Eastern Arc moist forest flora - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 47

relatively recent human disturbance as upper
montane orests on the Uzungwa and Rubeho
mountains are secondary, these having been
cleared for cultivation within the last few hundred
years (Greenway, 1973; Lovett & Congdon, 1989;
Lovett & Minja, 1990). The Usambara Moun-
tains are also topographically varied and so con-
tain more habitats than the steep, forested eastern
faces of other Eastern Arc mountains. There is
(or was) a relaively large area of the most
endemic-rich forest types in the Usambara with
undulaing plateaux at the alitude of montane
forest in the West Usambara and at the alitude of
submontane forest in the East Usambara. There
are more canopy tree niches available in the
Usambara as the forest canopy is diverse with
more ree species co-dominant than in the north-
ern Uzungwa mountains (Table 4.5). This can be
attributed to the bimodal rainy season covering
the northen Easten Arc, whereas in the south
the widespread dry season tolerant tree Pa rinari
excelsa is the most dominant species. The other
Eastern Arc mountains with a high rainfall and
short dry season are the Uluguru. Although they
are only partially explored, especially in the
southeast, and much of the lowland and submon-
tane forest on the wetter eastern side has been
cleared for culivaion, a large number of endemic
species have been recorded, especially in genera
like Impatins, Lasianthus and Polystachya. The
Uluguru also have a number of endemic genera,
not all of which are moist forest species (Table
4.6).
Eastern Arc endemic species can be divided
into palaeoendemics and neoendemics. Palaeo-
endemics are taxa which have survived in a
limited porion of their past territory, are relicts,
and rarely have close relaions in the same or
adajacent regions. Neoendemics are taxa in
rapidly evolving species complexes, which often
have their most closely related taxa in the same or
nearby regions (Rodgers & Homewood, 1982).
The origin of palaeoendemics pre-dates uplift of
the central African plateau and geomorphological
separaion of easten and westen biota, whereas
the origin ofneoendemics post-dates rejuvenaion
of the Eastern Arc faults and uplift of the moun-
tains. Neoendemics are predominately shrubs and

herbs; there are few trees. For example, each
Easten Arc mountain can have different endemic
species of Impatiens, Saintpaulia, Polystachya and
Zimmemannia, whereas palaeoendemic trees
such as Allanblackia stuhlmannii, Cephalosphaera
usambarensis, Lettowianthus stellatus and Polycerato-
caprus schfieri occur throughout the Arc.
Palaeoendemics with westen affinity are
represented in many different habits and habitats;
examples are given in Table 4.7. Despite the long
period of separation between east and west, many
eastern endemic species are so similar to their
western relaives that taxonomically they would be
better placed as subspecies or varieies. Examples
include Diospyros amaniensis which is closely
related to the widespread western Guineo-Con-
golian D. gabunensis; D. occulta which is hardly
distinguishable from D. troupinii; and Mammea
usambarensis which is closely related to M. afri- i-
cana. This has already been done or the Easten
Arc endemic subspecies of the westen Guineo-
Congolian species Pterocarpus mildbraedii subsp.
usambarensis and Greenwayodendon suaveolens
subsp. usambaricum.
Neoendemics are represented by taxonomically
difficult species complexes in a number of genera
which occur in a wide range of Eastern Arc forest
types (Table 4.8). They appear to have originated
from a number of different areas: Impatiens
entered the Easten Arc from the westen
Guineo-Congolian region by both the northern
and southen migraion routes (Grey-Wilson,
1980), whereas Saintpaulia and Zimmemannia
appear to have originated in Madagascar and then
speciated in the Easten Arc. Each Easten Arc
mountain has its own neoendemic flora, and dif-
fe rentiation has been described at a number of
taxonomic levels. For example, the Easten Arc
endemic ree Dypetes usambarica occurs from
southeast Kenya to the southen Uzungwa, and
has ive different varieies on different mountains
and in lowland and montane forests. The
relatively widespread moist forest shrubby tree
Heinsenia dieoilleoides has an Uzungwa endemic
subspecies muindinsis; and much of he variaion
in Impatiens and Saintpaulia is described at species
level.
Hybridisaion may play a role in the taxonomic