Eastern Arc moist forest flora - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 33

4 Eastern Arc moist fore st flora

JON C. LOVETT

Direct contact between the moist forests of West
and Central Africa and those in the east would
have been completely severed in the Miocene fol-
lowing uplift of the central African plateau, and
were probably limited before then. The age and
isolaion of these mountains is relected in the
unique nature of their flora, and it is this which
led them to be classified together as the Eastern
Arc mountains (Lovett, 1985, 1988, 1990a;
Beentje, 1988a). From north to south the main
mountain blocks are: the Taita Hills of Kenya,
and the Pare, Usambara, Uluguru, Nguru,
Rubeho (Usagara), Uzungwa, and Mahenge
mountains of Tanzania. The flora of these moun-
tains is characterised by a high level ofspecies and
generic endemism in comparison with forests
associated with mountains in Kenya, Ethiopia,
Malawi, Zimbabwe and other parts of Tanzania:
25-30% of the

Inroducion
In his classic paper on the vegetation of what is
now Tanzania, the geographer Clement Gillman
(1949) recognised the importance of the disjunct
arc of mountains in the east of the country as
condensers of moisture brought inland from the
Indian Ocean. At this they are remarkably effi-
cient: on the easten side ofthe mountains rainfall
can be well over 2000 mm per year; yet in the rain
shadow only a few tens of kilometres to the west,
it can be below 500 per year. In his list of

Indian Ocean condensers, Gillman included the
northern volcanic mountains of Meru and Kili-
manjaro, and then ran the arc southwards along
the disjunct crystalline block-faulted mountains to
the Kipengere range above Lake Nyasa. Here he
pointed out that high rainfall in the great
amphitheatre of volcanic and cystalline moun-
tains surrounding the northen end ofLake Nyasa
was largely attributable to convecion from the
lake surface.
The volcanoes of Meru and Kilimanjaro are
geologically recent, having been formed within
the last million years. However, the crystalline
block-faulted mountains of the Easten Arc are
very old, with iniiaion of faulting daing from
290-180 Myr BP (million years before present)
and reacivaion of the aults creaing the moden
mountains during the last 7 Myr BP (Griffiths,
Chapter 2). Indicaions are that the Indian Ocean
climate was comparaively stable during Pleisto-
cene climatic fluctuaions (Lovett, Chapter 3). It
can be supposed, then, that moist forests on the
eastern slopes of block-faulted mountains acting
as Indian Ocean condensers have been under a
stable high rainfall through the Pleistocene, and
possibly since before the end of the Miocene.

m

2000 Eastern Arc plant species
are endemic (Lovett, 1988) with 16 genera
endemic or near endemic to the forest and forest
edge.
Easten Arc orests have a wide alitudinal
range, from 300-400 m a.s.l. at the base of the
mountains to over 2000 m at the top. Low and
medium altitude forests can be very tall with
emergents to 60 m in height, whereas high
alitude ridge top forests may have a closed
canopy only 2-3 m tall. Because of local climate
or clearance for culivation not all the mountains
have a coninuous alitudinal range of forest from
base to peak. Some forests are obviously second-
ary, with signs of culivaion and habitaion under
a currently closed canopy. Past changes that may
be related to historical human disturbance are
also indicated by moribund individuals of pioneer
species with no regeneraion in otherwise mature
forest.

c.

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