Usambaras are part of the geologically ancient
Eastern Arc Mountain chain stretching along the
East African coast from southern Tanzania to
southern Kenya. These isolated mountains may be
regarded as a continental analogy of an oceanic
archipelago. They are covered with species rich
forests containing a high proportion of endemic
plants and animals.
The East Usambaras are
situated within 100 km of the Indian Ocean and
consist of a steep escarpment rising from less
than 300 m to an altitude of 900-1050 m, where
there is a deeply dissected plateau. Up to 2000
mm of rain a year fall during two distinct rainy
seasons (March-May and October-December).
The escarpments and the plateau were formerly
covered with forest of which less than half
persist to-day. Some stands of undisturbed forest
remain containing trees
as tall as 65 m and a number of endemic species. A permanent
forest plot has been established in a portion of forest that
was lightly harvested by pit-sawyers in the 1970s. Much of
the forest has been cleared for commercial crops
(e.g. tea), forestry plantations (Maesopsis
eminii in the uplands and Tectona grandis in
the lowlands) and agriculture.
Since the 1960s the pace of deforestation has
markedly increased mainly as a result of
increased human pressure.
During the first decade of the century the
German colonial power established a major
research station at Amani which included a
substantial botanic garden (over 300 ha and
nearly 1000 exotic species were introduced).
Amani has remained, throughout this century, a
major centre for research. Since 1997 the botanic
garden is part of the Amani Nature Reserve.
Until the early 1990s much of the forest
reserves were logged. Although the Forest
Reserves and the Nature Reserve are now free from
logging, their integrity is seriously threatened
by the spread of invasive
plants. Many of these introduced species
having been introduced to the botanic garden
chiefly prior to World War I.