This review provides an overview on the
vegetation zonation of a large part of the mountain systems of the
The Atlas and Jebel Marra are discussed
as examples for the dry North African Mountains. The Drakensberg
range is shown as representative of the mountain systems of Southern
The main focus of the review falls on
the Afrotropical mountains: Mt. Kenya, Aberdares, Kilimanjaro, Mt.
Meru, Mt. Elgon, Mt. Cameroon, Mt. Ruwenzori, Virunga, Simien Mts.,
Bale Mts. and Imatong.
A new nomenclature for the vegetation
belts of Afrotropical Mountains is proposed.
Key words: Africa,
mountain vegetation, vegetation belts, nomenclature, Mt. Kenya,
Aberdares, Kilimanjaro, Mt. Meru, Mt. Elgon, Mt. Cameroon, Mt.
Ruwenzori, Virunga, Simien Mts., Bale Mts., Imatong, Atlas, Jebel
landforms and soil conditions
is an old continent. Nowhere else is so much of the surface covered
by basement rocks, and the larger part of the geology of Africa is
indeed the geology of pre-cambrian material (Petters 1991). Large
areas, so called cratons, have remained essentially unchanged since
the early Proterozoic (~ 2000 Mio BP). The mobile belts, i.e. the
huge basins and swells between these cratons, are composed of equally
old rocks, but have been subject to deformation and partly to
metamorphoses, mainly in the Pan-African orogenesis (700 - 500 Mio
years BP). Today, relief differences are relatively small in these
areas, so the continent can be described as a large, uneven plateau,
which is tilted to the northwest. Extensive erosion surfaces covered
by ancient, heavily leached soils are more widespread in Africa than
on any other continent, and the huge relatively flat Savannahs are
well known. Inselbergs are very characteristic remnants of the older
surfaces in these plains, but far too low to merit treatment here.
well known as the flat surfaces are, however, some much more recent
geological features, namely the huge, isolated volcanoes of the
tropical part of the country, and the high fault mountains in the
Atlas region and in South Africa. In these three regions, true
high-altitude environments are found, with several ranges reaching
well above 4000 m. These are almost exclusively located in the
tropical parts, with Mt. Kilimanjaro (5985 m), Mt. Kenya (5198 m)
and Ruwenzori (5109 m) being the highest sites. Several other
mountains reach above the treeline, with the most extensive areas
being in Ethiopia. However, the High Atlas in Morocco is also high
enough to support truly alpine vegetation (4165 m).
occupies an intermediate position between the tectonic domains of
southern Europe and Africa. The evolution is complex, and the atlas
region was formed in several successive uplifting events. The
south-westernmost part, the Anti-Atlas contains Precambrian material,
but was largely formed in the late Proterozoic. Uplift occurred in
Hercynian times in close correspondence to some mountains in Europe,
and again mainly in late Cretaceous tectonic movements that formed
the several chains of the High Atlas and the Saharan Atlas. The
northern Riff and the Tell Atlas in northern Algeria are the
southernmost outposts of the Alpine orogen, but recent Tertiary
uplift affected also the older ranges, notably the Saharan Atlas.
Most of the material is acidic, but several ranges display calcareous
rocks, like the central High Atlas, part of the Saharan Atlas and the
Rif. Only the High Atlas reaches above 3000 m, while the neighboring
ranges hardly climb above 2000 m.
central Saharan mountains Hoggar and Tibesti rest on the relatively
stable West African craton. Part of the Tibesti is of ancient,
Proterozoic origin, but there has been extensive recent volcanism.
Both ranges are part of a chain of volcanoes of Cenozoic age that
extends southwards to the central African Mt. Cameroon and eastwards
to the East African Rift Valley System.
Cameroon is a huge single structure of largely Tertiary origin, but
has been active ever since with more than 10 major events in the 20th
century, some as recent as 1982 and 2000 (Ndam et al. 2001). The
series of volcanoes starting at Mt. Cameroon has been described to
continue up to the Jebel Marra in central Sudan. This is a massif
rather than a single volcano, which has been active from the Miocene
to the early Holocene (Miehe 1988). Lavas are predominantly basaltic
in all these ranges.
eastwards, the huge volcanic plateau of Ethiopia rests on ancient
material, but was largely formed in the tectonic processes during the
formation of the East African Rift Valley system. The EARS is the
prime example for a continental rift, and its huge structure extends
over more than 6000 km from northern Mozambique along all of eastern
Africa and through the Red Sea. Rifting started in the Cretaceous but
has intensified in the Tertiary, and is most likely a consequence of
tensional forces driving the two escarpments apart. Volcanism has
been intensive and complex, following the two main branches, the
Albertine Rift in the west, and the Gregory Rift in the east
Ethiopian Rift is an extension of the latter, mainly formed by
basaltic magmas in the Oligocene to Miocene, and reaching above 4000
m at several sites, notably in Simen Mts. in the northern part of
Ethiopia and the Bale Mts. in the southwestern. Relatively deep and
dry basins from the isolated volcanoes in east Africa proper separate
this group of well-connected ranges.
of these are relatively young and of Pliocene to Quaternary age. This
refers to mountains such as the Aberdares, which were formed from 3
Mio. BP onwards; Mt. Kenya was formed between 2-3 Mio. years BP
(Heinrichs 2001); Mt. Kilimanjaro is even younger at 1-2 and Mt. Meru
appears to be of similar age as are the Aberdare Mts. Lavas on the
huge shields are predominantly alkaline or basaltic, but intermediate
lavas formed several strato-vulcanoes.
mineral composition is similar for Mt. Elgon, which is however an
exception with respect to its age. Mt. Elgon lies at the border of
Uganda and Kenya, and is thus not strictly located in the Rift
Valley. Here volcanism commenced some 20 Mio years ago and lasted for
about 10 Mio years. The neighboring Cherangani range is an ancient
block of largely metamorphic rocks, but has been lifted to its
present height in the Miocene. These two ranges have been high
mountains, long before their neighbors in the eastern part of Africa,
and also those in central Africa had attained high altitudes.
Ruwenzori is a huge pre-Cambrian block, but has been lifted to alpine
heights later than 2 Mio years BP. The Kahuzi-Biéga west of
Lake Kivu has apparently experienced a similar evolution (Fischer
1996). The other high mountains along the Albertine Rift Valley are
exclusively of volcanic origin, with volcanism commencing as early as
some 13 Mio years BP in the Virungas. However, most of the activity
occurred in the last 2 Mio. years and continued to the present, as
was so strikingly demonstrated at the city of Goma, which was largely
destroyed during a catastrophic lava outpour in 2001.
young mountains are in striking contrast to the truly ancient Eastern
Arc Mountains in Tanzania and northern Kenya. They are composed of
granulite gneiss complexes and have been mountains during for over 25
Mio years. However, they barely reach 3000 m and lack an alpine belt
typical for the other mountains discussed here.
during the formation of the Rift Valley has formed much of the
present tropical part of African. Beyond Mozambique, mountains in the
southernmost part of the continent are fault mountains like the
northwestern Atlas system
slopes of the volcanic ranges have been subject to intensive
weathering and leaching, resulting in deep reddish soil profiles,
mainly Acri- and Nitisols, occasionally Ferralsols. The dominant soil
type of the upper montane belt are however well developed Andosols.
The Andosols have moderately acidic soil pH values, and are
exceedingly fertile and explain the intensive agriculture on the
tropical mountains. The alpine regions are partly covered by
Andosols, notably on the younger volcanoes, partly by various weakly
developed soils on glacial deposits such as Regosols and Rankers.
Very widespread are various Histosols that result from slow
decomposition of organic matter due to coldness and stagnant water.
They are characterized by low pH values (under 4 to 5.5) and generally
low nutrient availability.
