Climatic history and forest distribution in eastern Africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 26

and the area off the eastern African coast did not
decrease in temperature (Prell et al. , 1980).
However, a seabed core from near the mouth of
the Zambezi River showed a maimum
temperature difference of 3-4 °C between glacial
and interglacial periods during the last 135 000
years (van Campo et al. , 1990). Although the
southerly Indian Ocean monsoon was weaker
(Hamilton, 1988), coastal Tanzania and south-
eastern Kenya would have still been relaively wet
owing to rain originaing from the sill warm
Indian Ocean.
The temperature depression at about 18 000 BP
calculated from altitudes of glacial moraines on
high East African mountains is 6.7-9.5 °C and
pollen diagrams show a lowering of vegetation
zones by about 1000 m (Hamilton, 1988). Bonne-
fille, Roeland & Guiot (1990) suggest a
temperature decrease of 4 ± 2 °C with a mean
annual rainfall decrease of 30%, and date the
maimum last glacial advance to 21 500 BP when
the climate was cold but relatively moist. In
Burundi, at a site which is currently tropical for-
est, they deduce that presence of the tree-line and
alpine grassland at 2240 m indicates a lowering of
vegetaion belts by 1000-1500 m. Predictions of
temperature decreases during past glacial stages
in tropical Africa, from biotic and geomorphologi-
cal evidence, are prone to error, as the climate was
generally also much drier at the time. Rather than
there being a simple lowering of vegetaion belts,
it is more likely that there was a general restriction
in the distribution of lowland species and an
expansion in the range of species tolerant of
cooler and drier conditions (D. Taylor, in litt. ).
Ifthere had been a major depression ofvegeta-
ion types, or a much drier climate, in coastal
eastern Arica, then lower alitude wetter vegeta-
tion would have been eliminated. As the lower
alitude forest vegetation is rich in species of
restricted distribuion and isolated taxonomic
position, then the biological evidence suggests
that Indian Ocean temperatures and monsoonal
rainfall remained stable and that the lower
altitude forest zone in eastern Africa was not sub-
stantially affected by the climaic changes
observed on central African high mountains. It is

Pleistocene climaic luctuations
From the evidence of deep-sea cores it has been
suggested that there have been 21 glacials or near
glacial periods since 2.3 Myr BP (van Donk, 1976),
with the earliest fossil evidence in Africa (from
northeast Ethiopia) for a cooling and consequent
depression of vegetation belts being about 2.51-
2.35 Myr BP (Bonnefille, 1983). Climatic changes
resuling from the glacial periods in East Africa
have been reviewed by Hamilton (1982, 1988). It
is the most recent world glacial maximum, which
ended in equatorial Africa

14 000-12 000 yr BP,
that has provided the most evidence about effects
of the glacial periods on the East African climate.
This evidence has come from two main sources:
geomorphological fe atures, in particular the gla-
cial moraines on Mount Kenya, Kilimanjaro,
Rwenzori, Elgon, the Aberdares and Meru; and
fossil evidence, in the form of changes in pollen
levels and species composiion in pollen cores and
plant and animal macrofossils.
Before the advent of carbon-14 dating methods
it was thought that remains of high lake levels
indicated that the East African climate was cooler
and wetter during the glacial maimum, giving
rise to the 'pluvial theory' (Nilsson, 1931). Now it
is beyond doubt that the last ice age in East Africa
represented a period of dry cold climate. Lake
Victoria was almost non-eistent at around 14 370
BP (Kendall, 1969; Livingstone, 1980), so convec-
ional rainfall associated with it would have been
very much reduced. A drier climate also caused
water levels in Lakes Nyasa and Tanganyika to be
about 250-500 m lower before about 25 000 BP,
and the water level was low for tens of thousands
of years (Scholz & Rosendahl, 1988). Levels were
also low before about 12 000 BP (Scholz &
Rosendahl, 1988).
During the last glaciaion equatorial Atlanic
surface temperatures decreased by 4-5 °C with
greater reducion off northwest and southwest
Africa (CLIMAP, 1976; van Zinderen Bakker,
1982; Hamilton, 1988). This would have resulted
in decreased moisture coming into West and
Central Africa. In contrast, the Indian Ocean sur-
face temperatures were lowered by only 1-2 °C

c.