Natural vegetation of the sand seas of central and northern Saudi Arabia : a biogeographic study
This study is the first quantitative and qualitative attempt to evaluate the vegetation (the flowering plants) of the sand dunes of central and northern Saudi Arabia. Both the existence and the development of vegetation cover on the sand dunes are determined by the physical, thermal, hydrological and chemical properties of the sand, some of which are favourable to the existence and welfare of vegetation cover on the sand dunes and others of which have adverse effects. It is likely that the form of the vegetation currently covering the sand dunes dates from the decrease in humidity with the start of the present arid phase, some 6000 years ago. Plant species of the sand dunes have adapted to the rigorous abiotic environmental factors through various morphological and physical mechanisms. They have also adapted ecologically to this harsh environment. Biogeographical and floristic analysis of the flora of the research area has revealed that floristic diversity is very low. 165 plant species were collected from the main sand dunes and their periphery in the research area. Of these, only 22 species, or 13.33% of all species recorded, are considered to be true sand dune species. The research area has a considerable number of species from different plant geographical regions. The existence of many Saharo-Arabian species, and their high proportion of the total, confirm the designation of this region as Saharo-Arabian. Life form analysis of the flora of the sand dunes showed that 63.6% of the total species were therophytes (annuals and biennials). Perennial species accounted for 33.9%. 2.4% of species were those which may behave as annuals, biennials, or short-lived perennials. An outstanding feature of the 30 families presented is that only a limited number were of importance, with Compo sitae, Gramineae, Cruciferae, Caryophyllaceae, Leguminosae and ZygophyUaceae predominating both in the number of genera and species. Three plant community groups are strongly associated with the sand dune terrain. These community groups are communities of the sand dune habitats (sand dune bodies), communities of non-dune or shallow sand habitats (the ecotone communities) and vegetationless areas. Plant species of the sand dunes yield a wide range of productive benefits. Consequently, the vegetation cover of the sand dunes has been destroyed and degraded to some extent by disturbance of the natural environment resulting from human activities, such as overcultivation, overgrazing, woodcutting, recreation and associated activities.