Altitudinal belts in the African
Vegetation belts in the North African
respect to their geologic origin, mountains in northwest Africa are
closely related to European mountains like the Alps. Correspondingly,
climate, flora and vegetation display strong similarities to
Mediterranean mountains like the Sierra Nevada in Spain, and mediate
to the arid sites of central northern Africa.
are three principal complexes, namely the High Atlas and its
neighbors Anti-Atlas and Central Atlas; the Rif and Tell Atlas in
northern Morocco and Algeria; and the Saharan Atlas in Algeria and
Tunisia. These display large-scale climatic gradients, with
conditions getting progressively drier with distance to the coast, so
the Rif and Tell Atlas get most precipitation with annual totals
above 750 mm in the northern foot zone. In the Moroccan Atlas,
conditions get drier from west to east. These differences among
ranges are small in comparison to the large differences between the
northern and the south (-eastern) slopes within the mountain ranges.
The northern lowlands receive well above 300 mm, while the southern
slopes border the Saharan basin, where precipitation quickly drops to
less than 100 mm in the lowlands (Weischet & Endlicher 2000).
regions clearly receive more than lowlands with a precipitation
maximum at 2000 - 3000 m elevation. This is around 750 mm in the
High Atlas, and above 500 mm in all regions except for parts of the
Saharan Atlas. The Rif and the Tell Atlas receive annual totals of
above 1000 mm, which rise occasionally up to 2000 mm. Rainfalls occur
almost exclusively in winter, so the summit regions are at least
seasonally covered by snow, which might last until the following year
in moist conditions.
moisture availability and the usual altitudinal temperature lapse
rate control most of the vegetation patterns, but these are modified
by edaphic components. Like in the Alps, calcareous and acidic
bedrocks intersect, so vicariating plant communities are found
depending on the geological background. This analogy is however
limited, since the extremely harsh climatic conditions with a
pronounced seasonality and the general dryness of the summit region
induce excessive weathering. Thus the uppermost regions are covered
with scree, which move in the frequent freeze-thaw cycles. Soils are
weakly developed and conditions resemble those of dry ranges in the
Sahara or even Central Asia, with an open and rather sparse
vegetation cover completely different from the Alps.
impact has been tremendous in the entire region. Agriculture and
livestock keeping were introduced some 5000 BP, and, like the entire
Mediterranean region, most of the natural vegetation has been
replaced by various secondary plant communities, rendering inferences
of the potential character of the landscapes often different.
The Moroccan Atlas
exploration the area started equally late as in the seemingly much
more remote tropical parts of Africa and started in the 1870's. It
has however been intense and the general patterns are known since the
middle of the last century (Rauh 1952). The High Atlas displays the
typical situation. The northern foot zones are covered by open
shrublands with Zizyphus ssp., Lotus and notably Acacia
gummifera up to 900 m, where the dwarf palm Chamaerops humilis
demarcates the transition to forest vegetation. Tetraclinis
articulata forests are the natural community up to 1400 m, from
where they are replaced by various communities dominated by Quercus
ilex. Stands are dense and relatively shady with few companions
as Cistus laurifolius on acidic, and Buxus sempervirens
on calcareous bedrocks. Much of these forests have been replaced by
various scrub communities with Pistacia lentiscus and
Juniperus phoenicea in the lower regions, and by J.
thurifera in the upper montane belt. Impressive Cedar (Cedrus
atlantica) stands built a special forest community restricted to
the moistest parts of the Atlas, but are more extensive in the Rif
and in the Tell Atlas. Above 2800 m, J. thurifera forests
dominate up to the treeline in some 3100 m. Unlike in other areas of
Africa, progressively diminishing and finally shrubby growth of the
dominant species do apparently not characterize the treeline ecotone.
Instead, Junipers retain their single-stemmed growth up to the
treeline, with stands opening progressively and distance between
cushion plants characterize the lower alpine region up to 3600 m.
Cushions grow 0.5 to 1.0 m in height and have thick, often several
meters long root. The dominant species are the widespread Alyssum
spinosum, Bupleurum spinosum, joint by the flat thorny
cushions of Arenaria pungens in upper elevations. In contrast,
Erinacea pungens and Vellea mairei are restricted to
calcareous bedrocks. Cushions prefer weakly inclined to level slopes
where they can built almost closed stands, but have also benefited
from anthropo-zoogenic human impact on the upper montane Juniper
forest, where they from secondary stands on disturbed sites.
sites in the lower alpine zone near small brooks are the habitat of
communities called "Pozzines". These are meadows,
physiognomically similar to alpine meadows in Europe, and thus
colorfully flowering in summer. However, they are of very limited
spatial extent, and are restricted to soils with impeded drainage
over acidic bedrocks.
movement in the upper alpine region becomes increasingly strong above
3300 to 3600. The cushions become more and more restricted to the few
level sites, and plants truly adapted to scree slopes and rock
crevices take over. Again, we find vicariant taxa on acidic and
calcareous substrates, like Viola dyris var. orientalis on
calcareous rocks and Viola dyris var. typica on acidic
bedrocks. The latter is an endemic taxon of the Atlas, other examples
are Linaria lurida, L. heterophylla ssp. galioides
and Vicia glauca var. rerayenssis, all growing on
scree slopes. The relative importance of endemic species increases
with altitude in the Atlas, and is around 30% in the lower alpine
zone, while it reaches some 75% in the upper alpine and summit
regions. There is no truly nival belt and plants occur up to the
zonation on the southern slopes is somewhat different because of the
greater aridity. The foot zone is covered by succulent Euphorbia
species or open shrublands like on the northern slopes up to
1500. Juniperus phoenicea partly replaces Tetraclinis
articulata and the belt of Quercus ilex forests is smaller
and widely replaced by open Juniper scrub. Cedrus forests are
entirely absent. The timberline is formed by J. thurifera as
on the northern slopes, and the alpine vegetation is similar as well.
The Rif and the Tell Atlas
Rif reaches barely 2500 m and the Tell Atlas is even lower, but
affinities to the Moroccan Atlas are strong in the montane belt
(Knapp 1973), so the description is kept short. The most widespread
montane forest communities are also built by Quercus ilex,
which forms dense and rather shady stands. Quercus suber forms
extensive forests at the relatively moist coast of Algeria and
Tunisia up to some 1000 m asl., but small stands of the Oak are also
found in Morocco. Like Quercus coccifera, it has rather high
moisture demands and is restricted to the coastal regions below 1200
m. Much of the areas have been cleared for agriculture, especially
Olea europea plantations. The olive forms (semi-)natural
stands with Pistacia lentiscus in drier parts of the lowlands,
which might however be secondary in nature. Aleppo Pine (Pinus
halepenis), often mixed with Quercus ilex, dominates much
of the lower montane zone at 500 - 1500 m in the Rif and Tell
Atlas, but is also occasionally found in the Moroccan Atlas. Cedrus
atlantica forest is common in the moister ranges above 1500 m.
However, the moistest sites there are covered by deciduous oak
forests (e.g. Q. faginea) and coniferous forests with Abies
maroccana and A. numidica. Juniperus phoenicea
forests in the drier parts of the montane belt have suffered heavily
from anthropo-zoogenic disturbance but occur up to 2400 m under
natural conditions. Replacement communities include mixed scrub with
various Cistaceae, Fabaceae and Ericaceae species in the lower
montane belt, and shrublands with the dwarf palm Chamaerops
humilis and Rosmarinus officinalis in the montane belt.
no truly alpine vegetation in the Rif and in the Tell Atlas simply
because the ranges are too low.
The Saharan Atlas in northern
Saharan Atlas forms a second chain leeward of the Tell Atlas and is
thus relatively dry. Much of the natural vegetation has been replaced
by widespread steppes with Stipa tenacissima, that is
an important species of secondary grasslands in the western
Mediterranean region. It is accompanied by Lygeum spartum or
Artemisia herbae-alba, mostly on dense or compacted soils. The
natural montane vegetation closely resembles the situation in the
Moroccan Atlas described above, notably in the western part of the
country near the inter-state boundary. Communities with higher
moisture demands are almost exclusively absent, and forest with
Quercus ilex and occasionally Pinus halepensis would be
the zonal vegetation of much of the northern slopes. The southern
slopes are influenced by the arid Saharan climate and dry vegetation
types with succulents climb up well into the montane belt. Where the
mountains are high enough, alpine vegetation types occur, which are
also similar to the High Atlas. Thorny cushions are relatively
common, as are scree communities, but communities of moist rock
crevices are impoverished and Pozzines are entirely absent. The
alpine vegetation shows the typical differentiation of vicariating
species depending on the presence of acidic or calcareous bedrocks as
was described for the High Atlas.
