While it is true that
Zambia lacks the spectacular mountains of most of its neighbours, it has more intact
miombo woodland than any other country. It also has many large lakes, and vast
floodplains. The Zambezi and several of its tributaries are larger than any river to the
south of the Zambia. There are more large and spectacular waterfalls than in the whole of
the rest of the subcontinent, including five major waterfalls on the Kalungwishi river
system alone.
Zambia occupies a central position in the
Zambesian Region, which is the summer-rainfall belt of southern tropical Africa. Within
the region the natural vegetation is largely determined by altitude, rainfall and soils.
Common names. In countries where the English
language has been in use for centuries, such as the United States, South Africa and
Australia, many plants are known by common English names. Zambia has a vegetation quite
different from those countries and very few species have accepted English names. The
tendency to borrow names like "Wild Medlar" is of little help to local Zambians
who are never likely to see a medlar, and would not recognise the similarity if they did.
Ecologists in this country have preferred to use one of the local names. There is no
problem with "mopane" or "baobab", which are established
internationally, but there seems little point in coining a whole new system of names when
all plants already have names. It is true that some (but not all) of the botanical names
are difficult, but for the meantime there is little alternative.
Ecological zones. Zambia can conveniently be divided
into the following ecological zones:
- Plateau with higher rainfall
- Plateau with medium rainfall
- Montane
- Kalahari Sand with higher rainfall
- Kalahari Sand with medium rainfall
- Valley
Plateau
Most of Zambia consists of flat plateau at altitudes
ranging from about 1000m to 1500m. At this altitude the climate is mild, with maximum
temperatures rarely exceeding 35 degrees Celsius. The rainfall decrease
s from north to
south. The 1000mm isohyet corresponds approximately to the boundary separating the four
northern provinces from the five southern provinces. This also approximately demarcates an
important ecological boundary between the higher-rainfall miombo and medium-rainfall
miombo zones. In the higher rainfall areas the traditional staple crop is cassava
(although this is changing) and there is no tradition of keeping cattle. In the medium
rainfall areas, the traditional crop is maize (although recent droughts and loss of cattle
through disease have forced changes here too) and there is a long tradition of keeping
cattle.
MIOMBO WOODLAND.
It never fails to come as a surprise to newcomers to the region that many of the woodland
trees start their growth cycle in August and September, long before the onset of the rains
in November. The flush of new foliage, in spectacular shades of red, is a wonderful sight,
and the evening fragrance of the Brachystegia flowers three weeks later lends an air of
magic after the heat of the day.
Miombo woodlands are generally considered to be deciduous,
but they are neither strictly evergreen nor deciduous. They are best regarded as
semi-evergreen. Muputu Brachystegia spiciformis is evergreen in good years and on
the more favourable sites, and deciduous in dry conditions.
The name "miombo" is the plural for
"muombo", the Bemba name for Brachystegia longifolia, a tree which dominates
extensive areas of the Zambesian plateau. Miombo is regarded as woodland, in spite of its
closed canopy (with crowns touching), because of its light foliage which allows sufficient
light to reach the ground to support a continuous ground cover of grasses and other herbs.
Since this herbaceous ground cover dries out and burns most dry seasons, miombo woodland
is regarded as a "fire climax", a vegetation type which is maintained by regular
fires.
Some woodlands on steep or shallow soils are naturally
protected against burning, but retain their woodland structure because of the nature of
the soil. Other areas cannot burn because of heavy grazing pressure by cattle. Such areas
tend to become heavily invaded by shrubs.
Miombo woodland is defined as any woodland which is
dominated by species of three related genera in the family Leguminosae:
Brachystegia,
Julbernardia and Isoberlinia. Unlike most other leguminous plants, these do not
develop nitrogen-fixing nodules on their roots.
Two features which these trees have in common are the
characteristic mushroom-shaped crown, and that they disperse their seeds by the explosive
dehiscence of their pods. The violent twisting of the two valves of the pod flings the
seeds to a distance of up to 25 metres.
Miombo woodland is also rich in herbs and subshrubs.
Regular burning is necessary for their maintenance. Unburnt dead grass suppresses new
growth. Mowing has the same effect as burning, indicating that it is the heat of sunlight
on the ground which stimulates new growth. Grazing, however, can be detrimental to the
more sensitive herbs, such as orchids and milkweeds, Asclepiadaceae. Miombo
woodland provides poor grazing except during the rainy months, when the grasses are young.
