East African coastal forest botany - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 61

various stages of development and degradaion.
Variaion in the orest is correlated with hetero-
geneity in the physical environment. Four
idealised ransects rom west to east are given in
Figure 5.3, showing the major geomorphological
features associated with coastal forests (simplified
from Gregory, 1921; Oates, 1933; Teale, 1936;
Stockley, 1948; King, 1951; Miller, 1953;
Caswell, 1953, 1956; Thompson, 1956; Spence,
1957; Halligan, 1958; Dighton-Thomas, 1963;
McKinlay, 1963; Moore, 1963; Cooke, 1974;
Miyata & Saka, 1979). Exceping some inrusive,
volcanic peaks such as Mrima Hill in the coastal
range of Kenya, the coastal forests occur on
sedimentary rocks built up since the drift of
Madagascar away from Africa (Rust, 1973). Tilt-
ing and erosion have led to the exposure of rocks
daing rom the Karroo series in the Kenyan
coastal range, through Jurassic limestones and
Cretaceous sandstones, to Qu atenary deposits
such as the sands and raised coral reefs of the
coastal plain.
Rainall varies in monthly distribuion and total
amount throughout the coastal forests (Walter,
1952; Glover, Robinson & Henderson, 1954;
Griffiths, 1958, 1972; Walter & Lieth, 1967).
From the south to the north there is a general
increase in tendency towards two rainy and two
dry seasons, with a single dry season being
marked south of the Rufiji River. Reliability of
annual rainfall declines towards the north of the
Kenyan coast, but with anomalous high rainfall
(1085 mm) around the Witu forests near the Tana
delta (Dale, 1939; Moomaw, 1960). Average
annual rainfall is in some areas as low as about
950 mm (near Kilii, or instance). Alhough parts
of the offshore islands receive more than 1500
mm per year, no mainland coastal forest receives
more than an average of 1400 mm (1320 mm near
Tanga).
Hills atract signiicant orographic precipitaion
and support most of the moister patches of coastal
forest. The landward side of some hills (e.g.
Genda-Genda) is grassland whereas the seaward
side supports forest. In some areas, like the Pugu
Hills with an annual rainfall of 1236 mm (at
Kisarawe: Howell, 1981), the Moister forest tpes
are found in the valley bottoms. Elsewhere, for

example the Shimba Hills (with 1090 mm per
year) and the kaya forests, Moist forest is found
on higher land as well, with much of the adjacent
low-lying land supporing Dry orest.
There is a suggesion (map in Anon., 1947) of
low rainfall around Bagamyoyo (and the Msangasi
River) in the shadow of Zanzibar, which could
eplain the apparently abrupt change in the flora
across this area, described below.
Drainage, geomorpholoy and precipitaion
interact in their influence on orest ype, and
although soil type is often dependent on this com-
plex of relaionships (Milne, 194 7), soil
someimes exerts a more independent influence.
Calton, Tidbury & Walker (1955), for instance,
have suggested that past climates coninue to
exert influence on the present coastal vegetaion
by having helped create deep, heavily leached red
soils. These rather inferile, low phosphate soils
have persisted and now support coastal forests
disinct rom others, under similar condiions of
climate and geomorphology but with younger
soils. Similarly, alluvial soils, transported from
different areas, have a profound influence on tree
distribuion according to their nature and disposi-
ion, as outlined for the Tana flood plain by
Andrews et al. (1975).
The influence of rainfall alone on the vegeta-
ion is apparent from differences between patches
of forest in areas of similar geology but different
rainfall. Coral rag is a common landscape unit on
the coastal plain, very close to the sea. The forests
growing over it are very varied, with forests in
areas of greater rainall being of a Moister tpe
(Moomaw, 1960; Birch, 1963; Hall et al., 1984).
This web of physical factors has a profound
influence on the species composiion of the
coastal orests, but cannot account or all florisic
variaion. Part of the reason for this is the
influence of other environmental influences, par-
icularly the influence of humans.

Human influence

People have undoubtedly been influencing the
coastal ecology for millennia. Trade in many for-
est products, like gum copal (from Hy menaea ver-
ncosa) started even beore the arrival of he first