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The geographic distribution of Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan in Tunisia, is limited to the Bou-Hedma region (Bled Talah) where the National Park of Bou-Hedma, a UNESCO (1986) biosphere reserve, is located. We have tested the hypothesis that A. tortilis acts as a foundation species as a result of the improvement of soil conditions under its canopy (soil water availability and nutrient enrichment). The herbaceous community (floristic composition, species density, species richness) and soil characteristics (soil nutrients and soil water) were sampled using the quadrat method beneath and between 16 adult trees of A. tortilis with both northwards and southwards (south and north sides) and eight open areas among the trees. Results showed that A. tortilis trees improve soil water availability, the nutrient status of soil (organic matter, total carbon, total nitrogen, extractable phosphorus), and has a positive effect on the understorey vegetation under arid conditions. No significant differences in species richness were observed between the north subhabitats and the south subhabitats.