Since this is also the growing season of most other herbs, they are most vulnerable to
damage by trampling at that time.
MIOMBO FOREST PRODUCTS .
Miombo woodland produces a great range of valuable forest products. The chief source of
indigenous hardwood timber is mukwa, Pterocarpus angolensis, which is logged
by pitsawyers in almost every district in the country. Government restrictions on exports
have resulted in a serious undervaluation of mukwa which has led to wasteful usage.
Another very valuable product of miombo are edible caterpillars, ifinkobala, of
emperor moths Saturniidae, which are harvested in great quantities in certain
areas, and sold dried in the urban markets. The foodplant of the most important commercial
species is mutondo, Julbernardia paniculata. This species, which may well be
the most common tree of Zambia, is also the most important source of nectar for honey.
Unlike the other miombo dominants it flowers after the rains, thus providing a second
honey flow. Because there is little else in flower at that time the honey is less
contaminated than Brachystegia honey. Traditional bark-hive beekeeping has been practised
by the Lunda people of Mwinilunga and Kabompo districts for centuries.
HILLS AND ESCARPMENTS.
Steep slopes and rocky outcrops are generally protected from fires by the sparseness of
the grass cover. The good drainage on slopes ensures that the soil remains friable and
free from compaction. These areas are consequently rich in many of the more fire-sensitive
plant species. Smooth-barked trees and thicket clumps are characteristic features of the
vegetation. Hills of limestone and other basic rock types may develop deciduous thickets,
with pockets of rich herbaceous vegetation.
Characteristic of these hill slopes are the smooth-barked
species of Brachystegia B. bussei, B. glaucescens and B. microphylla, and the
white-barked Sterculia quinqueloba. Miombo species can only be supported where
there is sufficient soil to sustain them. Elsewhere deciduous species predominate.
DRY EVER-GREEN FORESTS. The best quality miombo woodland may grade into dense
evergreen forest, especially at dambo margins, or where there are laterite pavements. Like
other forest types they are protected from fire by having no flammable ground cover of
grasses and herbs. Two of the most characteristic species are mufinsa,
Syzygium
guineense subspecies afromontanum, which is frequently dominant, and mofu
Entandrophragma
delevoyi, which occurs as an occasional emergent, and is one of the tallest of
Zambia's indigenous trees. Some of the best of these forest, which are seriously
threatened by clearing for cultivation, are to be found in the southern parts of
Copperbelt Province. The best quality miombo woodland may grade into dense
evergreen forest, especially at dambo margins, or where there are laterite pavements. Like
other forest types they are protected from fire by having no flammable ground cover of
grasses and herbs. Two of the most characteristic species are mufinsa,
Syzygium
guineense subspecies afromontanum, which is frequently dominant, and mofu
Entandrophragma
delevoyi, which occurs as an occasional emergent, and is one of the tallest of
Zambia's indigenous trees. Some of the best of these forest, which are seriously
threatened by clearing for cultivation, are to be found in the southern parts of
Copperbelt Province.
DAMBOS.
Owing to the flatness of the terrain drainage on the plateaux is provided by flat-bottomed
valleys called dambos. The water level in dambos rises and falls with the seasons. This
annual rise and fall in the water level has a number of consequences. Since trees cannot
withstand flooding for any length of time the absence of trees marks the highest level to
which the water rises.
Owing to the flatness of the terrain drainage on the plateaux is provided by flat-bottomed
valleys called dambos. The water level in dambos rises and falls with the seasons. This
annual rise and fall in the water level has a number of consequences. Since trees cannot
withstand flooding for any length of time the absence of trees marks the highest level to
which the water rises.
Dambos are features of intense biological activity. It has
been observed that elephant prefer to dig for water at the dambo margins rather than
taking surface water from the channels. The reason for this is that the water entering the
dambo contains minerals, which are adsorbed by clay or taken up by living organisms in
their passage through the dambo soil.
Dambos which are permanently wet but have sufficient slope
to avoid being flooded, develop into acid peat bogs. These provide habitats for raffia
palms, Raphia farinifera, orchids and many other interesting plants. Denitrifying
bacteria deplete the soils of nitrogen compounds, thus providing habitats for
insectivorous plants, such as Drosera and Utricularia.