Dry mountains in central Sudan - the
Jebel Marra is a highly isolated volcanic massif in the center of
Africa, more than 1500 km away from the nearest coast and more than
1000 km distant to the next mountain, with the Simen Mts. in northern
Ethiopia being the most "closely" related range. The Jebel
Marra emerges from a pedi-plain in 1000-1200 m, with a semi-arid
climate with down to 280 mm of annual precipitation (Miehe 1988). The
mountain slopes receive more rain, and the southwestern side is
clearly moister than the eastern. The belt of maximum precipitation
is situated at around 2000 m asl., where totals can be as high as
1800 mm in extreme years, but 800 mm appear to reflect the average
conditions better. For the summit region (max. 2976 m Times Atlas
3071) 600 mm have been estimated.
vegetation shows correspondingly clear altitudinal gradients. Drought
deciduous woodlands cover the piedmont and Acacia scrub,
interspersed with lowland gallery forests along watercourses and in
moist ravines and valleys, mainly in the southwest. These can be
regarded as remote outposts of tropical evergreen forest. Typical
species include evergreen Ficus ingens, F. thonningii,
F. sur, Syzygium guineense and Trema orientalis,
indicating affinities to tropical montane forest described below..
dominant vegetation type in the submontane belt are, however,
communities with Combretum molle and Boswellia papyrifera,
which are replaced by wooded grasslands with Entada abyssinica
and Acacia albida as a transitional belt to the montane
transition from these submontane lowlands to the montane, sparsely
wooded grasslands occurs at some 2000 m. The principal tree species
is Olea laperrinei, which forms riverine forests above 1900 m
with Ficus palmata. The olive is also found in the Saharan
mountains and indicates the relationships to Hoggar and Tibesti.
However, the dominant vegetation are grasslands with
Andropogon-Hyparrhenia, and Themeda triandra-Conyza
hochstetteri. Scattered trees of O. laperrinei can be
found on scree slopes and in the grasslands, indicating that at least
part of these grasslands are suitable for tree growth. The upper
montane grasslands above 2800 m are composed of Pentaschistis
species and Festuca abyssinica. Occasionally, remnants of
ericaceous vegetation, notably E. arborea, occur in the upper
montane region. Ericaceous vegetation dominates the timberline
ecotone in much of tropical Africa, but the Jebel Marra is not high
enough for a true alpine belt.
with respect to the temperature and moisture availability most of the
montane belt should be suitable for dry afromontane forests dominated
by O. laperrinei, which have presumably been cleared by
large-scale woodcutting, grazing and fire. This would clearly
indicate parallels to the tropical mountains described in the
Vegetation belts in the tropical
African mountain regions
Mount Kenya - A typical example for
the altitudinal zonation of tropical African mountain vegetation
(Figure 1 a & b) is intermediate among the mountains in tropical Africa in
terms of geographical position and climate. Research there has
started early in the last century, so the available data are
relatively comprehensive. Hence, we will discuss the vegetation
zonation of Mt. Kenya in some detail, while descriptions for the
other mountains are kept shorter and focus on the differences to the
typical situation in central Kenya.
Kenya, located about 180 km north of Nairobi on the eastern side of
the Great Rift Valley, is a widely isolated volcano. The base
diameter is about 120 km; the northern foothills reach the equator.
With its main peaks Batian (5198 m) and Nelion (5188 m), Mt. Kenya
is, after Kilimanjaro (Kibo 5899 m), the second highest mountain in
Africa. It is broadly cone-shaped with deeply incised, in the upper
parts U-shaped valleys, indicating extensive former glaciations. Most
parts of Mt. Kenya are still covered with forests up to approximately
3400 m in the South and 3000 m in the North, where a sharp boundary
separates the forest from the lower alpine zone. Shape and location
of the upper treeline has been severely influenced by fires. The
present lower timberline is a result of extensive forest clearance
and agriculture with cultivation reaching up to 1800 m on the
southern, up to 2400 m on the eastern and western, and nearly up to
2900 m on the northern slopes. Without human impact, the mountain
would be almost completely surrounded by dense forests.
an important water catchment, Mt. Kenya contributes mainly to the
Ewaso Nyiro and Tana River drainage systems, and is therefore of
outmost importance for the water supply of about 50 % of the Kenyan
population. Moreover, since other power stations did not fulfill the
expectations hydropower from these rivers continues to generate more
than half of the country's production of electrical energy (Berger
1985; Decurtins 1985, 1992; Leibundgut 1986). Densely populated
farming areas surround the whole mountain, and large parts of the
forest suffered heavily from encroachment in the last decades. The
vertical zonation of the mountain has already been described in the
early 20th century (Engler 1895; Allaud & Jeannel
1914), but the first comprehensive accounts are from the 1950s
Fig. 1a. Vegetation zonation on Mt. Kenya, West-East Transect
Fig. 1b. Vegetation zonation on Mt. Kenya, North-South Transect
Supratropical evergreen montane
lowermost areas on the eastern and northeastern slopes from 1200-1500
m are covered with evergreen mountain forests, which closely resemble
forest types of central and western tropical Africa. Newtonia
buchananii, Lovoa swynnertonii and Chrysophyllum gorungosanum
are prominent tree species. These have survived only in
comparatively small remnants, while the principal forest formation
are typical Camphor forests, with Ocotea usambarensis, Aningeria
adolfi-friederici and Syzygium guineensis being the most
important canopy species. These evergreen submontane forests
(Bussmann 1994, 2001, Bussmann & Beck 1995a,b, 1999) are
two-storied, with huge specimens of the African Camphor tree, Ocotea
usambarensis in the upper canopy, and Xymalos monospora,
Lasianthus kilimandscharicus, Pauridiantha holstii and
Psychotria orophila in the lower tree layer and the
often-dense shrub stratum. Further characteristic species in the
upper tree layer are Strombosia scheffleri and Apodytes
dimidiata in the upper tree layer, while Tabernaemontana
stapfiana, Ochna insculpta, Macaranga kilimandscharica
and Peddiea fischeri form low trees and shrubs. Asplenium
sandersonii, A. elliottii and Panicum calvum occur
in the herb layer, together with Piper capense, Oplismenus
hirtellus, Plectranthus luteus, Begonia meyeri-johannis
and the ferns Dryopteris kilemensis and Blotiella
stipitata. Cyphostemma kilimandscharicum is a common
climber, and abundant fern epiphytes include Elaphoglossum lastii,
Trichomanes borbonica, Asplenium theciferum and
the lowermost eastern slopes of Mt. Kenya, forests with only one
closed tree layer are found, dominated by Newtonia buchananii
and many, often very tall palms (Phoenix reclinata),
which give these areas an appearance like an Andean cloud forest.
