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The interactions between species in six sites located in arid and semi arid rangelands of Nodushan were assessed studying association, covariation, segregation and dispersion patterns of plants. The results indicated that facilitation and competition between species were mainly affected by the life form of the species, environmental conditions and the species involved. The dominant chamaephytic shrub species (in particular Artemisia sieberi) ameliorate microenvironmental conditions and facilitate the establishment and survival of hemicryptophytic species (in particular the caespitose grass Stipa barbata). Establishment of different shrub species seems to depend mainly on soil type. Once established, they mostly are in competition with each other for limited resources on the basis of their similar life forms. (C) 2011 Elsevier GmbH. All rights reserved.