Water draining these peat bogs is often black owing to high
concentrations of tannins which are leached from the vegetation. Tannins are defensive
chemicals which inhibit the digestion of protein by animals. Tannin-producing plants are
prevalent in areas of nutrient deficiency, where competition for available nutrients is
particularly intense. Black-water rivers are well known to be deficient in animal life.
The grasses and sedges of poorly drained acid dambos are
extremely unpalatable to herbivores. However when the soils of these areas are disturbed
by heavy trampling, which breaks down the soil structure, palatable grasses may invade.
These areas attract grazing animals, thus extending the replacement of sedges by palatable
grasses.
Much of the upper and middle course of the Kafue River,
upriver of Kafue National Park, is flanked by
wide dambos, consisting of sedge grassland, whereas in the park the vegetation along the
river is mainly short grasses. The latter is kept in a palatable state by grazing and by
drainage provided by paths made by hippo to and from the river. Where these animals have
been eliminated by hunting the drainage is poor, the soils acid and the vegetation
unpalatable.
SWAMP
FOREST. Dambos in the higher rainfall area frequently have
patches of swamp forest (mishitu in Bemba). These, as the name implies, have wet
floors. Some are very rich in species while others consist of just one, musombo
Syzygium
cordatum. Some of the largest and richest occur in the Mpongwe area of Copperbelt
Province. Since the soils cannot be used for cultivation people do not make much use of
these forest, and rarely enter them. The dominant musombo does, however, produce fine
honey.
FLOODPLAINS.
The many extensive floodplains of Zambia have formed where large rivers cross flat
plateaux. The Zambezi River
and several of its
tributaries form a very extensive system of plains in Western Province and the western
part of North-western Province. The Kafue River has large floodplains on two of its
tributaries, the Lufupa (Busanga Plain) and the Lukanga, in addition to the Kafue Flats.
The Chambeshi River enters the Bwela flats near its source, and spills in the vast
Bangweulu swamp, which is drained by the Luapula
River.
These plains all have rich and distinctive floras. Large
areas of floodplain may be inundated for the whole period of the dry season during wet
spells, but may not flood at all during dry years. Such areas have two complementary
floras, one aquatic and the other adapted to dryland conditions. The aquatic flora
consists of a variety of species, such as the wild rice Oryza longistaminata, which are
rooted in the ground, and have stems which elongate to keep pace with the rising flood, so
that the leafy tips are always above water. The water may rise 4 metres or more. As the
flood recedes the vegetation lies down to form a dense mat.
Characteristic of the elevated levees and the higher levels
of the plains are groves of fan-palms, Borassus aethiopum and Hyphaene
petersiana, the former with a characteristic swelling on the bole. Termite mounds on
sandy floodplains frequently have the wild date-palm Phoenix reclinata.
TERMITE
MOUNDS.
Most woodlands and many dambos have large termite mounds, which are frequently covered
with dense woody vegetation. As a general rule the plants growing on these mounds are
forest species of trees and climbers, but the variety of termite mound vegetation is
enormous. What is nearly always apparent is that it is very different from the surrounding
vegetation. Yet there are exceptions even to this rule.
There are large tracts of miombo woodland on the plateaux
which lack termite mounds. The most extensive areas are the Kalahari Sands, where the
soils simply have not enough clay to support stable mounds. In many rocky areas, and on
steep slopes, where the soil depth is limited, mounds are generally absent. Mounds are
most consistently found at the dambo margins, where the optimal conditions of drainage and
an abundance of suitable clay exist.
Termite mounds undoubtedly have long lifespans, measured in
centuries. Old trees associated with mounds suggest this, and archaeological evidence
confirms it.
Some species are highly adapted to termite mounds, such as
the large succulent tree Euphorbia ingens, which superficially resembles some of
the New World cacti, but is in fact unrelated. Other trees, notably the Proteas, are never
to be found either on or even near termite mounds.
Termite mounds accumulate mineral salts. They are
frequently rich in lime even when the surrounding soils are deficient. This accounts for
the preponderance of lime tolerating plants on mounds. In some areas mounds are rich in
salt, and these attract animals which eat the soil. Even well fed cattle on commercial
ranches cannot resist salty termite mounds, eating away the soil to form a grotto and
eventually demolishing the whole mound.
