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Theoretical models predict that the relative importance of facilitation and competition may vary inversely across gradients of abiotic stress. However, these predictions have not been thoroughly tested in the field, especially in arid and semi-arid environments. In this study, we evaluated how the net effect of Acacia tortilis subsp. raddiana (syn. A. raddiana) trees on the herbaceous species varies across a gradient of water stress in arid Tunisian ecosystems. Our results show that the influence of trees on the herbaceous stratum is beneficial. An Acacia tree improves the richness of herbaceous species around it by two to three times. The positive effect of Acacia raddiana on species composition is characterised in particular by a better development of some species of high pastoral value such as Cenchrus ciliaris, Cynodon dactylon, Eragrostis papposa, Sisymbrium irio and Chenopodium album. We fitted the relationship between seasonal rainfall and the relative neighbour effect index, which varied across this gradient, to a linear model. Our results show that herbaceous plant cover, plant biomass, plant density and diversity are higher under a tree canopy and this positive interaction still increase with higher abiotic stress conditions. (C) 2010 Elsevier GmbH. All rights reserved.