Eastern Arc moist forest flora - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 51

vegetaion (Brenan, 1978). If there had been
widespread, severe Pleistocene droughts this
south-central African area might be expected to
be poorer in species than it actually is. However,
they may be responsible or the relaive species
poverty of forests in Malawi and Kenya.
Richness of the Eastern Arc moist forest flora
within the context of African vegetaio. can be
contrasted with its poverty in comparison to some
other ropical forests. There are only two palms,
Phoenix relinata and Elaeis guineensis; and four
laurels, Beilschmiedia kweo, Cyptocaya liebertiana,
Ocotea usambarensis and 0. kenyensis. Both of these
groups are well represented in Madagascar, the
Neoropics and Far East. To some extent, Mio-
cene loss of the southen temperate moist forest
flora, and lack of a north temperate moist forest
input, explains poverty in the montane forests.
However, survival of a substanial westen
Guineo-Congolian lorisic influence indicates
that the Easten Arc has been under a moist
climate since before the creaion of the east-west
coninental divide. Either there was a loss of spe-
cies before this and they were not replaced by
subsequent speciaion, or there never were many
species. Certainly the rate of speciation varies
enormously between different plants: compare
the eplosive speciaion in Impatiens with hardly
any morphological change in westen Guineo-
Congolian taxa isolated for perhaps more than 7
million years. Whatever factors created, and are
creaing, species in the Neotropics and Far East
are largely absent from the Eastern Arc. The high
number of endemics in the Usambara and Ulu-
guru mountains, which are under a per-humid or
almost per-humid climate, perhaps gives an indi-
caion that at least one of these factors is rainall.
A similar conclusion was reached by Genry
(1989) for Neoropical forests as an altenative to
speciaion resulting from the isolaing mechanism
of postulated Pleistocene refugia (Prance, 1987).
Whatever created or destroyed species in the
past, present areas of secondary forest resulting
rom culivation are species-poor in comparison
with undisturbed forest. Montane forests on the
Uzungwa plateau, Rubeho and Ukaguru moun-
tains, which show signs of habitation in the form
of pot-shards or culivaion ridges, are composed

of widespread species with no or few Easten Arc
endemics. Historical records indicate that these
forests were in eistence at the ime of the early
German adminisraion in Tanzania, nearly 100
years ago, and presumably they were there some
ime before then. The drasic changes in
microclimate that follow forest clearance must
inhibit regeneraion of moist forest dependent
endemics and it appears that the forests take a
very long ime to recover.

Summay
The Eastern Arc mountains act as condensers of
moisture originaing in the Indian Ocean, and
have been doing so since their iniial formaion
following the breakup of Gondwanaland. The
geological and climatological definiion of the
Easten Arc as ancient crystalline mountains
under the direct climaic influence of the Indian
Ocean is mirrored by a high level of species and
generic endemism. The affiniies of the flora are
primarily with the Guineo-Congolian region of
West and Central Africa, though direct contact
westwards would have been severed following
uplift of the central African plateau at the end of
the Miocene. The most recent uplift of the moun-
tains in the early Pliocene came after the late
Miocene climatic changes in southen Africa
which resulted in extinction of the southern
temperate forests. Consequently there was not a
temperate climaic refugium in the tropics or
southen temperate forest species, so he Eastern
Arc forests do not have a significant southern
origin. Species with southern distribuions often
occur on the forest edge and have relaives in
Madagascar. Geographical separaion from north
temperate forests has limited northern affiniies of
the Eastern Arc. Further inputs to the flora have
arrived by long-distance dispersal from overseas,
by hybridisaion with woodland species, or by
adaptation of woodland species.
Forest community associations vary con-
inuously with altitude, topographic and
laitudinal variations in rainfall; division of this
continuous variaion into forest types is arbitrary.
Geographical distribution of species is similar
within each of the different forest types as defined