Montane Vegetation
Zambia has very little montane vegetation. Four mountains
exceed an altitude of 2000 metres: the
Nyika Plateau,
which is mostly in Malawi, the Mafinga Mountains, also on the Malawi border, Mukutu, an
isolated block in Isoka District to the west of the Nyika Plateau, and another isolated
peak, Sunzu, south-east of Mbala.
Montane vegetation consists mostly of four types,
sub-montane forest, miombo woodland, macchia-type scrub and grassland. The Zambian Nyika
has two fine sub-montane forests, Chowo and Manyenjere, and Mukutu also has another.
Similar sub-montane forest occurs throughout northern Zambia, notably at the sources of
the Zambezi, Lunsemfwa and several other large rivers, and also along the upper
escarpments of the Luangwa and Luapula rivers.
High altitude miombo woodland is usually stunted, the trees
seldom growing more than about 6 metres high, and often as little as 2 metres. They are
thickly encrusted with lichens and epiphytic orchids.
The macchia-type scrub includes many shrubs in the families
Proteaceae: Protea and Faurea and Ericaceae: Erica and Agauria and
Compositae
(especially Helichrysum spp., the "everlastings").
Montane grassland is much more extensive than forest. It is
extremely rich in flowering herbs, which are seen as their best in the months after the
rains, March - May. After the fires these areas appear desolate, but without the fires the
grassland would turn to scrub, and would lose its herb flora.
Sub-montane areas, at elevations above 1400m, are much more
extensive. The flora is less distinctive than the vegetation of the high mountains, but is
nevertheless varied and rich. In the vicinity of the Kundalila Falls in Serenje District.
More than 360 species of orchids in five different habitats have been recorded
(Williamson).
The Kalahari Sands
Although the Kalahari Sand areas of western Zambia are part
of the southern African plateau the soils and vegetation are so different from those
elsewhere that they are always treated as a distinct entity by ecologists. Kalahari Sand
derives its name from the Kalahari Desert, which has undergone considerable expansions
many times during its geological history. The Kalahari Sands, which have been described as
the largest sand sea in the world, extends from the northern Cape Province, in South
Africa, to well north of the equator. They are recognised by microscopic examination of
the sand grains, which are rounded and pitted as a result of abbrasion while being blown
about. In the present Kalahari Desert of Botswana significant dune movement only occurs
where the annual rainfall is less than 150mm. The expansions of desert on either side of
the equator have depleted the African flora, which is not nearly as rich as South America
and other tropical regions.
The main differences between the Kalahari sands and the
rest of the Zambian plateaux are attributable to the very deep, free-draining soil with
virtually no clay or silt. Such soils provide an excellent growing medium for deep-rooting
woody plants. Since it is deficient in clay the soil can only hold nutrients where there
is organic matter. Exposure of the soil surface to the sun destroys much of the organic
matter and such areas tend to remain bare.
KALAHARI
SAND
WETLANDS.
On the west side of the Zambezi, where the relief is low, there are large plains which
barely rise above the high flood level. In
Liuwa
National Park near the Angolan border there are treeless plains where there is no tree
visible above the flat horizon in all directions.
Although the topland areas are deficient in clay and soil
nutrients this is not the case with the plains and dambos, which are of great importance
for grazing cattle and crop growing. In Sesheke and Senanga Districts the sand overlies
old river beds which are rich in clay and lime. These areas can usually be recognised by
the presence of termite mounds.
KALHARI SAND VEGETATION TYPES.
There are two types of dryland forest,
mavunda or Cryptosepalum forest,
which occurs mainly in the northern higher-rainfall areas, and mukusi
(Baikiaea
or Zambezi Teak) forest, which occurs mostly in Sesheke, Senanga and Kalomo Districts, as
well as in the adjacent areas of neighbouring countries.
MAVUNDA is classed as a dry-evergreen forest type, consisting of a very dense
evergreen shrub matrix, mostly about 4m in height, with a fairly light overwood in which
Cryptosepalum
exfoliatum subspecies pseudotaxus (mukwe) is the dominant tree. The two
main blocks of mavunda, occurring respectively to the north and south of the Kabompo
river, constitute the largest area of tropical evergreen forest in Africa (and probably in
the world) outside the equatorial zone. Small outliers of mavunda occur as far south as
Sesheke District.
