Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Tree-crop interactions within a Sahelian windbreak system

Auteur Brenner, Andrew J.

Université de soutenance : University of Edinburgh

Grade : PhD Doctor of Philosophy 1991

Résumé
The advantages of windbreaks in terms of reducing soil erosion and providing tree products for farmers in the Sahel are well known. There is less certainty about the effect of shelter on crop growth, the degree of competition for water and light between windbreak and crop, and almost no information on the amount of water a windbreak transpires over a year. Field measurements of millet (Pennisetum zyphoides) growth and microclimate behind a young double row neem (Azardirachta indica) windbreak at the ICRISAT Sahelian Centre, Sadoré, Niger, were carried out in 1988 and 1989. Measurements of tree transpiration were made using a commercial sap flow meter, along with measurements of tree heights, stem basal areas and leaf areas. The reduction in wind speed produced by the windbreak changed over the season, as a result of the reduction of the porosity of the lower half of the windbreak by the growth of the millet crop on the windward side of the windbreak. Maximum shelter was found at 6h at the beginning of the cropping season and 3h during the middle and end of the season, where h is the height of the windbreak. Factors influencing the magnitude of shelter are discussed. Shelter increased both air temperature and water vapour pressure. At the start of the season millet growth and development was delayed by shelter. However, as the season progressed shelter caused a more favourable climate for crop growth. It is suggested that crop response to shelter occurs because of (i) increased plant temperatures increasing rate of plant development and, (ii) a decrease in the vapour pressure deficit at the leaf surface which increases stomatal conductance and the solar radiation conversion ratio. Leaf area of the neem was linearly related to stem basal area. So windbreak water use was estimated by normalizing tree transpiration to stem basal area and multiplying by the average stem basal area per unit length of windbreak. Transpiration rates of trees decreased as the dry season progressed. Yield reductions as a result of windbreak-crop competition did not occur at distances exceeding 1.5h from the windbreak during the two years of trials. Belowground competition and above-ground competition influenced biomass yields to a similar extent. Grain yield reduction by above-ground competition could not be attributed solely to a reduction in PAR. Optimum windbreak spacing based on 1989 results was calculated as 10 - 15h, which would provide grain yields that compensated for land area taken out of production and increased stover by 25 %. The relative transpiration rates of the windbreak and crop were investigated. Averaged over the season the windbreak used up more water per unit land area than the millet. If the windbreaks were spaced 100 m apart a windbreak would use up 60 mm more water per year than would be used by a millet crop and bare soil, approximately 11 % of the annual rainfall.

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