Once, these forests covered large tracts of land especially on the
eastern slopes of Mt. Kenya, and formed the lowermost submontane
forest belt in transition to the savannah lowlands, but these forests
have long ago been cleared for cultivation.
western side of Mt. Kenya, evergreen submontane semi-deciduous
forests, where drought resistant species like Calodendron capense or
Croton megalocarpus are common, follow woodlands dominated by Acacia
drepanolobium from 2000-2300 m.
the northern part of Mt. Kenya, the woodlands lead directly into very
drought-resistant xeromorphic forests, which are almost entirely
dominated by Pencil Cedar (Juniperus procera) and Wild Olive
(Olea europaea subsp. africana). A Bamboo belt is
absent there, and even the narrow cloud forest belt is heavily
interspersed with Cedar. Several authors mentioned a so-called
"forest gap" on the drier northern slopes of the mountain
(e.g. Hutchins 1909; White 1950). Whether this gap is natural or man
made has been a matter of debate. According to the statements of
long-term residents of the area, clearing the forest for farming
purposes has widened the gap. In the late 1970's, the Kenyan
government for farmland excised some areas of the Mt. Kenya Forest
Reserve. In many smallholder fields (shambas) of this area, forest
trees or remnants of trees are still present, indicating clearly the
former extension of the forest belt. Climatic conditions are less
probable to be responsible for the gap since in an even drier area
exactly north of the gap on Mount Kenya, the Ngare Ndare forest with
extensive stands of Juniperus procera is found. Therefore,
there are no reasons to assume a natural reason for the existence of
the forest gap. Most probably, a formerly closed forest belt must be
forests and a variety of very different forests can be also found,
indicating affinities to the mountain forest communities of Central
and Western Africa. Lovoa swynnertonii, a very tall and
dominant canopy tree is a typical species, growing together with the
shrubs Rawsonia lucida, and with Heinsenia
diervilleoides and Rinorea convallarioides. In
many places Uvariodendron anisatum, with its beautiful
and very fragrant white leaves, grows abundantly in the lower tree
stratum and in the shrub layer.
Camphor forests are found mainly on the very wet southeastern and
southern slopes of Mt. Kenya on altitudes between 1550-2550 m. They
grow on humic Niti- and Acrisols and receive an annual rainfall of
1500-2500 mm. Moist forests in the lower and middle submontane region
of southeastern Mt. Kenya are dominated by the evergreen species
Syzygium guineense (Myrtaceae) and Aningeria
adolfi-friederici (Sapotaceae), reaching a height of up to 50 m.
Furthermore, the shrubs Drypetes gerrardii (Euphorbiaceae) and
Allophyllus cuneatus (Sapindaceae), as well as the lianas
Adenia gummifera (Passifloraceae) and Jaundea pinnata
(Connaraceae) are characteristic species. The number of
epiphytic mosses is limited, but a wide variety of epiphyllous and
epiphytic liverworts and lichens occur.
stands suffered from heavy exploitation, and have been replaced by
secondary vegetation types, leaving untouched evergreen forests only
in steep ravines or in remote areas, which are difficult to access.
Macaranga kilimandscharica dominates the canopy of these
secondary forests, and often forms dense pure stands. It is a very
fast growing species suppressing the regeneration of other trees.
Orotropical montane forest - Evergreen
broad-leaved and evergreen xeromorphic montane forests
cedar forests (Juniperus procera) grow on humic Acrisols
(Speck 1986) at rainfall totals between 700 and 1500 mm. They
represent the typical vegetation of the altitudinal range between
2500 and 2950 m of the drier exposures (Bussmann 1994, 2001, Bussmann
& Beck 1995a, 1999). Evergreen-broadleaved forests cover the
moister southern and southeastern slopes, at lower altitudes between
2150-2650 m. Juniperus procera itself is rare or completely
absent, due to heavy logging and suppressed regeneration in the
closed stands. The dominant tree species in these two-storied stands
are Cassipourea malosana (Rhizophoraceae) and Olea capensis
subsp. hochstetteri, the East African Olive; both forming the
upper canopy. Lepidotrichilia volkensii in the lower tree and
shrub strata, and Ilex mitis, the African Holly are further
important woody species.
forest, in contrast, is rather open and one or two-storied, with J.
procera growing about 50 m tall under favorable conditions. Other
important tree species, mainly of the lower canopy, are the Wild
Olive (Olea europaea subsp. africana) and Podocarpus
latifolius. Mosaics of dense grass layers with Stipa dregeana
and Brachypodium flexum, interspersed with herbs (Sanicula
elata, Isoglossa gregorii and Achyranthes aspera),
and low shrubs (e. g. Berberis holstii) are also
characteristic of these open cedar forests.
Orotropical Bamboo forest
forests dominated by the African Bamboo (Sinarundinaria alpina)
which are found on various East African mountains, constitute a very
distinct formation (Bussmann 1994, 1997, Bussmann & Beck 1995a,
1999, Bussmann 2001, Bytebier & Bussmann, 2000). Sinarundinaria
alpina is the overwhelmingly dominant species in all strata, but
Impatiens hoehnelii, Pseudocarum eminii as well as
Selaginella kraussiana, Cyperus dereilema, and
Anthriscus sylvestris are common companions. Bamboo forests
are restricted to a relatively narrow ecological range, mainly
depending on soil temperatures of 10-15 °C and the presence of
very deep volcanic soils, namely humic Andosols. On Mt. Kenya, they
are found in an almost closed belt around the entire mountain,
interrupted only on the dry northern slopes. Stands are somewhat more
extensive on the very wet southern parts.
steep slopes of the western, northwestern and eastern side of Mt.
Kenya, in areas with lower rainfall and higher soil temperatures,
often huge individual trees protrude from the dense Bamboo layer in
some 10-15 m height. The dominant species of the tree stratum is
always Podocarpus latifolius, which is also common in the
surrounding montane forest. Towards the hygric limits of the Bamboo,
mainly on the dry northwestern parts of the mountain, Juniperus
procera and Olea europaea subsp. africana are found
as characteristic members of the tree stratum. Trails and heavily
trampled resting places of big game occur everywhere in these
forests, and seem to facilitate regeneration of trees in the
otherwise extremely shady stands of Sinarundinaria alpina.
Orotropical cloud forest
forest types, namely tall and malacophyllous forest with Hagenia
abyssinica occupy this vegetation zone, and lower stands built by
various species with sclerophyllous, "ericoid" leaves. The
so called "ericaceous belt" forms the transitions zone
towards truly afroalpine vegetation within the timberline ecotone. In
contrast, the malacophyllous forests extend from 2900 to 3300 m
a.s.l., where low clouds and mist are frequent. The dominant tree is
the Kosso tree, Hagenia abyssinica (Rosaceae), often
accompanied by the Giant St. John's Worth, Hypericum revolutum.
Bothriocline fusca is a common shrub, while Polygonum
afromontanum, Stephania abyssinica, Cinereria deltoidea
and Carduus afromontanus are common herbs. Pure
Hagenia-Hypericum forests with a low, dense shrub layer of
Hypericum are characteristic for the wetter southeastern to
western slopes of Mt. Kenya. Here, important herbaceous companions
are Luzula johnstonii, the large Lobelia bambuseti, Rubus
frieseorum, Agrostis schimperiana, Poa schimperiana,
Helictotrichon milanjeanum, and one of the few
succulents, Uebelinia rotundifolia.
the drier northern part of Mt. Kenya, the Hagenia-dominated
forests are replaced by mixed forests with Podocarpus
gracilior and broad-leaved species, clearly differentiated by
the high frequency of Juniperus procera, Olea europaea,
Nuxia congesta and especially Rapanea melanophloeos.
Hagenia abyssinica is only a co-dominant canopy species and
Hypericum revolutum is mostly absent.
some 3300 m upwards, open ericaceous communities gradually replace
Hagenia forests. They grow at the upper limit of forests, but the
timberline has been lowered by several hundred meters due to fires
and undulates now at 3100 to 3300 m. True Erica excelsa
forests are poorly developed on Mt. Kenya, and most often the
ericaceous belt is formed by remnant stands of ericaceous scrub with
Erica trimera (formerly Philippia, Oliver 1987), E.
arborea, Conyza vernonioides, and the astonishingly
"ericoid" Asteraceae Stoebe kilimandscharica (Rehder
et al. 1988). The latter, together with Protea kilimanscharica
often indicates regular disturbance by high altitude fires. Fires
are usually discontinuous, so the scrub is interspersed with various
grassland types that are already afroalpine. Thus, ericaceous and
afroalpine communities generally form patchy mosaics and transition
communities rather than clear altitudinal belts.
