Abstract
Riyadh region is one of the hardest habitats in Saudi Arabia with a hyper-arid climate. This study was conducted to investigate floristic composition and plant life forms of Riyadh region and their ecological significance. Work aimed to determine the prevailing plant families and biological spectrum of their components that reflects the phytoclimate and adaptation to hyper-arid conditions of the region. Work involved field surveys of different locations in Riyadh region where plant specimens were collected and identified. Collected plant species were then listed according to their families and data were used to assess the contribution of different plant families to the flora of the region. Study of life forms was conducted to classify recorded floristic elements into categories that reflect environmental conditions prevailing in the region. Prevailing plant families were Asteraceae (17.4%), Poaceae (11%), Brassicaceae (9.9%), and Fabaceae (7%). These percentages reflect wide ecological ranges especially for Asteraceae and Poaceae. High presence of species belonging to Astreaceae and Poaceae can be attributed to their adaptation to harsh conditions as well as to effective wind dispersal strategies of their diaspores. While the most frequent life form classes were therophytes with 52%, and chamaephytes with 30%, phanerophytes and hemicryptophytes represented 9% and 8%, respectively. Predominance of therophytes and chamaephytes over other life forms is a response to hyper arid climate with insufficient rainfall and the nature of region of few available microhabitats that can support high percentage of perennials.