MUKUSI
FOREST
is deciduous and occurs in an area of much lower rainfall than the
mavunda forests. Again it consists of a dense thicket with a lighter overwood, but the
species are entirely different. The overwood species is mukusi (Zambezi teak), with one or
two other species coming into forests of slightly inferior quality.
is deciduous and occurs in an area of much lower rainfall than the
mavunda forests. Again it consists of a dense thicket with a lighter overwood, but the
species are entirely different. The overwood species is mukusi (Zambezi teak), with one or
two other species coming into forests of slightly inferior quality.
Zambezi (or Rhodesian) teak supported a major timber
industry in the first half of the century, supplying enormous quantities of railway
sleepers and parquet flooring. Without this timber resource the rail through to the
Copperbelt and Lubumbashi, in Zaire, would undoubtedly have taken much longer to complete.
Production of mukusi timber peaked at 100,000 cubic metres per annum in the 1930s and
again in 1964. Since then there has been a steady decline, and a recent inventory
undertaken by the Japanese aid agency found no further exploitable reserves in the prime
teak forest areas of Sesheke District.
As with other indigenous African timbers, exploitation has
destroyed the forests, with little hope of recovery. Any opening up of the forest results
in the invasion of grasses and fires. Mukusi will survive as a woodland tree, but in this
habitat it never achieves the same size as in the forests. The German aid agency has
embarked on a 15-year programme to try to find out how to rescue what remains of the teak
forests.
KALAHARI WOODLAND,
DAMBOS.
PANS.
Most upland Kalahari sand carries woodland vegetation which is similar to miombo. On the
deep sands of the plain east of the Zambezi there are a few major rivers in the form of
wide dambos. These are highy productive areas. Also on this plain are numerous
"blow-outs", or wind-scoured pans, which are remnants from previous desert
conditions. Many are seasonal swamps. These dambos and pans of the Kalahari Sand provide a
fine thatch grass, Loudetia simplex (mwange in Lozi) for which Western
Province is famous.
Most upland Kalahari sand carries woodland vegetation which is similar to miombo. On the
deep sands of the plain east of the Zambezi there are a few major rivers in the form of
wide dambos. These are highy productive areas. Also on this plain are numerous
"blow-outs", or wind-scoured pans, which are remnants from previous desert
conditions. Many are seasonal swamps. These dambos and pans of the Kalahari Sand provide a
fine thatch grass, Loudetia simplex (mwange in Lozi) for which Western
Province is famous.
The Bulozi floodplain. This vast area of wetland is
one of the most important areas for the production of fish and cattle in southern Africa.
The depth of flooding of the Zambezi and its tributaries varies considerably from year to
year, which makes both cropping and cattle keeping somewhat dependent on chance. In years
of high rainfall the floods recede slowly and the cattle suffer, because the grazing off
the plain is of poor quality.
Western Province is the source of some of the finest crafts
in southern Africa. The most skilled are the Mbunda and Nkoya people, both of whom use
mukenge,
the root fibres of the tree Combretum zeyheri for weaving basket work which is much
sought after on international markets. These tribes also make very fine bows and arrows,
which are still used in hunting.
The Valleys
Several deep rifts traverse the eastern and southern parts
of Zambia, forming the southern end of the great East African rift system. These rifts, or
troughs as geologists prefer to call them, vary in depth. The two deepest are the valleys
of Lakes Tanganyika and Malawi, which, with Lake
Baikal, are world's deepest lakes.
The valleys of the middle Zambezi and the Luangwa and its
tributaries, the Lukusashi and Lunsemfwa, are all approximately 300m in depth. The Kafue
Flats form yet another valley trough, although the altitude, about 975m, is only slightly
less than the surrounding plateaux, and there are only minor escarpments.
All of these valleys have been formed by down-faulting. The
rocks of the valley floors date from the Karroo period. Fossil bones of mammal-like
reptiles (Therapsids), which preceded the dinosaurs, have been found in a few places.
Soils derived from the Karroo sandstones generally have a
higher mineral content than those derived from the basement complex of the plateaux, and
the contrast in the vegetation is sharp. The Luapula valley is not part of the rift valley
system and belongs to the basement complex. Its natural vegetation is not miombo, and
although it resembles the vegetation of the other valleys in structure, the species are
quite different.