Altotropical grasslands and
tropical Africa, the altitudinal belt above the timberline has been
termed "afroalpine" (Hauman 1955). Communities intersect
with ericaceous vegetation, and are mainly dominated by tussock
grasses and stands of Giant Rosette Plants. Fortunately, a vegetation
map is available, so the spatial distribution of communities is well
known (Rehder et al. 1988; Beck et al. 1990). "Open Moorlands"
are particularly extensive on the western slopes of Mt. Kenya at
altitudes from 3400-3800 m, often found at clearings of ericaceous
vegetation. The dominant species are grasses, namely the large sedge
Carex monostachya, interspersed with tussocks of Festuca
pilgeri in drier places. Most characteristic are the large,
cabbage-like ground rosettes of Dendrosenecio brassica and
Lobelia keniensis. The Giant Rosette "tree"
Dendrosenecio johnstonii subsp. battiscombei is common
in the lower moorlands as well as in remnant ericaceous stands.
"Upper Moorland Zone", extending from about 3800 m upwards
as high as 4550 m comprises two very characteristic vegetation units.
Where moisture availability is sufficient, "Upper Alpine
Wetlands" are formed by Dendrosenecio brassica and
Lobelia deckenii ssp. keniensis. In contrast to lower
altitudes, sclerophyllous dwarf shrubs like Alchemilla johnstonii,
Alchemilla argyrophylla subsp. argyrophylla and Helichrysum brownei
often replace grasses. Sedges are less important than in the
typical plant community above 4300 m is the "Dendrosenecio
woodland", in which D. brassica is replaced by the Giant
Groundsel, Dendrosenecio keniodendron, growing together with
Lobelia telekii. From 4500 m onwards, the Giant Groundsels
recede, and an open layer of Festuca pilgeri, interspersed
with Lobelia telekii, Festuca abyssinica, Carduus chamaecephalus
and Senecio keniophytum extends to the subnival zone.
Occasional specimens of vascular plants where found up to the summit
region in above 5000 m (Rehder et al. 1988). In contrast to the
situation on Kilimanjaro, where low precipitation creates a real
alpine desert, closed vegetation on Mt. Kenya is more limited due to
freezing and continuous solifluction, although moisture availability
probably has some influence (Beck 1994).
Mt. Kenya's neighbors: Mt. Kilimanjaro,
Mt. Meru, Aberdare Mts. and Mt. Elgon
they are separated by hundreds of kilometers of lowlands, the
neighboring mountains show a vegetation zonation very similar to Mt.
Kenya. These striking similarities have long been known to
phytogeographers (Hedberg 1951; Hedberg 1964), and have let to the
general classification scheme that provided the base for the previous
chapter. The vegetation belts are very distinct, as are differences
between sides of the mountains, where drought-resistant forest types
on their northern escarpments replace very humid elements on their
southern and southwestern slopes.
differences are particularly pronounced on Mt. Kilimanjaro (Figure
2). The southern slopes are unusually moist with an annual
precipitation of around 900 mm in the southern foothills, around 2000
mm at 1500 m asl. and well above 3000 mm between 2000 and 2300 m. The
summit region is, in sharp contrast, an alpine desert with well below
200 mm annual precipitation (Hemp 2002). Although no exact
measurements are available, the northern slopes are clearly drier as
indicated on the available vegetation map and the accompanying
descriptions (Hemp 2001). Below 1000 m dry savannah surrounds the
entire mountain, but on the northern slopes savannahs with Acacia
drepanolobium and forest plantations climb above 2000 m altitude,
followed by a small belt of xeromorphic forests with Cedar and Olive
up to 2800 m.
the moist south between 1000 - 1800 m, evergreen submontane
deciduous forests has largely been replaced by dense agriculture.
Remnants of the lowermost submontane forests are characterized by
Olea europaea and at drier sites by O. capensis, but
from 1700 m upwards, lower montane evergreen Camphor forests and
other broadleaved forests dominated by Cassipourea malosana replace
these. In proper montane forests above 2000 m Podocarpus
latifolius becomes increasingly common. The most striking feature
of these forests is the abundance of epiphytes, in particular
Hymenophyllaceae and other fern groups, which give the stands the
appearance of a true elfin forest. On all sides of Kilimanjaro, dense
Erica excelsa forests occupies the belt between 2700-3100 m
(South) and 2800-3400m (North). Lower down, in the central montane
belt, such forests form replacement communities at disturbed sites
(Hemp & Beck 2001). Cloud forests with Hagenia abyssinica and
Hypericum revolutum are restricted to special sites such as
boulder streams and moist valleys. Above 3100 m, Erica forests
are gradually replaced by ericaceous scrub mixed with altotropical
moorlands, where subspecies of Dendrosenecio johnstonii and
Lobelia deckenii grow as vicariant taxa to Mt. Kenya (Knox 1993).
Above 3900 m, subnival Helichrysum scrub takes over up to some
4600 m, where it becomes to dry for growth of vascular plants. The
summit region is truly nival and devote of vegetation.
Fig. 2. Vegetation zonation
on Mt. Kilimanjaro
northern flanks of the neighboring Mt. Meru (Figure 3) are covered by
deciduous open woodlands, which are followed by a small band of cloud
forest with Hagenia abyssinica, Hypericum revolutum and Gnidia
glauca between 2500-3000 m (Hedberg 1955; Hedberg 1964?). Higher
up, these are replaced by evergreen ericaceous scrub up to 3700 m,
which forms the transition to altotropical moorlands, where Carex
monostachya, Lobelia deckenii and Dendrosenecio johnstonii
dominate. On the southern slopes of this mountain, however,
evergreen montane xeromorphic forest with Podocarpus latifolius,
Cassipourea malosana and Olea capensis grows at altitudes
from 1700-2300 m. These merge gradually into a distinct Bamboo belt
that is replaced by typical cloud forest at 2550-2700 m. The zonation
of the uppermost regions shows the same features as the northern
Aberdare Mts. form the next neighboring mountain range west of Mt.
Kenya. Climate and vegetation were comprehensively described by
Schmitt 1991, who also provides a vegetation map. Climatic conditions
are similar to Mt. Kenya, with highest precipitation in the southeast
and a maximum in the montane belt of around 2200 mm. The vegetation
zonation is also largely similar, with Cassipurea malosana and
Olea capensis in the supratropical mountain forest. Above 2500
to 2700 m, forests with Cedar, Podocarpus latifolius and
riverine forests with Afrocrania volkensii dominate. An almost
closed belt of Bamboo, from which specimens of P. latifolius and
Nuxia congesta emerge, follows these. Above 2900 m cloud forests with
Hagenia abyssinica, Hypericum revolutum and Rapanea
melanophloeos take over. Erica excelsa becomes important
towards the upper limit of montane forests on the main plateau in
some 3100 m altitude. Thus, most of the upper region of the Aberdare
Mts. is well in the altitudinal range of ericaceous forests and
scrub, so that grasslands are largely secondary and promoted by fires
as well as grazing by wild ungulates. The vegetation gives a very
patchy impression, and altitudinal belts can hardly be inferred
except towards the summits from 3600 m upwards. Ericaceous vegetation
includes well-developed cloud forest of E. excelsa, and scrub
with E. trimera, Stoebe kilimanscharica and Cliffortia
nitidula. The altotropical grasslands are dominated by Festuca
pilgeri, Koeleria capensis, the C4 grass Andropogon
lima, and at moist sites sedges (Carex monostachya,
Cyperus kerstenii). Again, vicariant subspecies of the
Dendrosenecio johnstonii and of the Lobelia deckenii groups
grow in the afroalpine zone.
