Diversity and endemism of the western Ethiopian escarpment - a preliminary comparison with other areas of the Horn of Africa.

The vegetation on the western Ethiopian escarpment is still fairly intact, but imperfectly known. The escarpment-area agrees with the area of White’s Undifferentiated woodlands (Ethiopian type). It has characteristic vegetation, which consists mainly of deciduous woodland, with combretaceous trees, e.g. Anogeissus leiocarpus, Combretum hartmannianum, or the frankincense tree, Boswellia papyrifera, and dense thickets of the lowland bamboo Oxytenanthera abyssinica. Other vegetation types are wooded grassland, riverine forest and swamps. Potential threats to the vegetation and flora of the western escarpment are mentioned. The flora of the western escarpment consists of at least 950 species, of which at least 27 are endemic or near-endemic. Most of these occur in the Benshangul Gumuz National Regional State, a core area of the western Ethiopian escarpment. A number of new endemic species have been discovered in Benshangul Gumuz in recent years, especially petaloid monocots. Genera with western escarpment endemics include Ceropegia (Asclepiadaceae), Vernonia, Bidens and Laggera (Asteraceae), Combretum (Combretaceae), Crinum (Amaryllidaceae), Chlorophytum (Anthericaceae) and Aloe (Aloaceae). With regard to diversity the western escarpment ranks at a low position when compared with Ethiopian and Eritrean Flora regions, while with regard to number of local endemics it ranks higher than most Ethiopian and Eritrean Flora regions, but lower than Flora regions in Somalia. With regard to the relative number of local endemics the western escarpment ranks lower than nearly all Flora regions in Somalia, but higher than nearly all Ethiopian and Eritrean Flora regions. Possible reasons for the development of the endemism in a transition zone between the Nile Valley and the Ethiopian highlands are discussed. The new endemic species Chlorophytum herrmannii Nordal & Sebsebe and Chlorophytum serpens Sebsebe & Nordal from the region are described in an appendix, and the new combination Drimiopsis spicata (Baker) Sebsebe & Stedje for an endemic species is made.