Valley vegetation consists of complex mosaics. It is
affected by the drainage pattern and soils, and also by large herbivores which are
concentrated in these nutrient-rich areas. Deciduous thickets commonly occupy the well
drained sites. The banks of rivers and lagoons have riverine fringing forest. The slopes
between the thickets and the riverine forest are frequently covered with mopane woodland.
Grassy plains occur on cracking clay soils. Lagoons, which are frequently formed as ox-bow
lakes, have a rich variety of aquatic vegetation. At the lower end of the mopane slopes
there are frequently large termite mounds covered with forest species. These mounds are
often partly or completely encircled by pans, which hold water for several months into the
dry season. These pans begin as wallows and are extended as more mud is carried out on
animals using them.
MOPANE AND MOPANE WOODLAND.
Mopane Colophospermum mopane is a very distinctive species familiar to anyone who
has visited the Luangwa or Zambezi valleys, where it forms extensive pure stands on the
valley floor and lower escarpment slopes. Most people associate mopane with the hot dry
valleys, but it also occurs quite extensively on the southern plateau. Its distribution in
Zambia is strongly correlated with Karroo sandstone, dating from the Triassic period,
which occurs in a number of down-faulted troughs in the much older surrounding basement
complex.
Mopane Colophospermum mopane is a very distinctive species familiar to anyone who
has visited the Luangwa or Zambezi valleys, where it forms extensive pure stands on the
valley floor and lower escarpment slopes. Most people associate mopane with the hot dry
valleys, but it also occurs quite extensively on the southern plateau. Its distribution in
Zambia is strongly correlated with Karroo sandstone, dating from the Triassic period,
which occurs in a number of down-faulted troughs in the much older surrounding basement
complex.
In the drier parts of its distribution range mopane may
dominate most soil types, but on the plateau it is confined to particular soil types,
which are alkaline and contain high concentrations of sodium salts. The clays in these
soils swell on absorbing water, and rapidly become completely impervious.
These conditions are unfavourable to the growth of most
trees, and the few species that do tolerate them must be adapted to take up water rapidly
for the short time it is available. In fact mopane develops a superficial root system
which is able to suppress perennial grasses, and it is not uncommon to observe isolated
trees in a circle of taller perennial grass, with only sparse annual grasses and herbs
under the trees. This suppression of perennial grasses actually promotes surface runoff
and soil erosion, and gulleying is a common feature of mopane woodland areas.
The soil characteristics of mopane woodland are in complete
contrast to those of miombo woodland, which conserves both soil and water. Yet mopane can
grow on deep, well-drained soils, and many of the finest specimens are on such soils. In
these circumstances it develops a deep taproot like its woodland associates. Yet it is
evident from its distribution that it cannot compete with Brachystegia species and the
other miombo dominants.
Besides miombo and mopane there are other woodland types,
mostly of minor occurrence. They are generally more open in structure than miombo, and
lack the characteristic miombo dominants. They occur in situations which are either too
dry for miombo, or become too wet during the rains, or suffer from fires too severe for
the miombo species to tolerate.
ANIMAL-MODIFIED VEGETATION.
The unnaturally high population densities of herbivores in the more popular national parks
have greatly modified the natural vegetation. The constant trampling and browsing of
animals effectively prevents any but the best protected species from getting started. Most
trees become established as seedlings in dense thickets, which are not attractive to large
animals. In fact we often associate elephant, black rhino and buffalo with dense thickets,
but this is an artificial situation brought about by hunting with firearms, which has
forced these animals to take shelter in the dense forests and thickets. When left in peace
these animals they choose more open habitats and avoid the thickets.
Trees such as the baobab,
Cordyla africana, and several
other fruit trees become established in thickets, but once these trees mature and their
ripe fruits fall to the ground, elephants and other heavy herbivores attracted to the
fruits open up the thicket and transform it into a parkland, which is what we see in many
of the best game viewing areas in the Luangwa and Lower Zambezi national parks. Since
there is no way that these trees can replace themselves their habitats are not
sustainable.
Other fruit trees are able to regenerate without the
protection of thickets. These include Acacia tortilis, which forms its own spiny thickets,
and the two fanpalms, Borassus aethopum and Hyphaene petersiana, which has very coarse
foliage which resists browsing.
©Mike Bingham
