In the Mediterranean dry ecosystems, Fabaceae species are considered as a major factor for natural and unnatural restoration of degraded rangelands and therefore play a crucial role in soil and water conservation, maintaining ecological balance in terrestrial ecosystems. The central objective of the current research study, conducted during growing season, in an afforested steppe (South Tunisia), was to determine the effects of the shrub species Retama raetam on floristic composition, total plant cover, species density, dry matter (DM) yield and soil characteristics in protected sites (inside the park of El Gonna) and unprotected sites (outside the park). In each study site, two microsites were distinguished, namely the canopied microsite (under the shrub crown) and uncanopied microsite (in open area). The herbaceous community (floristic composition, species density, total plant cover, dry matter) and soil characteristics (soil nutrients and soil water) were sampled using the quadrat method beneath 60 adult shrubs of R. raetam (30 in protected sites and 30 in unprotected sites) and 60 open areas among the shrubs (30 in protected sites and 30 in unprotected sites). Results revealed that R. raetam improves soil water availability, the nutrient status of soil (organic matter, total carbon, total nitrogen, extractable phosphorus) and the understorey vegetation under arid conditions. Certain palatable species are frequently found under shrubs. In the protected sites, these species are being replaced by less desirable species, which emphasizes the importance of conservative stocking rates and proper pasture management. Our work provides additional evidence that shrubs have the potential to increase the diversity of arid systems at the landscape level. Additionally, grazing by large herbivores influences negatively the dynamic of vegetation under arid bioclimate.