Eastern Arc Mountains and Coastal Forests of Tanzania and Kenya ...

In terms of plant biogeography, the hotspot straddles two ecoregions: Eastern Arc Forest andNorthern Zanzibar-Inhambane Coastal Forest Mosaic (WWF-US 2003a, b). These twoecoregions are mostly discontinuous but do meet in the lowlands of the East Usambara,Uluguru, Nguru and Udzungwa Mountains as well as in the Mahenge Plateau (WWF-US2003a,b; Burgess pers. com.). A considerable proportion of species (e.g. nearly 60 percent ofplants) are found in both ecoregions and the distinction between them has been a matter of somedebate (Lovett et al. 2000). However, each of these forest types contains an impressive numberof strict endemics. Lovett et al. (2000) conclude that the forests in these two ecoregions are verydifferent, with differences in altitude and rainfall leading to a steep gradient of speciesreplacement with elevation.The Eastern Arc MountainsThe Eastern Arc Mountains stretch for some 900 km from the Makambako Gap, southwest ofthe Udzungwa Mountains in southern Tanzania to the Taita Hills in south-coastal Kenya (Figure2) (Lovett & Wasser 1993; GEF 2002). They comprise a chain of 12 main mountain blocks:from south to north, Mahenge, Udzungwa, Rubeho, Uluguru, Ukaguru, North and South Nguru,Nguu, East Usambara, West Usambara, North Pare, South Pare and Taita Hills. The highestpoint (Kimhandu Peak in the Ulugurus) is more than 2,600 m in altitude, but most of the rangespeak between 2,200-2,500 m (GEF 2002; WWF-US 2003a). Geologically the mountains areformed mainly from Pre-Cambrian basement rocks uplifted about 100 million years ago(Griffiths 1993). Their proximity to the Indian Ocean ensures high rainfall (3,000 mm/ year onthe eastern slopes of the Ulugurus, falling to 600 mm/year in the western rain shadow) (GEF2002). Climatic conditions are believed to have been more-or-less stable for at least the past 30million years (Axelrod & Raven 1978). The high rainfall and long-term climatic stability,together with the fragmentation of the mountain blocks, have resulted in forests that are bothancient and biologically diverse.The original forest cover (2,000 years ago) on the Eastern Arc Mountains is estimated at around23,000 km 2 , of which around 15,000 km 2 remained by 1900 and a maximum of 5,340 km 2remained by the mid-1990s (Newmark 1998; GEF 2002). At that time the Udzungwas containedthe largest area of natural forest (1,960 km 2 ), followed by the Nguru, Uluguru, Rubeho, EastUsambaras, South Pare, West Usambaras, Mahenge, Ukaguru, North Pare and Taita Hills (6km 2 ). These and the following estimates of forest status and losses in the Eastern Arc8