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The influence of Acacia tortilis on its understory vegetation was analysed seasonally and in relation to grazing. Plots were sampled under tree canopies and open areas located in protected zones inside the Bou-Hedma National Park (Southern Tunisia, lower arid climate) and in un-protected areas. Species responses were monitored during two growing seasons (a wet year and a dry year). These surveys were then analysed on the basis of plant cover, species richness. In both un-protected and protected areas, the influence of the tree canopy on the herbaceous strata can be considered as positive, even though it is more significant inside the park. A. tortilis allowed the establishment of new species probably due to an improvement in soil fertility and microclimate. The positive effect of the tree on its understory vegetation was especially noticeable in protected areas during the two years. In un-protected areas, the positive effect of the Acacia remained significant throughout the dry year but disappeared in the wet year (interaction community x year). Moreover, the existence of an interaction between the community type and the management demonstrated the existence of a mitigation of the positive effect of Acacia by grazing. (C) 2011 Elsevier Ltd. All rights reserved.