Fig. 3. Vegetation zonation on Mt. Meru
the boundary to Uganda, Mt. Elgon and the Cherangani Mts. form the
easternmost part of these semi-humid mountain ranges. The Cheranganis
are heavily cultivated and grazed on all elevations, so there is
hardly any natural montane forest left. With some 3600 m altitude
they are not high enough for extensive natural grasslands, so much of
the present altotropical grasslands appear to be secondary (Mabberley
contrast, Mt. Elgon is the largest solitary volcano with a full set
of altitudinal belts comparable to Mt. Kenya (Wesche 2002, vegetation
maps provided by van Heist 1994a; van Heist 1994b; Neville &
Wesche in press). The western and southwestern slopes are clearly
moister than the eastern part of the mountain, with a maximum of
precipitation between 2300 and 2700 m asl. (1500-2000 mm on the
western slopes, 1200-1500 on the eastern). Hardly any natural
vegetation has survived the intense agriculture below 2200 - 2500
m, but higher up, large supratropical forests with scattered Camphor,
Aningeria adolfi-friederici, Podocarpus latifolius and
riverine Afrocrania forests survived. The eastern side is
covered by dry orotropical forest with P. gracilior, Olea
capensis and scattered Cedars. The Bamboo belt is almost closed
except in the northernmost part. Cloud forests with Kosso and tree
heather have mainly survived in the western part. Above the present
timberline, mosaics of ericaceous vegetation with E. trimera
and Stoebe kilimandscharica, Helichrysum and Alchemilla
dwarf scrub and altotropical Festuca pilgeri grassland
form the vegetation up to some 3700 m. Above this, afroalpine
grasslands, Helichrysum scrub and Dendrosencio
woodlands extend up to the summits. Mt. Elgon carries a separate
species within the Dendrosenecio johnstonii complex, with one
subspecies occurring up to 4000 and a second one between 4000 and
4300 m, resembling D. keniodendron on Mt. Kenya.
The interlacustrine highlands:
Ruwenzori, Virunga Volcanoes and Kahuzi-Biéga
mountains at the western branch of the Rift Valley form a separate
group due to their perhumid rather than semi-humid climate. They
border the huge rainforests of the Congo basin, so their lower slopes
are covered by luxurious tropical evergreen forest with a high
richness in species. Comprehensive vegetation surveys have been
produced for Kahuzi-Biéga (Fischer 1996, including land cover
map), the eastern half of the Ruwenzori range (Schmitt 1992; Osmaston
1996, land cover map by van Heist 1999, glaciological map by Osmaston
& Kaser 2001), and for parts of the Virunga Mts. ((Snowden 1933;
Biedinger 1995; Karlowski 1995).
the eastern side is generally drier, the vegetation of Ruwenzori
(Figure 4), as of Mt. Cameroon, shows no clear distinction between
the humid Western and drier Eastern side of the mountain, whereas on
the Virunga Volcanoes (Figures 5 and 6) exposition differences are
pronounced. The evergreen submontane forest belt of Ruwenzori
stretches from 1000-1600 m. Common and typical tree species are
Ocotea usambarensis, Aningeria adolfi-friederici and Syzygium
guineense. On Muhavura and Karisimbi, submontane forest occupies
both western and eastern slopes from 1300 to 2000 m. At altitudes
between 2000-2300 m evergreen supratropical mountain forests with
Olea capensis, Cassipourea malosana and Podocarpus
latifolius replace lowland species in all three regions. In
striking contrast to East Africa, no Cedar (Juniperus procera)
can be found on any mountain in central Africa. Bamboo
(Sinarundinaria alpina), mostly in stands interspersed with P.
latifolius, is a prominent feature on Ruwenzori (from 2300 -
3000 m) and on the Virungas (2400-2600 m on Karisimbi, and stretching
up as high as 2900 m on western Muhavura).
Ruwenzori, evergreen Erica forest follows immediately after
the Bamboo belt and forms extensive stands at 3000-3900 m. The
situation on the Virunga Volcanoes is different. On Western Karisimbi
(Figure 5), a small fringe of evergreen cloud forest with Hagenia
abyssinica and Hypericum revolutum separates Bamboo and
ericaceous belt between 2600-2700 m, from which ericaceous forests
can be found up to 3800 m. The cloud forest belt is better developed
on the humid eastern slope, where it extends from 2600-3500 m,
followed by ericaceous scrub up to 3800 m. The drier Muhavura
(Figure 6) shows an exactly inverted cloud forest distribution: on
the more humid western slope, Hagenia prevails from 2900-3300
m, followed by the ericaceous vegetation up to 3700 m, whereas on the
dry eastern escarpment the cloud forest zone is completely replaced
by afroalpine grassland (Karlowski 1995).
Fig. 4. Vegetation zonation on
altotropical moorlands always occupy the alpine zone of the Central
African Mountains. Tussocks of Festuca abyssinica and Carex
runssoroensis, as well as the giant rosette plants Lobelia
wollastonii and Dendrosenecio johnstonii occur on all
three mountains, whereas Lobelia stuhlmannii grows only on the
Virungas. Due to its much higher elevation, upper alpine dwarf scrub
is only found on the uppermost parts of Ruwenzori, where Helichrysum
stuhlmanii forms isolated thickets. The Kahuzi-Biéga range
is not high enough for a true afroalpine zone, so the summit region
is covered by ericaceous scrub and Deschampsia flexuosa grassland.
The latter indicates the regular presence of fires.
Fig. 5. Vegetation zonation
on Mt. Karisimbi, Virunga Volcanoes
Fig. 6. Vegetation zonation on Mt. Muhavura, Virunga Volcanoes
The westernmost outpost - Mt.
highlands in Cameroon form a widely isolated archipelago in western
central Africa. The highest peak is Mt. Cameroon (4095 m, Figure 7),
an active volcano that had its most recent eruption in 1982. The
climate is extraordinary moist, with up to 10000 mm annual
precipitation in the foot zone and below 2000 mm in the summit region
(Cable & Cheek 1998, including vegetation map). Mean air
temperature is around 20°C at 900 m and estimated to be around 4°
in the summit region. Snow is rare, even on top of Mt. Kilimanjaro.
Soils in the mountains are mostly young and therefore fertile, so
large areas have been cleared for agriculture. About a third of Mt.
Cameroon is now under legal protection, and here natural vegetation
has survived partly down to the sea level (Cable & Cheek 1998).
The small remnants of lowland forests are rich in various species,
and have the highest number of endemics of all vegetation belts (29
vs. 49 for the entire range). Pluviotropical evergreen rain forest
dominate the larger part of these altitudes, with tree species like
Schefflera mannii or Syzygium guineense frequently
found in the canopy, and Allophyllus bullatus forming a second
layer. These forests merge gradually into submontane evergreen forest
from 800 m upwards. Species composition, however, changes only
slightly, being characterized by Syzygium staudtii,
Schefflera abyssinica, and still Allophyllus bullatus
(Richards 1963; Hall 1973). From 2000-2600 m orotropical cloud
forest is the dominant forest type with species found on all tropical
African mountains like Nuxia congesta, Prunus africana,
Xymalos monospora, and Hypericum revolutum. These form the
evergreen cloud forest, together with Rapanea neurophylla.
Bamboo is completely absent from Mt. Cameroon, but occurs other
ranges in western central Africa. Hagenia abyssinica does also
not occur on Mt. Cameroon, although all other species show clear
relationships to the Hagenia-dominated upper montane forests
common in other areas of sub-Saharan Africa. At all altitudes, large
parts of the forests have been replaced by grasslands due to high
volcanic activity and thus frequent fires. For the same reason, scrub
and thickets are frequently found all over the mountain, with
Marantaceae dominating below 2000 m and ericaceous species like Erica
mannii, Agauria salicifolia and Myrica arborea forming
a patchy ericaceous belt towards the timberline ecotone. Dense
altotropical tussock grasslands reach up to 3400 m, where large
Poaceae as Andropogon sp., Festuca abyssinica, Deschampsia
mildbreadii and Loudetia camerunensis dominate,
interspersed with small tree islands (Photo). True giant rosette
plants, Dendrosenecio or Lobelia species are absent
from Mt. Cameroon, though Crassocephalum mannii and Lobelia
columnaris show resemblances to this growth form. From 3400 m
upwards to the summit region, vegetation cover becomes more open and
impoverished in species (Maitland 1932). There are no clear
vegetation differences with regard to slope exposition.
