03 January 2016

Southwest Arabian endemic area

With sparkling mountain streams, forests drenched in mist and incredible high-altitude agricultural terracing, this afrotropical ecoregion is one of the most fascinating and unusual in Arabia. Plant diversity and endemism are very high here, with over 2,000 plant species and about 170 endemics. The woodlands are rich in animal life, sheltering approximately 34 mammal species, 245 bird species, 41 reptile species and 7 amphibian species. Overgrazing, erosion of poorly maintained agricultural terraces, deforestation and hunting are the major threats here. It is situated in the southwest Arabian highlands above 2,000 m and includes part of the Asir Mountains of Saudi Arabia and most of the western highlands of Yemen. In the west, a steep escarpment drops to the Tihamah plain on the Red Sea coast. To the east is a high plateau, with the mountains then sloping more gently to the inner desert regions and sands of the Rub’al-Khali (Empty Quarter). The escarpment mountains are the principal topographic feature here; they run in a north-south direction, parallel to and overlooking the Red Sea. The rugged mountainous landscape contains several peaks over 3,000m, including Jebal Sudah. The Asir Mountain chain is the highest land in the Arabian Peninsula, which tilts from west to east. East of the mountains the land slopes gradually to the Arabian Gulf. The mountains are composed mainly of sedimentary rocks such as limestones, sandstones and shales that are of Jurassic, Cretaceous and lower Tertiary ages. These overlie a basement complex of Pre-Cambrian granitic igneous rocks. The climate of the region varies considerably depending on altitude, aspect and season. The highlands receive variable rainfall caused by the southwest monsoon, which brings damp oceanic winds. A tradition of high-mountain agriculture spanning two thousand years has produced a spectacular terraced landscape on the steep mountain slopes. However, this has eliminated much of the forest and woodland cover and only scattered patches of woodland now survive. In Saudi Arabia, the vegetation of the Asir Mountains has remained largely intact, although the situation is deteriorating. In some places the vegetation shows a distinct zonation, with an evergreen forest or scrub above about 2,000m and largely Afroalpine vegetation above 2,500m. At these higher altitudes a lush cloud forest exists, including Juniperus procera, usually festooned with the lichen Usnea articulata, the woody shrub Euryops arabicus and Lavandular on the north facing slopes. On the more barren south facing slopes, Aloe sabae and Euphorbia are common. The flora of southwest Arabia has strong affinities with parts of Africa, particularly East Africa. Juniperus procera, found above 2,500m in the Asir Mountains, is also very abundant in and characteristic of the East African highlands. There, it is a dominant plant in some montane or subalpine vegetation units and is well known in Eritrea, Somalia, Ethiopia, Kenya and Tanzania at almost the same altitudes.