The Eastern Arc Mountains are Pre-cambrian basement rocks, which have been faulted during the Miocene (Iversen 1991). The area consists of several mountain blocks with a high number of endemic species and genera (Burgess et al. 1998a). The more prominent rocks of the Arc are the Taita Hills (Kenya), North and South Pare Mts., East and West Usambara Mts., Nguu Mts., Nguru Mts., Ukaguru Mts., Rubeho Mts., Uluguru Mts., Malundwe Hill, Udzungwa Mts. and Mahenge Mts. (all in Tanzania). About 25% of the ca. 2,000 plant species and 66% of the 82 amphibian and reptile species occurring in the Eastern Arc are endemic (Lovett 1988; Howell 1993). The area is influenced by the Indian Ocean monsoon (Lovett 1993a) and the mountain blocks are separated by dry woodland and savannah. Marine drill-core data suggest that the coastal waters of East Africa were less influenced by Pleistocene climatic fluctuations (1-2°C) than other oceans (Prell et al. 1980). The stable temperatures of the Indian Ocean may have supported the high rainfall pattern of the Eastern Arc and the persistence of evergreen forests. The present climate of the Eastern Arc is mainly influenced by the seasonal movements of the intertropical convergence zone, which crosses the equator twice a year (Hamilton 1982). Subsequently, two rainy seasons occur in the East Usambara Mountains, while the rainy seasons merge to one long rainy season further South (e.g. in the Udzungwa Mts.). Secondly, the rainfall pattern is modified by oceanic currents and the positions of the mountains. The currents of the Indian Ocean (South Equatorial current) bring warm and humid waters to the East African coast, increasing the rainfall close to the coast and at the Eastern slopes of the mountains (Wasser and Lovett 1993). The Eastern Arc Mountains are known to receive the highest rainfall in East Africa (up to 2,000-3,000 mm per year). The temperatures are exceptionally low, with frosts occurring frequently above altitudes of ca. 2,000 m during July and August (Lovett 1998). The cool climate is mainly caused by cloudiness, which decreases the quantities of incoming radiation (Kenworthy 1966). Parts of the Usambaras and Ulugurus have an almost per-humid climate (Lovett 1993a).
The vegetation zones of the Eastern Arc mainly follow the altitudinal and moisture gradients (Lovett 1993b). The floristic variation with the altitude is continuous without any abrupt changes (Hamilton 1998). Forests naturally extend from the lowlands of the Eastern slopes up to 2,400 m above sea level. However, at the lower slopes many forests have been cleared. At low altitudes the forest vegetation is rather similar to the lowland coastal forests, which occur fragmented along the Eastern coast of Africa (Burgess et al. 1998b). The lowlands between the Southern (and inland) parts of the mountain chain are mainly covered by miombo woodland (composed of the tree genera Brachystegia, Isoberlinia and Julbernardia) and receive an annual rainfall of 750-1,000 mm with a long dry season in between (Hamilton 1982).
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