Fig. 7. Vegetation zonation on Mt. Cameroon
The Sudano-Ethiopian Mountains:
Imatong, Simien, Bale
striking contrast to the mountains in the strictly tropical part
Africa, the Sudano-Ethiopian mountains emerge from dry vegetation and
semi-arid surroundings. Thus, the mountains of southern Sudan and
Ethiopia are characterized by steep climatic gradients and
correspondingly, by clear altitudinal changes in the vegetation
zonation. All ranges are widely isolated from the eastern African
mountains, with the Imatong Mts. in southern Sudan being the least
foot zone receives some 800 mm, while at 1900 m asl. some 2200 mm
have been recorded; the highest figure in Sudan. The lower slopes of
the Imatong Mts. (Figure 8) are covered with lush evergreen
submontane forest, where Ocotea usambarensis, Olea welwitschii and
Chrysophyllum fulvum are common species (Jackson 1956). Above
2000 m, these forests merge into evergreen montane xeromorphic forest
with Podocarpus latifolius and Olea capensis. Above
2600 m, a mixture of evergreen Bamboo and evergreen cloud forests
with Hagenia abyssinica replaces these. The highest zones, above 3000
m, are covered with dense thickets formed by Erica arborea and
Myrica salicifolia up to the summit region.
Fig. 8. Vegetation zonation in the Imatong Mts.
by far largest area of afromontane and afroalpine environments is
found in Ethiopia. This huge country displays notable differences in
climate, and consequently relatively moist ranges as the Bale Mts. in
the southern part are found, as well as extremely dry areas like the
Simen Mts. in the north. The Bale Mountains (Figure 9) form the
single largest afroalpine area, with a flora and vegetation being
transient between East Africa and the drier northern part of Ethiopia
(Weinert 1983). The southern foot zone emerges from dry
Combretum-Commiphora savannah at 390 mm annual
precipitation, but the southern Harenna escarpment receives around
850 mm at 3000 m asl. The central part of the range is a huge plateau
in some 4000 m asl., where conditions become progressively moister
towards the northern slope, which receives around 1000 mm (Miehe &
Miehe 1994). This induces a clear zonation of very well pronounced
vegetation belts. The lower southern slopes are covered with dense
evergreen submontane deciduous forest from 1450-1900 m, where
Podocarpus gracilior, Filicium decipiens and Celtis
africana as well as C. gomphophylla form an open canopy
(Lisanework & Mesfin 1989; Bussmann 1997). The undergrowth is
often dominated by very dense thickets of wild coffee (Coffea
arabica). Mid-altitudes from 1900-2300 m are covered by evergreen
submontane forest (with Ocotea kenyensis, Aningeria
adolfi-friederici and Olea welwitschii as most
characteristic tree species), and thus resemble the Imatong Mountains
closely. Above 2300 m, Cassipourea malosana, Schefflera abyssinca
and Croton macrostachyus form the evergreen montane
forest, which slowly gives way to cloud forest communities with
Hagenia abyssinica and Hypericum revolutum from 2400 m
onwards. Large areas of Bamboo are found between 2800-3100 m
altitude. Cultivation has been more intense on the drier northern
slopes, so only remnants of the natural vegetation survived. Dry
orotropical Cedar and Podocarpus forests have been described
up to 3000 m (Weinert 1983), but cloud forest with Kosso and Giant
St. John's Worth is restricted to a band from 3000-3400 m.
vegetation with E. trimera sl. start to dominate the
vegetation from this zone upwards to 4000 m. As elsewhere, the
vegetation of the ericaceous belt has been severely altered by human
influence, so mosaics of forests, scrub and afroalpine communities
result. On the central Sanetti plateau, woody vegetation is
restricted to sheltered sites as outcrops or boulder streams up to 4
up to 4000m. Instead, Helichrysum scrub and to a lesser extend
tussock grasslands prevail up to the highest peaks (Menassie &
Masresha 1996). Many of the grass and shrub species demonstrate clear
affinities to the eastern African Mts. (e.g. Pentaschistis
pictigluma, Helichrysum citrispinum), but the Giant
Lobelias are present with only one endemic species (Lobelia
rhynchopetalum), and the widespread forest species Lobelia
gibberoa. Giant Groundsels are not found in Ethiopia. Strangely,
an endemic thistle (Echinops longisetus) has developed a stem
here resembling giant groundsels.
Fig. 9. Vegetation zonation of the Bale Mts.
of the Ethiopian highlands would bear montane forests if untouched,
hence remnants of these forests still occur in the central part of
the country (Tamrat 1993). The plant communities are still strikingly
similar to those described for Mt. Kenya, but differences increase
north-westwards, until the dry Simen Mts. (Figure 10) form the second
largest afroalpine region in Ethiopia. Here, the precipitation regime
finally becomes unimodal; the foot zone is rather dry but near the
timberline 1500 mm have been measured (Hurni 1982). Savannahs below
2000 m are followed by evergreen montane forest with Cedar, Olea
europaea, Rapanea simensis and finally Kosso between
2000-3500 m (3300 m in the southwest). Between 2900 and 3700 m
ericaceous vegetation takes over, but here Erica arborea is
the only tree heather; joined by Hypericum revolutum, as well
as Nuxia congesta in the lower elevations (Klötzli 1975;
Verfaille 1978; Nievergelt et al. 1998). Above this belt, Festuca
macrophylla, Carex erythrorhiza, and Lobelia
rhynchopetalum occur, the latter with an inflorescence up to 5m
tall. In the highest regions, stands are gradually replaced by
Festuca abyssinica and finally by Helichrysum citrispinum
scrub up to the summit region.
Fig. 10. Vegetation zonation in the Simien Mts.
Maloti-Drakensberg: Mountain vegetation
in southern Africa
Maloti-Drakensberg forms the highest part of the Great Escarpment
which bounds the interior plateau of southern Africa. The geological
structure is relatively homogenous and characterized by stratified
Jurassic flood basalts in the upper portion, which cap the underlying
fine-grained Clarens Sandstone (c. 1800-2200 m) and other
formations of the Karroo Supergroup. For approximately 200 kilometers
the towering scarp of the High Drakensberg constitutes the boundary
between the eastern highlands of Lesotho and South Africa. An abrupt
and rugged barrier ranging from the foothills at about 1400 m to
numerous peaks, buttresses, pinnacles, and cutbacks of the escarpment
edge lying roughly between 2800 and 3300 m marks the declivity
towards South Africa. Some long spurs (the so-called Little Berg) run
out at right angles to the escarpment. The mountain divide consists
of two parts: the northern Berg, a north-east-facing portion between
Mont aux Sources (3282 m) and Giant's Castle (3314 m),
and the southern Berg, a south-east-facing part of the escarpment
between Giant's Castle and Wilson's Peak (3310 m). Towards
the west, the summit plateau gently slopes down to the foothills and
lowlands of Lesotho below 2000 m. Rounded slopes, small cliffs
and deeply incised valleys characterize the treeless plateau. The
highest peak Thabana Ntlenyana (3482 m) lies about 4 km back
from the edge of the escarpment (Hilliard & Burtt 1987; Killick
annual precipitation in this high mountain area ranges from 1240 mm
at the foot of the escarpment (1370 m) to a maximum of about
2000 mm at an altitude of 2300 m. The escarpment edge
receives about 1600 mm (rainfall data from the Cathedral Peak
area after Killick 1963). Letseng-la-draai (3050 m), located in
the rain shadow receives 713 mm, and Mokhotlong Town (2377 m)
about 35 km to the west of the escarpment receives only about 575 mm
(Killick 1978b). Apart from these steep gradients in rainfall
distribution, the humidity conditions are generally characterized by
strong seasonal differences. Almost 80 % of the annual
precipitation falls between October and March, resulting in generally
humid conditions during the growing season (Killick 1978b). Frequent
thunderstorms and recurrent periods of fog, which usually do not
extent into Lesotho for more than 3 km contribute to summer
humidity. Snow falls frequently between April and September and is
usually restricted to the summit plateau and nearby slopes of the
escarpment. Snow cover seldom lasts for longer periods, in
exceptional years it may last for two months. Recurrent droughts
force plants to sustain water stress. Generally, the southern
Drakensberg is colder and drier than the northern portion. The
temperature regime in the upper valley sections near the escarpment
is characterized by a long frost period, lasting for approximately
180 days (Grab 1997a). Mean annual temperature is 5.7°C in
Letseng-la-draai, and 11.5°C in Mokhotlong Town. Absolute minimum
temperature exceeds -20°C at the highest altitudes. Therefore,
periglacial conditions with Thufur, needle ice, patterned ground, and
soil movements are common at higher elevations (Grab 1994, 1997b,
1998). Prominent differences between the warmer and drier north
facing slopes and the colder and moister southerly slopes result from
different solar radiation income, especially during winter (Granger &
Schulze 1977). Obviously, differences of snow cover duration and soil
moisture are ecologically most important and mirrored by a more lush
vegetation of the southerly exposures. Most plant species of the
Drakensberg are adapted to intense solar radiation, long periods of
low temperature and drought, and solifluction due to frost heaving.
