Geology Reference
In-Depth Information
Tricalysia allocalyx
(Robbrecht,
1982)
and
Xy lopia
sp. B. Most widespread Southen species are at
the northen limit of a range stretching from far
south of the Rufiji River
(47%:
ST). Some spe-
cies, like
Croton steenkampianus,
reach as ar south
as Natal. Like members of the Dry forest element
many of these are ypical of thicket or other non-
forest vegetaion. Although they often occur in
Dry forest as relics from earlier successional
stages, they are more specialised than forest pion-
eers like
Trema.
It is likely that many of these
widespread Southen species previously
flourished in a woodland, forest and thicket
mosaic (Engler's 'Sachsenwald') of a type never
well developed around the Northen forests, on or
near gently undulaing, someimes seasonally
waterlogged coastal sediments. The edges and
recently closed gaps of the forested parts of this
mosaic are today the best known habitat of such
species, but provide an uncertain refuge for them.
A few Southen species have relaives that are
more widespread in the Zanzibar-Inhambane
region, although only a few have close relaives in
the Northen forests
(Cynometra
sp. cf.
webberi
and
Tricalysia ovalifolia
(varieies) ). Southen spe-
cies, with relaives in basement complex forests
and not typical of Northen coastal forests,
include
Lasiodiscus holtzii
(related to
L. usam-
barensis)
and
Garcinia acutifolia
(closely related to
the Kimboza endemic
G. bifasciculata).
Some
pairs of Southen taxa are closely related; for
example,
Sapium tiloculare,
a Pugu Hills
endemic, is at least closely related to
S. amatum;
and
Pseudoprosopis euyphylla
is a widespread
Southen species with the Pugu Hills populaion
differeniated under subsp.
puguensis
(Brenan,
1984).
Several species have close relaives in the
Guineo-Congolian region, including
Diospyros
mafiensis
(White
&
Caveney,
1980), D. verrucosa
(White,
1955); Millettia impressa
subsp.
goetzeana
and
M. puguensis
(Gillett,
196la); Playspalum
inopinatum
(Gillett,
1960); Baphia puguensis
(Brummitt,
1968;
Soladoye,
1985);
and
Com-
bretum harrisii
(Wickens,
1976).
The genera
Tessmannia
and
Guiboutia,
found in some
Southen forests, especially the Kichi Hills (K.
Vollesen, personal communicaion) and elsewhere
Most Northen species, on the other hand, are
more isolated, geographically or taxonomically,
from their closest relaives. These include
Mkilua
ragrans
(the only species in the genus);
Savia
aenii
(although other sections of this genus
occur on Indian Ocean islands);
Aristogeitonia
monophylla,
which is related to a riverine species
in Angola (Shaw,
1945, 1971); Cola oaoloboides
(Brenan,
1978);
and
Caesalpinia alei
(Brenan,
1963).
Such distant relaionships suggest evolu-
ion of the Northen element over a long period,
although quirky long-distance dispersal followed
by rapid divergence caused by the founder effect
has possibly contributed. The evoluion of some
of the Northen Dry forest-savanna species is no
doubt best understood in terms of a history of
isolaion of vegetaion other than coastal forest,
possibly based in what is now the Somali-Masai
region.
The Southen element
By conrast with the Northen element, many spe-
cies in the Southen element occur in Dry forest,
thicket or woodland, someimes in associaions
similar to some in the North. The only exceptions
come from the Moist (or riverine) forest in or near
the Pugu Hills, but few of even these species are
confined to Moist forest.
The Pugu Hills forest is mainly on steep slopes
and ridge tops, with Moist forest in the valleys and
milder slopes (see Fig.
5.6). Dia/ium holtzii-
Baphia kirkii
forest on milder slopes,
Scorodophloeus
forest on steep slopes and forest or
thicket on some ridge tops are rich in species
endemic or nearly endemic to the Pugu Hills
(21 %
of the Southen element: SP in Appendix
5.1).
These include the small trees
Baphia pugun-
sis
and
Diospyros capriconuta,
the liane
Millettia
puguensis,
and the forest grass
Humbetochloa
greenwayi.
An unnamed
Sapium
sp. and
Tricalysia
bridsoniana
var.
panensis
are known only from
Pande forest, a rather different Dry forest nearby.
More widespread, Southen, Dry forest species
are common in the Southen forests. These
include 'Usaramo endemics'
(26%
of Southen
element: SU), unknown south of the Rufiji River
and
iel
Hills, for example
Combretum harisii,
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