a b s t r a c tThe present study aims at describing and analysing the floristic composition and vegetation types, as well as determining the effect of recent land uses on the vegetation structure. It aims also at identifying the alien plants species and elucidating the impact of these species on the plant diversity and community structure of the study area. One hundred and ninety stands were selected monthly for this study, 145 species were recorded (69 perennials and 76 annuals) related to 83 genera, 40 families in 9 identified habitats in El-Omayed Biosphere Reserve (coastal sand dunes, salt marshes, saline depression, nonsaline depression, inland ridges, inland plateau, irrigation canals, road sides and cultivated lands). Therophytes were the most represented life form. Three habitat groups resulted after the application of TWINSPAN and DCA as classification and ordination techniques: 2 represented the natural habitats and one represented the urban and cultivated habitats. Group I represented coastal dunes and salt marshes GII: saline depressions, non-saline depressions, inland plateau and inland ridges and GIII: irrigation canals, road sides and cultivated lands. Coastal dunes had the highest species richness (α-diversity), followed by cultivated lands, while inland plateau had the lowest; but saline depressions had the highest species turnover (β-diversity). Non-saline depressions had the highest relative evenness, while saline depressions had the highest relative concentration of dominance. Coastal dunes had highest values of calcium carbonates and calcium ions, and salt marshes had the highest salinity, pH, potassium and sodium contents, but cultivated lands had the highest values of silt, clay and organic matter. The diagram resulting from CCA showed an influence of most soil variables, except nitrogen, calcium and potassium. Twenty two species were recorded for the first time in the study area. The recent land use (overgrazing, wood cutting and collecting, construction of summer resorts and irrigation canals and agricultures) led to the emergence of new invasive species, which may severely affect the plant diversity and community structure of this hot spot of biodiversity in Egypt.