The frequency of strong winds, especially in spring and summer is
considered to be "a powerful factor militating against tree growth"
by Hilliard & Burtt (1987).
position and altitudinal zonation: Problems of nomenclature
vertical sequence of the Maloti Drakensberg has been differentiated
into a montane (approximately 1280-1830 m), subalpine
(1830-2870 m) and alpine belt by Killick (1963, 1990). Other
studies like those of Herbst & Roberts (1974) and Morris et al.
(1993) follow this altitudinal division. White (1983) includes the
"afroalpine" belt as an "archipelago-like region of extreme
floristic impoverishment" within the "afromontane region" On
the other hand, Hilliard & Burtt (1987) reject the possibility to
include the slopes and summits of the Maloti-Drakensberg in an
"afromontane" and "afroalpine" region on the basis that
floristic affinities with adjacent lowlands of southern Africa are
stronger than with the tropical mountains of Tanzania and Kenya. They
argue for the recognition of the area as a distinct phytochorion, the
"South-eastern Mountain Regional Mosaic" instead. In the most
recent overview of the vegetation of southern Africa, Cowling &
Hilton Taylor (1997) include the Drakensberg and Lesotho mountains
together with the mountains of the Cape Folded Belt in an
"afromontane region". In his latest paper Killick (1997)
describes the treeless vegetation of the summit plateau as tundra.
The proposed nomenclature postulates a low-lying laural belt with
supralaural forests up to approximately 2000 m and largely
tree-deficient grass-savannas on the upper mountain slopes.
Altitudinal zonation of vegetation
triandra grasslands with Trachypogon spicatus, Hyparrhenia
hirta, Harpochloa falx predominantly covers the slopes between
approximately 1600 and 2750 m. Large tussocks of
Merxmuellera macowanii (1830-2550 m) and Festuca
costata (below 2900 m) occur along the streams. Depending on
the kind of fire regime some of these grass slopes up to about 2400 m
contain open Protea savannas, including 3-5 m high trees
of Protea caffra and P. roupelliae with rounded crowns,
and the fire-resistant dwarf species Protea dracomontana.
Locally, 2-4 m high Aloe ferox (=A. candelabrum),
characteristic for drier conditions can be found as a grassland
constituent on steep rocky north-facing slopes in the southern
Drakensberg (Loteni valley) (Hilliard & Burtt 1987, Killick
1990). Small forest groves with the 12-18 m high Yellowwood
(Podocarpus latifolius, up to 1900 m) and Olinia
emarginata (up to 2150 m), often festooned with Usnea
are confined to narrow valleys and southern aspects along stream
gullies. These forests are larger and more numerous on the northern
Berg (Hilliard & Burtt 1987). The forest margins are
characterized by the Rosaceae Leucosidea sericea, the
commonest woody plant on the Drakensberg (up to 2100 m),
Buddleia salviifolia and Bowkeria verticillata,
or by up to 2.5 m high Miscanthus capensis-Cymbopogon validus
grasslands and the Berg bamboo Thamnocalamus tessellatus
(1800-2000 m). The altitudinal distribution of forest
margins roughly extends from the valley floors to the lowermost
basalt cliffs at about 2000 m; some tree clumps persist up to
2400 m (Hilliard & Burtt 1987). Evergreen thickets with the
Thymelaeaceae Passerina drakensbergensis (2100-2450 m on
the northern Drakensberg), the Ericaceae Phillipia evansii
(1800-2300 m), the Compositae Euryops tysonii
(1800-2500 m) and the Cupressaceae Widdringtonia nodiflora
("Subalpine Fynbos" after Killick 1963) are limited
in extent because of recurrent grass fires. Forest in Lesotho only
exists to a limited extent of 1 km2 (Leslie 1991, Low &
Rebelo 1998) with the largest stand in Tsehlanyane (2100 m),
which accommodates a 6 m high Leucosidea sericea forest.
vegetation of the summit plateau of Lesotho between approximately
2850 m and the highest peaks comprise grasslands, dwarf scrub
communities, wetland communities along the streams and moister
slopes, and open scree and rock communities, colonized by small and
scattered turf patches along the basalt outcrops. These vegetation
types are best developed from December to the end of March. The
grasslands are usually dominated by the Gramineae Merxmuellera
disticha, Festuca caprina, Pentaschistis oreodoxa, Harpochloa
falx and the Cyperaceae Scirpus falsus in moister
places. Subtropical Themeda triandra grasslands are
confined to the lower slopes and warmer north-facing aspects below
3000 m. Generally, species of temperate affinity like Festuca
caprina are more abundant on the cooler aspects and at higher
altitudes. These mixed grasslands are distributed as mosaics, often
interspersed by dwarf scrubs and patches of up to 1 m high
Merxmuellera drakensbergensis stands at water surplus sites.
The evergreen tussock grass is adapted to low temperatures through
accumulation of dead and decaying material that acts as an insulator
at the base of the plant. The most common dwarf scrubs are
Helichrysum trilineatum and Erica dominans, covering
considerable areas with low heathlands, especially in the
vicinity of Mont Aux Sources. Apparently, this led Killick (196,
1979) to consider Erica-Helichrysum heathland to be the climax
community of the uppermost altitudinal belt. The cushion-forming
succulent Euphorbia clavarioides is limited to steep rocky and
mostly north-facing slopes (Hilliard & Burtt 1987). Furthermore,
the two karroid invader species Chrysocoma ciliata and Pentzia
cooperi (Morris et al. 1993) together with Artemisia afra
dominate the dwarf scrub communities between Sani Pass and
Mokhotlong. Wetlands and flat marshy grasslands in the riverheads
contrast to the surrounding vegetation. The low-growing wetland
vegetation contains a mixture of Gramineae (Agrostis sp., Poa
annua) and Cyperaceae (Scirpus falsus, S. ficinioides,
Isolepis fluitans and Schoenoxiphium filiforme). Pools and
silt patches, often waterlogged in summer form a habitat for aquatic
communities with Limosella inflata, Aponogeton ranunculiflorus and
Crassula inanis surrounded by Merxmuellera
drakensbergensis tussocks and hygrophilous forbs like
Ranunculus meyeri, Kniphofia caulescens, Moraea alticola
frequently accompanied by Trifolium burchellianum (van
Zinderen Bakker 1955, Killick 1978c, Backéus & Grab 1995,
Schwabe 1995). Thufur are often colonized by Athrixia
financial support of DFG (German Science Foundation) for large parts
of the fieldwork of this study is gratefully acknowledged. The author
wishes to thank Karsten Wesche and Wolfgang Nuesser for their
contributions on the vegetation of the Atlas and the Drakensberg, and
the regeneration of Erica dominated forests on the
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