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freestanding massif in the world. Highlands are areas
that rise above surrounding land with a relatively
flat top; the Kenyan and Ethiopian Highlands are
examples of such features, the latter being the most
well-known in Africa (UNEP, 2014).
The mountains support a variety of ecosystems,
including Afro-alpine moorlands, forests, scrublands
and grasslands. The latter three are the most common
type of mountain ecosystems, while Afro-alpine
moorland ecosystems are only found at elevations
above 3,000 metres. Forests cover a vast area of East
Africa’smountains andahighproportionareprotected
due to their importance for biodiversity conservation
and as water catchment areas. The mountains are
home to some of the most diverse tropical montane
forests in the world, located in areas such as the
Eastern Arc Mountains, the Mau Escarpment, the
Albertine Rift, and the Eastern Highlands (Alweny
and Gatarabirwa, 2014; UNEP, 2014). These montane
forests are also the most important ‘water towers’ in
East Africa, providing water for millions of people
in highland and lowland communities as well as to
important transboundary rivers (UNEP, 2010).
Situated near or directly on the equator, East Africa is
dominated by a tropical climate, but variations occur
between locations and elevations. While vast areas
of Kenya and Ethiopia and some areas of Tanzania
are semi-arid or arid, most of the mountainous areas
have a tropical climate, which are moderated by
high elevations and mountain formations (UNEP,
2014). Although it is situated entirely within the
equatorial zone, Rwanda, for example, enjoys a cool
climate due to its high elevation – nearly all of the
country is situated over 1,000 metres above sea level
(REMA, 2011). The combination of cooler and wetter
climates in these mountainous areas, coupled with
fertile volcanic soil makes the mountain regions very
adequate for agriculture. As agriculture is the main
source of income for East Africa, the population
densities in mountainous areas are generally much
higher than in lowland areas (UNEP, 2014).
The majority of the region has two distinct rainy
seasons: the ‘long rains’ from March to May, and
the ‘short rains’, which occur sometime between
September and December, depending on the year.
The movement of the Inter-Tropical Convergence
Zone (ITCZ) over the equator is the main driver
of these seasonal rains. Climate variability in the
region is mainly caused by changes in the sea-surface
temperatures of the tropical eastern Pacific Ocean and
the Indian Ocean. The El Niño–Southern Oscillation
(ENSO), caused by changes in the eastern Pacific
Ocean, occurs roughly every fifth year resulting in
either El Niño or La Niña conditions. During El Niño,
ocean temperatures are warmer than normal and lead
to wetter conditions in East Africa, while La Niña,
which occurs when the ocean becomes cooler than
average, brings about drier conditions. Changes in the
temperature of the western Indian Ocean, known as
the Indian Ocean Dipole (IOD), have similar effects
on the region with higher sea-surface temperatures
resulting in wetter conditions over eastern and
southern Africa (Christensen et al., 2013; Shanahan et
al., 2013). The ITCZ is especially sensitive to changes
in the temperature of the western Indian Ocean,
which affects the onset as well as the duration of rainy
periods (McSweeney, New and Lizcano, 2012a).
Simien Mountains National Park, Ethiopia