Eastern Arc moist forest flora - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 44

Africa and southwest Ethiopia, most are wide-
spread: 41 also occur south of the Limpopo, 31
occur in the Kivu-Rwenzori mountains and 49
occur in the westen Guineo-Congolian region.
Fewer ree species reach Kilimanjaro (92) than
he more distant Kenyan highlands (ll5). There
are a number of possible eplanaions for this:
Kilimanjaro is of relaively recent origin, with the
oldest lavas one million years old and major
volcanic mudflows occurring in the lower and
upper Pleistocene (Griffiths, Chapter 2). Alter-
naively, Easten Arc species cannot grow on
volcanic soils, or the Kilimanjaro forests may be
seconday, following culivaion on the erile,
well-watered slopes.
As with southerly afiniies, although there are a
great many Easten Arc species occurring to the
north, it appears that few of them originated
there. Herbaceous genera such as Alchemil/a and
iola, which have predominately temperate dis-
ibuions, are represented in the Easten Arc
flora by Achmilla cptantha, A. ellenbeckii, A.
johnstonii, A. kiwuensis, iola abyssinica and .
minii. All of hese species are Afroalpine plants
(Hedberg, 1957), occurring in damp upland moor
and extending only into moist forest in bamboo
thicket and disturbed areas. The Alfroalpine
region has srong norhen and souhen temper-
ate links (Hedberg, 1951, 1961, 1 965; Killick,
1978) in conrast to the largely tropical nature of
the moist forests.
Other northen elements occur in dry forests
and thickets. For example, Juniperus is a north
temperate genus of about 50 species which
survived Pliocene and Pleistocene climaic fluc-
tuaions in the easten Mediterranean (Kerfoot,
1966, 1975) and which is represented by one spe-
cies, Junipeus procera, in easten and southen
ropical Africa (Hall, 1984; Keroot, 1961, 1964,
1966). Another dry montane orest ree with
northen afiniies, Punus africana, is widespread
throughout Africa south of the Sahara. The genus
Punus contains about 400 species which are
mostly north temperate. The two African species
(one doubfully disinct) occupy a ransiional
posiion between Punus and Pygeum with Prunus
africana being closely related to the Asiaic Prunus
pygeoies (Kalkman, 1965). Another montane dry

orest temperate link is he tree Hagenia ayssinia,
a monotypic genus resricted to the East African
highlands in the subgenus Poterieae which is
predominately temperate and herbaceous (Kalk-
man, 1988).
An ancient northern link is found in Conus
volkensii (formerly in the monoypic African
Af rocrania). Co nus is a north temperate genus of
about 65 species. . volkensii and four related
species are though to be derived from the fossil
Dunstania based on fruits from England's early
Teriay (Eyde, 1988). It differs from the other
species of Co nus in being the only one with spiny
pollen, the only dioecious Conus, and belonging
to the group with red fruits, though having blue-
purple fruits. The most closely related species is
. sessilis from Califonia, and the split from other
members of Conus probably dates from the
Palaeocene or early Eocene 50-60 Myr BP.
Barriers to northen moist forest temperate
influence in the Eastern Arc flora have been enor-
mous. Africa did not complete its northward
migraion and union with Eurasia unil the middle
Miocene 17-18 Myr BP (Axelrod & Raven, 1978).
Closure of the Tethys Sea prevented the flow of
oceanic currents around northen Africa, bringing
aridiy to the Sahara and the Arabian Peninsula.
This is in marked conrast to both the Neoropics
and the Far East, where moist tropical regions are
more closely connected to moist north temperate
regions by areas of high rainfall. For example, Far
Easten montane forests have florisic ainity at
family and genus level with the warm temperate
and subtropical forests of south China and
southen Japan (himore, 1984).

Originsrom oveseas

Surprisingly few Easten Arc ree species occur
outside coninental Africa. Of the 274 trees
analysed only 24 species reach Madagascar and
nearby islands, eight extend to the Far East and
three occur in the Neoropics. The weak afiniies
with Madagascar contrast with the srong ai-
niies to the westen Guineo-Congolian region.
This demonsrates that the Mozambique Channel
is a formidable barrier to dispersal.
The ainiies that do eist between Mada-