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A field experiment was conducted in an afforested Stipa tenacissima L. steppe in arid southern Tunisia to evaluate the effect of three tree species (Acacia salicina Lindl., Pinus halepensis Mill. and Eucalyptus occidentalis Endl.) on understorey vegetation and soil nutrients. For each tree species, two subhabitats were distinguished: under the canopy, and out in the open. Organic carbon, total N, available P and pH were higher under the canopies of the three tree species than out in the open, and the effect was more pronounced in the top 10cm of soil. Similarly, plant cover, biomass, richness and diversity were significantly higher under tree canopies. Some species such as Plantago amplexicaulis Cav., Helianthemum kahiricum Del. and Artemisia campestris L., which use large amounts of soil nutrients, showed a strong preference for areas under the canopy. Among the three tree species, Acacia salicina had the strongest positive effect on soil nutrients and understorey vegetation, and, thus, may be more useful for restoring arid areas and creating areas of enhanced soil nutrients than Pinus or Eucalyptus.