Le potentiel de l'agroforesterie pour augmenter la production primaire dans les zones sah�lienne et soudanienne de l'Afrique de l'Quest

Abstract

The article presents a critical evaluation of agroforestry systems as regards their potential to increase primary production in the Sahelian and Sudanian zones of West Africa. The suggestion that trees would always and everywhere be profitable for the region will be counterproductive, the basis for disappointments and a waste of money. One has to consider carefully which properties of woody species could serve which objective, where and under what circumstances.Primary production is limited by water availability in the north Sahelian zone only+ elsewhere in the region nutrient availability is critical. Woody species influence the water balance via rainfall interception+ the influence on evapotranspiration and the influence on water infiltration. The ultimate result for grasslands and crops depends upon local conditions: positive effects of windbreaks should be particularly attributed to protection against mechanical stress. Processes that influence nutrient availability under trees are those acting via redistribution, those reducing nutrient losses and those increasing soil fertility. Of the latter processes, serious limitations exist in the region concerned: nitrogen fixation is limited by phosphorus shortage mainly and deep rooting is very limited and so is possible uptake of minerals from deep soil layers.The positive influence from woody species on soil fertility and primary production varies with average annual rainfall and soil type; its importance increases more than proportionally with rainfall going south. This positive influence is basically linked to the redistribution of nutrients and water, the internal nutrient cycling and the related enlarged plant-litter-soil nutrient cycle. Taking advantage of its effects is difficult and risks further depletion of soil fertility. Moreover, since woody species generally compete with the herb layer+ it will be difficult to develop agroforestry in such a way that the positive influences are not overridden by negative ones. The potential to increase nutrient availability by agroforestry systems is limited and windbreaks are more beneficial under conditions which are rare in the region.More research is needed to determine under which conditions introduction of woody species may be beneficial. 42 R~rmeLe potentiel de l'agroforesterie pour augmenter la production primaire dans les zones sahOlienne et soudanienne de l'Afrique de l'Ouest Une 6valuation critique de l'agroforesterie est prrsentre, en ce qui concerne la possibilit6 d'augrnenter la production primaire dans les zones sahrlienne et soudanienne de l'Afrique de l'Ouest. La supposition que dans cette rrgion des arbres seraient toujours et partout avantageux, n'est pas productive, mais plut6t une cause de drceptions et un gaspillage de ressources. I1 faut se demander quelles proprirtrs des esprces ligneuses pourraient servir quels objectifs, off et sous quelles conditions. Seulement dans la zone nord-sah+lienne la production primaire est limit~e par la disponibilit6 en eau; aiUeurs c'est la disponibilit6 des 6lrme...

Effects of tree clumps on soil characteristics in a humid savanna of West Africa (Lamto, Côte d'Ivoire)

“…Tree clumps can modify soil water content dynamics as they act on both soil water inputs and outputs. Kessler and Breman (1991) have reviewed three field processes controlling moisture availability under woody species: rainfall interception, evapotranspiration and water infiltration. According to the level of each of these processes, tree canopy effects can lead to various, complex and contrasting situations.…”

Single-tree influences on soil properties in agroforestry: lessons from natural forest and savanna ecosystems

“…While soil amelioration occurs beneath the canopies of certain species, increasing the tree density may not extend these benefits (Kessler and Breman, 1991;Alstad and Vetaas, 1994). Such a benefit may occur in ecosystems where trees enhance soil fertility by increased inputs from external sources such as Nfixation, or interception of wet or dryfall nutrients.…”

“…Such a benefit may occur in ecosystems where trees enhance soil fertility by increased inputs from external sources such as Nfixation, or interception of wet or dryfall nutrients. In semi-arid savannas or parklands, however, where trees often enrich soil through nutrient redistribution (Kessler and Breman, 1991;Belsky et al, 1989;Campbell et al, 1994) via lateral root scavenging or as deposition foci of animals, the upper limits on tree density are set by the finite amount of nutrients available in the system. Similarly, low water availability in natural and managed semi-arid systems limits tree and shrub density.…”

Growth performance of exotic and indigenous tree species in saline soils in Turkana, Kenya

“…In the Sahel, low successes of tree planting trials have been attributed to the poor performance of exotic species compared to indigenous ones (Catterson et al, 1987). The exotic tree species may have faster initial growth and produce more biomass than the indigenous species (Gosseye, 1980;Taylor & SoumareH , 1984;Kessler & Breman, 1991), but given that information on survival is usually lacking, choice of exotic over indigenous species cannot be justified (Sinha & Ghial, 1997). Rather, tree-planting trials need to assess growth performance of tree species as criteria for selection.…”

“…Rehabilitation of saline soils in arid rangelands requires careful selection of tree species that tolerate salts, going hand in hand with the improvement of soil water by appropriate water-harvesting methods to increase productivity (Gindel, 1973;Evanari et al, 1982;Kessler & Breman, 1991;Minhas et al, 1997). In the arid zones of central Turkana, north-western Kenya, where 15-20% of the rangelands are affected by soil salinity (Van Bremen & Kinyanjui, 1992), the effect of water harvesting on growth performance of trees in saline soils has not been evaluated.…”