East African coastal forest botany - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 75

gscaiense. These non-tree species are often
found in thickets where the canopy trees men-
ioned above are inconspicuous or absent.
However, savanna shrubs (analogous to type 4
trees, above) are not included if, as is usually true,
they die out very rapidly in closed forest. Several
Dry forest species including Asteranthe asterias,
Clerodendrum capitatum and Strychnos henningsii
are quite variable in appearance, someimes in a
way apparently correlated with habitat but often
without clearly deined subspecies or varieies.
Some Dry element species, such as Ticaysia
ovalolia, have a more disconinuous variaion
pattern. Six infraspecific variants of Dry forest
species are included the geographical elements.
The Dry coastal forests are more difficult to
compare with forests outside the Zanzibar-
Inhambane regional mosaic than is the case with
the Moist forests. In the Guineo-Congolian
region and is transiion zones small patches of
structurally similar forest of limited extent are
known. Cy nometra megalophyll-Manilkara obovata
Dy evergreen coastal forest is found in the
Sudanian transiion zone of the coastal plains of
Ghana (Hall & Swaine, 1981). Close to the Volta
lake, Ta lbotiella gentii-dominated forest (see
Swaine & Hall, 1981) is similar in physiognomy to
caesalpinioid-dominated East African coastal for-
est. Indeed . gentii is strikingly similar in its veg-
etaive appearance to Cy nometra webberi. It is
possible that other isolated pockets of Sudanian
dry forest may be similar to coastal forest as well,
but overlap in species composition is negligible.
Most Zambezian Dry evergreen forest has been
destroyed by culivaion and fire (White, 1983)
and it is not clear how similar in structure these
would be to the coastal forests.
Closer to the coast, Brachylaena huillensis,
Ae elia quanzensis, Scoroophloeus ischeri and
Manilkara sulcata are found in Somali-Masai
scrub forest (White, 1983), and some species of
the Dry Mariime-Riverine category occur there
along rivers (Tamarindus indica, Acacia robusta).
Half of the Dry forest element are coastal
endemics, and a further 10% are Easten species.
No other ecological category of forest species has
such a high proporion ofcoastal endemics except
the 'Moist-High' category. The percentage of

endemic Dry forest species is even greater if one
excludes trees of more disturbed forest (especially
type 4, above), most of which are found in the
Zambezian region.
Dry Mariime-Riverine species also show a
greater tendency to be coastal endemics than
Moist Maritime-Riverine species, although the
former more often have an oceanic distribuion.
Many species that occur in a variety of coastal
communiies are very limited in their associaion
elsewhere on the coninent, and vice versa. For
example, Asteranthe asteris and Scoroophloeus is -
cheri are common in most coastal forests, yet
Asteranthe is a genus and S. ischei is a species
endemic to the Zanzibar-Inhambane regional
mosaic. Yet the trees Ce/tis mildbraedii and
Paamacrolobium co eruleum are rare in the coastal
forests but widespread on a coninental scale.
This may arise from several factors, including the
'Jack-of-all-trades or master of one' dilemma and
the fact that widespread species are often adapted
only to a narrow band of the conditions available
locally. Gleasonian influences, which are men-
tioned below, have a separate and important
influence on this broad scale and more locally.

Geographical elements in the coastal
forest flora

Geographical elements are defined entirely within
the context of the coast. The Msangasi River is a
convenient, rather arbitrary boundary, chosen to
separate as many species of restricted distribution
as possible. Species found only north or only
south of this river constitute the Northen and
Southen elements, respecively.
Few rare species or species of restricted dis-
ribuion straddle this river. Most of these are
Mariime-Riverine species of specialised ecology,
thriving alongside Vitellariopsis kirkii in the
riverine thicket or forest ypical of the many
watercourses draining the coastal plateau into the
Pangani and adjacent rivers. Examples include:
Stuhlmannia moavi, an evergreen tree, represent-
ing a genus endemic to the coastal area; Polys-
phaeria braunii (V erdcourt, 1980) and By ttneria
fu ticosa, which have close relatives elsewhere in
the coastal forests; and Anisoyla blepharospala