Geology Reference
In-Depth Information
Africa plateau. The early Miocene east-west
watershed was slightly east of its present posiion
(Andrews
&
van Couvering, 1975) and the cenral
highlands of the westen rift were not formed
unil the Pliocene (Griffiths, Chapter 2). The
strong tropical Atlanic rain-bearing climate must
have penetrated across the present-day arid cor-
ridor over a relaively flat, eroded mid-Teriary
land surface. Even if rain forest did not extend
coninuously from west to east, biological contact
could have been through gallery forests. At the
end of the Miocene, after 10 Myr
BP,
the cenral
plateau was raised to its present level and major
rifts developed. After this time, access to the
Easten Arc from the western Guineo-Congolian
forests would require lowland species to disperse
over mountains and montane species to disperse
across valleys. During the Pliocene there were
warm wet periods (Lovett, Chapter 3) which
would have at least facilitated movement of readily
dispersed moist forest plants tolerant of a range of
forest ypes. Thus, although the main phytogeo-
graphic patten was established by mid-Teriary
geomorphological changes, it sill could have been
supplemented by dispersal during suitable cli-
matic condiions.
Table 4.2.
Nu mbes of species occurring in dferent
Easten Arcorest ty pes with southerly distributions.
274 sp ecies were included in the analysis, of wh ich
167 or 61
%
have southerly distributions
Number of species
a
Forest ype
2a 2b
3a 3b
4a 4b
Dry lowland
22 12 32
3
27
2
15
Lowland
17 17 33
8
32
3
21
Submontane
19 23
42
5
53 11 40
Montane
33 27
60 11 49 14 78
Upper montane 19 5
23
6
29
5
34
Dry montane
36 9
44
4
44
5
49
74
49 120 20 120 24 127
Total
No te:
a
l. South of the Limpopo River.
2a. South of the Zambezi River, but not south of the
Limpopo River.
2b. South of the Zambezi River, and also south of the
Limpopo River.
3a. South of 14° S, but not south of the Zambezi
River.
3b. South of 14° S and also south of the Zambezi River
and Limpopo River.
4a. South of the Sonwe River, but not south of 14°
s.
4b. South of the Songwe River, and also south of l 4°
S, the Zambezi River and the Limpopo River.
Affinities with the south
A high proporion of Easten Arc species have
southerly distribuions. Of the 274 ree species
considered, 167 (61 %) occur to the south,
represening 80% of species occurring outside
the Easten Arc (Table 4.2). Of the southen spe-
cies, 76% occur south of the Songwe River and
north of 14° S; 71% occur south to the Limpopo
River; but only 44% extend south of the
Limpopo. Many southern species also occur west
of the Easten Arc with 47% occurring
west of
the central rift and 34% reaching the Kivu-
Rwenzori mountains. Only 19% are confined east
of the arid corridor. Of those species which occur
south of the Limpopo River, 60% also occur west
of the cenral rift.
Few Easten Arc ree species appear to have
originated in southen Africa. Ancient southen
links are found in the predominately southen
hemisphere genus
Podocarpus,
which is represen-
ted by three species in montane and upper mon-
tane forests. More modem southen afiniies are
predominately in the dry edges of upper montane
forest:
Dais cotinifolia, Diospyros whyteana
and
Schflera umbelera
reach their northen limits in
the Uzungwa;
Ptaeroylon obliquum
has its north-
en limit on the West Usambara;
Kigelaria afi -
cana
has its northen limit on Kilimanjaro;
Aphloia
theifimnis
reaches Kenya; and
Ha lleia lucida
reaches the Hom of Africa. All of these species
occur south of the Limpopo River and do not
occur west of the arid corridor. Some of these
species have Madagasacan connecions, for
example the polymorphic monotypic genus
Aphloia
also occurs on Madagascar. The genus
Dais
has two species, one southen and
southeasten African, the other in Madagascar.
Ptaeroylon
is placed in the family Ptaeroxylaceae
by Pennington
&
Syles (1975), the only other
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