Climatic history and forest distribution in eastern Africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 23

3 Climatic history and fore st
distribution in eastern Africa

JON C. LOVETT

three time periods: (i) the breakup of Gondwana-
land about 100 Myr BP (million years before
present) and the tectonic events following it; (ii)
uplift of the central African plateau during the
Miocene; and (iii) Pleistocene global climatic
fluctuaions during the last 2.3 Myr. Reviews of
the relationship between biological distribution
patterns and climaic and geological events are
given by Bonnefille (1984); Flenley (1979);
Hamilton (1976, 1982, 1988) and Wickens
(1976). The geological events are reviewed by
Griffiths (Chapter 2) and present-day climate is
described by Griffiths (1972) and Kingdon (1971)
with speciic details oneasten Africa inHamilton
(1989) and Pocs (1974, 1976).

Introducion
The African climate has been far from stable.
Equatorial rainfall is created by oceanic solar
heaing generating the intertropical convergence
zone (ITCZ), so rainfall and hence forest growth
is in part dependent on the relative position of the
solar equator. The continent was south of its
present position at the end of the Cretaceous with
the equator running through the present-day
Sahara, suggesting that the rainfall was high in
what is today a desert. The amount of rain carried
inland by the ITCZ is dependent on oceanic cur-
rents which are in turn modified by positions of
coninents; or example, Antarctica creates the
cold Benguela current which brings aridity to
southwestern Africa, and Madagascar casts a
monsoonal rainshadow on southeast Africa.
Ocean currents are also affected by periodic vari-
ations in the Earth's orbit, the Milankovitch cycle,
which is thought to be responsible for repeated
Pleistocene global cooling and warming (lmbrie &
Imbrie, 1980). Finally, rainfall on the continent is
affected by posiions of mountains and lakes
which create their own weather systems within the
overall ITCZ and oceanic current climates.
Climaic fluctuaions are partly responsible for
eastern African moist forest biota distribution pat-
terns. The three main patterns to elucidate are:
apparently ancient links to the Guineo-Congolian
forests of western and central Africa; apparently
recent links to the western Guineo-Congolian
forests; and high degree of endemism within the
eastern African forests. The climaic history of
eastern Africa can be conveniently divided into

Breakup of Gondwanaland and
northward drift of Africa
In the Triassic to early Cretaceous there were two
main land masses, a southern Gondwanaland and
a northen Laurasia. They were separated by the
Tethys sea and possessed markedly different bio-
tas. Evidence of this comes from plant fossils
(Roche, 1974) and the north-south split of coni-
fe r genera (Sneath, 1967): African examples are
Widdrintonia and Podocarpus to the south and
Juniperus in the north. Africa was south of its
present posiion in the centre of Gondwanaland
flanked by South America to the west and India
and Antarcica to the east. At this time much of
central and eastern Africa would have been dry as
it lay in the centre of Gondwanaland and it is
unlikely that it had an extensive moist forest
cover.

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