Abstract
Peripheral populations have significant conservation values due to their evolutionary potential and regional ecological implications. Astragalus, the largest genus in the flora of Saudi Arabia, has an extensive distribution range in West Asia. However, the distribution of Astragalus species in Arabian Peninsula is restricted. The present study analyzes the floristic diversity and ecology of some of the peripheral populations of Astragalus species occurring on the edge of their distribution range in central Saudi Arabia and highlights its common associations through TWINSPAN, DCA and CCA approach. Five 'Raudhas' (meadows) in the central region have been selected to quantify the density and abundance of Astragalus species. Although these populations in no way dominate the selected habitats as a whole, at least a few species exert a local dominance in some stands. 180 species were recorded from 150 stands, including 14 species of Astragalus and 15 very common associated species. 12 plant communities were recognized after application of TWINSPAN and DCA programmes. Based on the data obtained from 150 species-rich stands, we also assessed the relationships between edaphic-gradients and plant relationships using CCA. The results show that the pattern of distribution and existence of Astragalus species are highly influenced by both edaphic factors such as soil texture, pH, organic matter, etc. and competition among Astragalus species and associated species. As the peripheral populations of Astragalus in all ecologically divergent habitats of the central and eastern regions are potentially significant for future speciation events, protection of such populations have impending importance in the evolutionary process.