<strong>Questions</strong> What are the relative contributions of short-term canopy and long-term soil effects of a savanna tree species in explaining differences in biomass, species density and composition of understorey plants (1) between trees and open areas, and (2) below trees between the northern and southern sides of the trees? <strong>Location</strong> Acacia tortilis tree savanna ecosystem, southern arid Tunisia (Bou Hedma National Park). <strong>Methods</strong> We measured environmental conditions (soil and microclimate) on the northern and southern sides of the trees and in open areas between trees. We also manipulated light on the southern side of the trees with a shade cloth, and cross-transplanted soils between north and south sides of the trees while testing for soil disturbance. The response of the vegetation was analysed with CCA and univariate analyses. <strong>Results</strong> Differences in species composition, richness and biomass of understorey communities between trees and open areas were mostly due to long-term soil effects of A. tortilis, mainly nutrients and water, whereas differences below trees were due to short-term effects of the trees, in particular on light and temperature conditions. Additionally, the soil disturbance induced by our soil transplantation treatment modified species composition and density, likely because of disruption of the soil crust. <strong>Conclusions</strong> Our study provides additional evidence that savanna trees have the potential to increase the diversity of arid systems at the landscape level. We showed that two main mechanisms are involved: (1) trees improve soil conditions, in particular nutrients and water, resulting in differences between trees and open areas; (2) within trees, there is heterogeneity of light and temperature conditions under the crown, resulting in vegetation differences.< Réduire