East African coastal forest botany - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 78

in coninental Africa, are absent from the North-
ern coast. It is hard to envisage many of the
legumes being dispersed other than across land,
even other than within forest. The Southen ele-
ment, therefore, may include remnant species or
lineages from a dry forest or thicket belt spreading
to what is now the Guineo-Congolian or Zam-
bezian regions, but which has been almost
destroyed by fire (as discussed by Hamilton,
1976). Such species may have found a safer
refuge under the more seasonally dry condiions
of the Southen coast than further north.
Although afiniies of the Southen element
seem strongest with more central regions of con-
inental Africa, the only close relaive of the
Southern grass Hu mbertochloa greenwayi occurs on
Madagascar in steep limestone ravines by the sea
(Camus, 1961 ). The affinities of the Southen
element are as diverse as those of the Northen
element, with perhaps a greater bias away from
lineages that thrive on the coastline of coninental
Africa. This may be because of a less extensive or
less Moist limestone/coral rag forest along the
Southern coast; it may also be because of
inadequate sampling of Southen riverine vegeta-
ion, as suggested by the recent discovery of
Foetidia af ricana (an Oceanic genus) in the area.

1. Habitat availability. The pattern in the physical
environment determines the potenial range or
species and thereby creates the fundamental
trends in variaion of the forests. The coastal
landscape is complex and patchy, and so are the
coastal forests. Many species are restricted to, or
strongly favour, certain habitats; for instance, the
Northen endemics Saintpaulia rupicola and
Euphorbia wakieldii grow only on Jurassic lime-
stone outcrops. Hard sedimentary rocks support-
ing forest are more exposed towards the north,
whereas in the south, Dry forest of a variety of
types is better developed. The variation in rainfall
pattens from south to north probably has some
influence on the south-north florisic trends. The
possibility that Zanzibar steals the rain desined
for the coast and makes a barrier to the free
migration of some species has been suggested
above.
The Kimboza Forest on metamorphosed lime-
stone under a high rainfall west of the Southen
coastal forests is similar to some of the Northen
coastal forests. However, absence of the Northen
element from Kimboza Forest, despite the occur-
rence in Kimboza of close relaives of the North-
en element, suggests that environmental
determinism alone cannot explain all the obseved
patterns. If, as seems likely, isolaion has con-
ributed to differences between Kimboza and the
Northen coastal forests, then one can expect that
the isolaion between the Northern and Southen
coastal forests, which are sometimes more dispa-
rate environmentally, will also have contributed to
their differences. Similarly, Dry forest in the
North lacks Southen species. Such considera-
ions lead one to the conclusion that environmen-
tal factors do not explain all the pattens in the
coastal forests.

Discussion

The wide range of floristic associaion in the
coastal forests invites consideraion of the factors
causing such diversity. It does not seem likely that
the coastal forests have arisen simply through
fragmentaion of a previously more extensive,
more homogeneous community, even though
within certain areas this surely applies. Rather,
even before human intervenion there must have
been a patchwork of communiies, much of which
was even then not forest. The eistence i f many
non-forest species (e.g. Rothmannia ravae and
Phillipia mafiensis) which are endemic to the
mosaic supports this view.
The two main causes of variation in any com-
munity are habitat availability and 'Gleasonian' or
logistic/historical factors. Other factors, like dis-
turbance, play a subordinate role.

2. History and logisics. Isolaion and limitaions
on dispersal in a changeable and patchy environ-
ment will have contributed to the development of
different taxa and different communiies in the
Northern and Southen coastal forests, as well as
on more local levels.
T axa such as Saintpaulia rupicola and Oyanthus
py romis subsp. longitubus have apparently
diverged within blocks of coastal forest from close