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An experimental study was undertaken on a complex ecological system in the semi-arid Sahelian region of Senegal. The effects of tree cover on the above- and below-ground phytomass of the herbaceous layer were studied. The results show that : herbaceous above-ground production was 1.5 to 4 times greater under the canopy than in the open sun light ; understory root phytomass was about two-fold greater than the root phytomass of the open grassland ; and the root/shoot ratio was 2.5 higher in the open than in the shade. Increased herbaceous layer productivity was interpreted as a result of better climatic conditions and greater soil fertility within the canopy zones. By promoting the development of herbaceous vegetation, trees increase the long-term production potential of the zone and, in consequence, improve conditions for livestock husbandry. (Résumé d'auteur)

Three experiments were carried out in Senegal to assess the effect of four factors, clone, soil type, volume of containers and plant age. on N2 fixation by Casuarina equisetifolia using isotope dilution (ID). A value (AV) and total N difference (TND) methods. Three clones grown in 151. containers differed significantly in their ability to fix N2 and also to absorb soil N, the best clone (β) fixing 3.27 (ID) and 3.85 (AV) and the less active 2.51 (ID) and 2.73 (AV) g N2 per tree during 1 yr. N2 fixation by clone β was markedly affected by soil N: it decreased from 7.60 (AV) in Camberene soil (6.010% N) down to 4.64 (AV) g N2 fixed per tree in Bel Air soil (0.025–0.030% N) during 1 yr. The amount of N2 fixed was much higher in larger containers. In the case of 1 m1 containers with Camberene soil, the amount of N2 fixed (AV)per tree (cloneβ) was 40.44 g during the first year and 84.43 g during the 2 first years of growth. In experiments 1 and 2, ID and AV estimations were significantly less than ID or AV estimations, but, in experiment 3, AV and TND estimations were similar. Problems raised by the estimation of N2 fixation are discussed. Inoculation with Frankia strain ORS 021001 always markedly increased plant biomass expressed as dry weight and total N.