In sub-Saharan Africa, woody plants make a large contribution to the livelihoods of rural populations, and are an integral part of cropping systems: tree parks, tree fallow, bush fallow. In a context of increased climate risk, all initiatives seeking to improve the sustainable management and production of these agroforestry systems are welcome.

Tree parks: systems combining cereals and trees are widespread in the West African savannah landscapes; the best known are the Faidherbia parks (Faidherbia albida) and the shea tree parks (Vitellaria paradoxa).

Cereal/bush association: Guiera senegalensis and Piliostigma reticulatum bushes, which are naturally present in the fallow lands of the sub-arid zone, are traditionally cut and burned prior to cropping. Over the last few years, some studies have tended to show that keeping these species in the fields creates a special pedoclimate and leads to an increase in associated cereal yields.

Senegal 

The groundnut basin in the centre of the country (around 30% of national territory) is the main agricultural region in Senegal. The main food crops are millet (P. glaucum), groundnut (A. hypogaea L.), sorghum (Sorghum bicolor), cowpea (Vigna unguiculata), and maize (Zea mays) in the southern part. The region has a unimodal rainfall pattern varying in time and space, with episodic droughts and frequent poor harvests. The rainy season lasts from July to October, in the form of generally short and heavy scattered showers. The region is greatly marked by a North-South pedoclimatic gradient, which will be taken advantage of in our study.

Keur Mactar site

The first study zone is located in the northern region of the groundnut basin, just outside the town of Thiès either side of the RN3 road heading to Bambey. Annual rainfall is around 450 mm; the climate is Sahelian. It is an ideal zone for Faidherbia albida parks. In addition, as part of a multi-institutional project (PIRE project notably involving IRD), an experimental site set up by ISRA (Institut Sénégalais de Recherches Agricoles) at Keur Matar (14°45'N, 16°51’W, alt 55 m) is being used to study a combination of millet and Guiera senegalensis, which is the most widespread native bush in the region. This bush, which is planted in the trial at a density of 240 plants/ha has an extensive superficial root system in the first 45 cm and an unbranching main root descending to a depth of over 2.5 m, enabling a hydraulic lift process that is apparently of benefit to the associated crop.

Nioro-du-Rip site

The second study zone is located a few kilometres north of the town of Nioro-du-Rip, in the south of the groundnut basin. The average annual rainfall is around 750 mm; the climate is of the Sudanian type. In this zone, ISRA has also set up an experimental design (13°45'N, 15°47'W, 22 m alt.) being used to test a combination of millet with the locally most densely represented native bush, Piliostigma reticulatum. The density in the design is 185 plants/ha. The root system of the bush is well developed and quite deep so that the crop can benefit from this association.

Niger

The site is located in the Sahel with an agricultural dominance, in Niger, in the watershed of the Dantiandou temporary water course (5,645 km²) within an endoheic hydrographical network mainly draining towards temporary or permanent ponds. The average annual rainfall is 560 mm (1905-2004). The geology of the region is dominated by the sandy sedimentary formations of the Iullemmeden basin. The non-cultivated vegetation consists of shrub steppes dominated by G. senegalensis. Over 60% of the territory is covered exclusively by pearl millet (Pennisetum glaucum) food crops, without amendment or motorization. This basin, which has been equipped for 20 years for measurement of all the water balance terms and for 7 years for the energy balance, is marked by both climatic variations (1970 and 1980) and widespread farming, to the detriment of the woody cover, which degrades soils and leads to severe gulley erosion. A model expresses the combination of hydrological and plant processes. It has been used to assess the relative importance of such impacts. During the rainy season, evapotranspiration decreased over the study period and appeared to be more controlled by the nature of the plant cover than by rainfall.  During the same season, plant productivity (natural and cultivated) increased with the shift from fallow to field, mainly due to the replacement of the natural vegetation (grasses and trees) by millet and rejuvenation of the woody cover in the fields. In particular, the overall hydrological consequences of this change in use remains to be understood.

Benin 

The site is located in the Sudanian band, in North-West Benin, 450 km from Cotonou in the watershed of the Donga permanent water course (586 km²). The hydrographical network is functional, with intermittent flow linked to the rainy season. The climate is of the dry tropical type, with a dry season and a rainy season with an average rainfall of 1,200 mm/year (1950–2005). The vegetation is a mosaic of dry forests and savannahs, with a dense cover of tall herbaceous perennials and a more or less dense tree stratum. The basin is almost entirely covered by a mosaic of rainfed food crops (yam, maize, millet, sorghum, cassava, groundnut), mainly under shea tree (Vitellaria paradoxa) and Néré (Parkia biglobosa) parks, and fallow lands of highly varying ages. This basin has been equipped for 10 years for measurement of all the water balance terms on the main types of plant cover. The first results indicate a seasonal replenishment-depletion signal for the groundwater, which suggests the depletion in the dry season might mainly be due to take-up by trees (transpiration). It is this “trade-off” between production diversification and water availability (soil and water tables) that will be particularly analysed during the project.   

A socio-economic survey intended to show the contribution made by livestock farming in reducing vulnerability has been used to establish the typology of the means of subsistence depending on the activity system. The region is attractive and has many forest-agriculture transition zones alongside farming land cleared much longer ago. Women’s incomes govern children’s access to healthcare and education, due to the family structure making women responsible for these aspects. This factor partly explains the chronic infant malnutrition seen despite quite good food availability. This is why the place of women in the shea nut supply chain, as operators but not owners, will be at the heart of the project’s socio-economic activities at this site.