VEGETATION ANALYSIS AND SPECIES-DIVERSITY ALONG AN ALTITUDINAL GRADIENT IN THE CENTRAL HIJAZ MOUNTAINS OF SAUDI-ARABIA

The study deals with the distribution, habitats, and ecological charac teristics of natural vegetation along a transect crossing the: central part of Hijaz Mountains, Saudi Arabia. Fifty stands were classified o n the basis of leading dominant species, using agglomerative cluster a nalysis, into five distinct and recognizable vegetation groups (vegeta tion types). A total of 86 plant species were recorded. The present st udy provides evidence that the vegetation shows discernible zonation. The annuals increase, and the biennials and perennials decrease along the elevation gradient from lower to higher elevations. This appears t o be correlated, at least partly, with altitude. Species diversity var ies from one vegetation group to the other as well as between stands o f the same group. The vegetation type of Ficus salicifolia is the most widespread, diversified and consequently, the most stable vegetation cover in the study area; it, therefore, represents the climatic climax community. Human interference has a prominent role in changing the ph ysical environment and inevitably destroys the precarious balance amon g the components of the ecosystem. The environment has deteriorated, a nd retrogressive: changes in the vegetation are recorded.