Vegetation, species diversity and floristic relations along an altitudinal gradient in south-west Saudi Arabia

Analysis of vegetation along an altitudinal gradient in south-west Saudi Arabia has disclosed some generalities in vegetation characters and species diversity. The complex interaction of different environmental factors in relation to altitude leads to variation of habitat types and different plant communities and vegetational belts. Vegetation along the altitudinal gradient showed three zones. At low altitude (0–500 m a.s.l.), the Tihamah coastal marshes, plains and foothills have discrete plant communities on discontinuous substrates and habitat types. At high altitude (above 2500 m), the Asir highlands have discrete communities determined by topographic form (slopes, ridges and crests). The intermediate altitudes (500–2500 m) have continuous plant communities. Vegetation in the intermediate altitudinal belt attains the highest species richness and diversity with relatively high evenness. The studied area is under the influence of two floristic regions: The Sudano-Zambesian (especially Afroriental and South Arabian Domain) and Irano-Turanian region, with extension to Mediterranean (mainly Eastern Mediterranean) and Saharo-Sindian region (Middle and Western).