The tropical rain forests of Africa are divided by a corridor of arid land that runs from the Horn of Africa, through Kenya, Tanzania, Zambia, Zimbabwe and Botswana to the Namib Desert of Namibia (Werger, 1978). The arid corridor reaches the sea on the east coast of Africa in Mozambique, where Madagascar casts a rain shadow, and in the north where the corridor covers much of Somalia. A narrow strip of relatively high rainfall lies between the deserts of the Somalia coast in the north, the Madagascar rain shadow in the south and the woodland of the central African plateau to the west. It is this humid area of eastern Tanzania, Kenya and southern Somalia that we define here as eastern Africa (Figures 1.1 and 1.2).

The area of tropical rain forest in eastern Africa is not large; it is approximately 10000 km2, a mere 0.1% of the estimated 10 million km2 of tropical rain forest in the world (Mabberley, 1983). Unlike the vast west and central African forests, the forests of eastern Africa are highly fragmented – discrete islands associated with localised areas of high rainfall, surrounded by a sea of comparatively arid woodland. Contrasts between these wet and dry areas are pronounced. For example, on the eastern slopes of the Uluguru Mountains of Tanzania there is a per-humid climate where more than 100 mm of rain falls each month of the year and the annual rainfall exceeds 3000 mm.