Eastern Arc moist forest flora - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 34

Forests of the Easten Arc have been collected
botanically for over 100 years (Gillett, 1961).
Hildebrandt visited the Taita Hills in 1877, and
later Holst, Engler, Buchwald, Heinse, Goetze,
Kummer, Albers, Eick and Scheffler collected in
the Tanzanian mountains between 1890 and
1900. Subsequent important collectors in the for-
ests between 1920 and 1960 included Schlieben,
Greenway, Bruce, Drummond and Hemsley,
Semsei, and Proctor. A number of types from the
early German collecions were desroyed in 1943
when the Berlin-Dahlem Botanical Museum was
bombed (Hiepko, 1987), but fortunately most had
duplicates in other herbaria. Recently, there has
been renewed interest in the forests with major
collecions being made in the Taita, Usambara,
Nguru, and Uzungwa mountains (Iversen, 1987;
Beentje, 1988a; Lovett, Bridson & Thomas,
1988; Pocs, Temu & Minja, 1990).
In this chapter a brief descripion is given of the
Easten Arc environment drawing attenion to the
cooling effect of the Indian Ocean and cool forest
microclimate. Some of the classificaion systems
that have been proposed for, or are applicable to,
the forests are then reviewed and a system pro-
posed with descripions of the various forest
types. Finally, the affiniies and origin of the
Easten Arc flora are discussed and related to
geological and climaic history.

ted in the forest by changes in species dominance
and alitudinal ranges. Mist and cloud occur
throughout the year and are an important source
of dry season moisture in higher alitude forests
where a dense epiphyic bryophyte cover (Pocs,
1976a) or microphyllous leaves (Kerfoot, 1968)
exract amospheric moisture through occult
precipitaion.
Dry season is the coolest ime of year, with
frosts occurring above 1800 m during July and
August. The upper limit of forest is determined
by regular occurrence of frosts, which is 2100-
2450 m in the Uluguru (Pocs, 1976b) and just
over 2000 m in the Usambara (Moreau, 1935).
Frosts have been recorded as low as 1500 min the
Usambara Mountains (Moreau, 1935); frost
damage in forests has been observed at 1800 m in
the Uluguru (Pocs, 1976b); and in the Uzungwa
at Mufindi a paricular thicket tpe is associated
with frost pockets at 1800 m (T. C. E. Congdon,
personal communicaion). The lapse rate is
relaively high: Pocs (l 976b) gives a igure of
0.6 °C per 100 m in alitude. Higher igures have
been recorded in the cool dry season in the north-
en Uzungwa forests with a dayime lapse rate of
1 °C per 100 m (Huddersfield New College,
unpublished data). Moreau (1935) gives a hot
season temperature maimum lapse rate of 1.7 °C
per 100 m for the Amani-Sigi scarp in the East
Usambara and 1 °C per 100 m for the West
Usambara scarp. Proimity to the Indian Ocean
has a cooling effect and temperatures in the East
Usambara are 4-5 °C lower at 700 m and 2-3 °C
lower at 500 m than those in other parts of
Tanzania at similar alitudes (Hamilton, 1989a).
The forest itself creates a remarkably cool
microclimate. Pocs (1974) showed that
temperatures within a forest on Bondwa peak in
the Uluguru Mountains differ markedly from
those outside at the hottest ime of day. The soil
surface temperature in submontane forest at 1430
m was 30 °C less than a maize field at the same
alitude; and upper montane forest at 2020 m was
20 °C cooler than open summit at 2125 m.
The forest soils are mostly weakly acidic humic
ferrisols becoming mor-like at high alitudes.
Soils in the East Usambara Mountains have been
described by Hamilton (l989b). Basic soils occur

Enironment

Rainfall and dry season length are the main cli-
maic factors determining forest limits in the
Easten Arc, with the excepion of rost which
determines the upper alitudinal limit. The forests
occur on east-facing slopes where rainfall is over
1000-1500 per year. In the northen part of

the range there are two peaks n rainfall with the
short rains during October to December and the
heavier long rains from March to May. Some
areas (notably parts of the Usambara and Uluguru
mountains) have an almost per-humid climate
with most months receiving more than 100

m

of
rain. Further south there is a marked dry season
of several months June to September) and a sin-
gle rainy season (November to May, peaking in
April). These different rainfall regimes are retlec-

m