once a month during the dry season, until the sample shrubs lost their leaves in their deciduous cycle. We used linear models to characterize the daily Ψ cycle of the shrubs, and of the populations during each season. For each population, three sources of variation were considered: the shrub number, the date and the hour in day. Date and hour in day effects are presented as trend surface models defined by the degrees of polynomials and the status of shrub effects. The status of shrub effects characterizes the variability in accounting for environmental conditions. The group of plant populations that explain the least variability in every model as well as the least total variance among the shrubs and between seasons were in locations with the best water supply. Two other groups were defined according to the median and the highest explained variability of Ψ , respectively. For all stations, the most temporal variability is explained by models accounting for the interaction between shrubs and the temporal variation, with the least variability for models that do not account for the shrub effect at all. Models accounting only for the additive effect of shrubs explain intermediate values. The models used herein indicate the large range of variability in G. senegalensis water status, both for inter- and intra-populations. This high physiological plasticity must contribute to the strong species-dominance stability in its current distribution area. © 2003 Elsevier B.V. All rights reserved.

Two vibrodrill and six core drill cores were recovered in Lake Assom near Tibati, south of Adamawa. From eight radiocarbon ages on samples from the two vibrodrill cores and on the basis of sedimentological analysis, a reconstitution of the environmental history of this site for the late Holocene has been done. These results suggest the Lake Assom has remained swampy or even dry since 2800 years BP. Spectacular deposition of compact muddy intraclasts strongly suggests that this situation was suddenly interrupted close to ≈1700 years BP when the lake was inundated by water. This deposition can be compared to that of coarse materials observed at the base of the erosion profile “Basse Terrasse”. In spite of this present filling trend, the lacustrine state of the site seems to still pertain. Observations done on the lake banks and the occidental swampy plain, compiled with those from the well in the centre of the small adjacent depression of Ngwana, enable us to better understand the recent functioning of the lake. The Mandjara river certainly played a major role in the sedimentation of sandy gley, and later in the high energy reworking of older coarse intraclasts at ≈1700 years BP. Lake Assom seems to have originated from the dissolution of a more extensive iron crust covering the hole region of Mbella Assom. The presence today of two adjacent depressions is due to local morphology following changes in the lake during the dry period. On the other hand, palynologic studies show no indication of the proximity of any humid forest to this wooded savannah over the past 4500 years BP.

Sur les rives du Mayo Boula au sud de Maroua au Cameroun septentrional les sédiments anthropiques des sites de Goray et Salak ont été prélevés sur les coupes des puits spécialement creusés dans ce but. Dans l'un et l'autre cas les résultats pourraient être décourageants. Les sédiments cendreux sont abondants et souvent dépourvus de pollen ; mais quelques niveaux en particulier de Salak ont fourni des palynoflores assez riches. Par rapport au contenu floristique d'un fragment du mur d'une case en ruine les différences enregistrées permettent de supposer aux abords du XIème siècle un couvert plus dense et varié (des sols plus humides aussi) où #Bombax$ (#B. buonopozense$) existe et semble disparaître juste au moment de l'arrivée du sésame. (Résumé d'auteur)

The actual evapotranspiration ( E a ) was followed from 1989 to 1992 in a thorn scrub stand in the northern Ferlo region (North Senegal, West Africa) for an Acacia tortilis grove, a Balanites aegyptiaca grove (every grove possessed its characteristic herbaceous layer) and a herbaceous zone outside the tree crown shade (annuals). ( E a ) was derived from the water balance equation method (neutron probe measurements). The soil water profiles (soil water content and potential plotted to soil depth) were established, giving an account of preferential water uptake zones and the rooting depths of trees and annuals. The mean seasonal actual evapotranspiration of the thorn scrub was 1.92 mm day −1 during the rainy season (period 1), 1.62 mm day −1 during the ‘deferred’ season (period 2), 0.62 mm day −1 during the cool dry season (period 3) and 0.09 mm day −1 during the hot dry season (period 4). The daily maximal actual evapotranspiration of the groves was higher than that of the herbaceous zone and the Acacia tortilis grove's maximal daily actual evapotranspiration was higher than that of the Balanites aegyptiaca grove: 5.04, 4.15 and 3.98 mm day −1 (period 1), 4.28, 3.76 and 2.89 mm day −1 (period 2), 1.73, 1.79 and 1.34 mm day −1 (period 3), 0.93, 0.46 and 0.36 mm day −1 (period 4) for the Acacia tortilis and Balanites aegyptiaca groves and the herbaceous zone, respectively. For soil depth to 3.5 m taken into account in the water balance equation, by the end of the growing year, soil moisture differed by 20.8 mm between tree and grassland zones in favor of grassland. Had this water been used by trees, this thorn scrub could have produced 13.72-0.8 kg DM ha −1 year −1 more wood.